 Chapter 18 Part 1 of Principles of Geology. This is the LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. Principles of Geology by Charles Lyall. Chapter 18 Part 1. Reproductive effects of rivers continued. Deltas formed under the influence of tides, basin and delta of the Mississippi, alluvial plain, river banks and bluffs, curves of the river, natural roughs and snags, new lakes and effects of earthquakes, antiquity of the delta, delta of the Ganges and Brahmaputra, head of the delta and Thunderburns, islands formed and destroyed, crocodiles, amount of fluvia tile sediment in the water, artesian boring at Calcutta, proofs of subsidence, age of the delta, convergence of deltas, origin of existing deltas, not contemporaneous, grouping of strata and stratification in deltas, conglomerates, constant interchange of land and sea. In the last chapter several examples were given of the deltas of inland seas where the influence of the tides is almost imperceptible. We may next consider those, marine or oceanic deltas, where the tides play an important part in the dispersion of fluvia tile sediment, as in the Gulf of Mexico, where they exert a moderate degree of force, and then in the Bay of Bengal, where they are extremely powerful. In regard to estuaries, which are now termed negative deltas, they will be treated off more properly, when our attention is specially turned to the operations of tides and currents, chapters 20, 21, and 22. In this case, instead of the land gaining on the sea at the river's mouth, the tides penetrate far inland beyond the general coastline. Basin and Delta of the Mississippi A Louis Plain The hydrographical basin of the Mississippi displays, on the grandest scale, the action of running water on the surface of a vast continent. This magnificent river rises nearly in the 49th parallel of North latitude and flows to the Gulf of Mexico in the 29th, a course, including its meanders, of more than 3,000 miles. It passes from a cold climate, where the hunter obtains his furs and peltries, traverses the temperate latitudes, and discharges its waters into the sea, in the region of rice, the cotton plant, and the sugarcane. From near its mouth at the baleys, a steamboat may ascend for 2,000 miles, with scarcely any perceptible difference in the width of the river. Several of its tributaries, the Red River, the Arkanzas, the Missouri, the Ohio, and others, would be regarded elsewhere as of the first importance, and taken together, are navigable for a distance many times exceeding that of the mainstream. No river affords a more striking illustration of the law before mentioned than an augmentation of volume does not occasion a proportional increase of surface. Nay is even sometimes attended with a narrowing of the channel. The Mississippi is half a mile wide at its junction with the Missouri, the latter being also of equal widths. Yet the United Waters have only, from their confluence to the mouth of the Ohio, a medial width of about half a mile. The junction of the Ohio seems also to produce no increase, but rather a decrease of surface. The St. Francis, White, Arkanzas, and Red Rivers are also absorbed by the mainstream with scarcely any apparent increase of its width, although here and there it expands to a breadth of one and a half or even to two miles. And arriving at New Orleans, it is somewhat less than half a mile wide. Its depth there is very variable, the greatest at high water being 168 feet. The mean rate at which the whole body of water flows is variously estimated. According to Mr. Forshee, the mean velocity of the current at the surface somewhat exceeds two and one fourth miles an hour when the water is at a mean height. For 300 miles above New Orleans, the distance measured by the winding river is about twice as great as the distance in the right line. For the first hundred miles from the mouth, the rate of fall is 1.80 inch per mile. For the second hundred two inches, for the third 2.30, for the fourth 2.57. The alluvial plain of the Mississippi begins to be of great width below Cape Gerardo. Fifty miles above the junction of the Ohio, at this junction it is about 50 miles broad, south of which it contracts about 30 miles at Memphis, expands again to 80 miles at the mouth of the weight river, and then after various contractions and expansions protrudes beyond the general coastline in a large delta about 90 miles in width from northeast to southwest. Mr. Forshee estimates the area of the Great Plain as above defined at 31,200 square miles was a circumference of about 3,000 miles exceeding the area of Ireland. If that part of this plain which lies below or to the south of the branching off of the highest arm called the Achafalaya be termed the delta, it constitutes less than half of the whole being 14,000 square British miles in area. The delta may be said to be bounded on the east, west and south by the sea on the north, chiefly by the Broad Valley Plain which entirely resembles it in character as in origin. The east and west boundaries of the alluvial region above the head of the delta consists of cliffs or bluffs which on the east side of the Mississippi are very abrupt and are undermined by the river at many points. They consist from Beton Rouge in Louisiana where they commence as far north as the borders of Kentucky of geological formations newer than the Cretaceous, the lowest being Eocene and the uppermost consisting of loam, resembling the loaves of the Rhine and containing freshwater and land shells almost all of existing species. These recent shells are associated with the bones of the mastodon, elephant, tapir, millidon, horse, ox and other quadrupedes, most of them of extinct species. I have endeavored to show in my second visit to the United States that this extensive formation of loam is either an ancient alluvial plain or a delta of the Great River formed originally at a lower level and since upheaved and partially denuded. The Mississippi in that part of its course which is below the mouth of the Ohio frequently washes the eastern bluffs but never once comes into contact with the western. These are composed of similar formations but I learned from Mr. Forshee that they rise up more gently from the alluvial plain. It is supposed that the waters are thrown to the eastern side because all the large tributary rivers entering from the west have filled that side of the Great Valley with their deltas or with a sloping mass of clay and sand. So that the opposite bluffs are undermined and the Mississippi is slowly but incessantly advancing eastward. Curves of the Mississippi The river traverses the plain in a meandering course describing immense curves. After sweeping round the half of a circle it is carried in a rapid current diagonally across the ordinary direction of its channel to another curve of similar shape. Opposite to each of these there is always a sandbar and swearing in the convexity of its form to the concavity of the bend as it is called. The river by continually wearing these curves deep returns like many other streams before described on its own track so that a vessel in some places after sailing for 25 or 30 miles is brought round again to within a mile of the place once it started. When the waters approach so near to each other it often happens at high floods that they burst through the small tongue of land and insulate a portion rushing through what is called the cutoff so that vessels may pass from one point to another in half a mile to a distance which it previously required a voyage of 20 miles to reach. As soon as the river has excavated the new passage bars of sand and mud are formed at the two points of junction with the old bend which is soon entirely separated from the main river by a continuous mud bank covered with wood. The old bend then becomes a semi-circular lake of clear water inhabited by large garfish, alligators and wild foe which the steamboats have nearly driven away from the main river. A multitude of such crescent shaped lakes scattered far and wide over the alluvial plain the greater number of them to the west but some of them also eastward of the Mississippi bear testimony of the extensive wanderings of the great stream in former ages. For the last 200 miles above its mouth the course of the river is much less winding than above. There being only in the whole of that distance one great curve that called the English turn. This great straightness of the stream is ascribed by Mr. Foreshape to the superior tenacity of the banks which are more clay in this region. The Mississippi has been incorrectly described by some of the earlier geographers as a river running along the top of a long hill or mound in a plain. In reality it runs in a valley from 100 to 200 or more feet in depth as ACB in figure 24. Its banks forming long strips of land parallel to the course of the main stream and to the swamps G, F and D, E lying on each side. These extensive morasses which are commonly well wooded though often submerged for months continuously are rarely more than 15 feet below the summit level of the banks. The banks themselves are occasionally overflowed but are usually above water for a breadth of about two miles. They follow all the curves of the Great River and near New Orleans are raised artificially by embankments or levees A, B, figure 24 through which the river when swollen sometimes cuts a deep channel or crevice inundating the adjoining low lands and swamps and not sparing the lower streets of the Great City. The cause of the uniform upward slope of the riverbank above the adjoining alluvial plain is this When the waters charged with sediment pass over the banks in the flood season their velocity is checked among the herbage and reeds and they throw down at once the coarser and more sandy matter with which they are charged but the fine particles of mud are carried further on so that at the distance of about two miles a thin film of fine clay only subsides forming a stiff, anxious black soil which gradually envelops the base of trees growing on the borders of the swamps. Waste of the banks It has been said of a mountain torrent that it lays down what it will remove and removes what it has laid down and in like manner the Mississippi by the continual shifting of its course sweeps away during a great portion of the year considerable tracts of alluvium which were gradually accumulated by the overflow of warmer years and the matter now left during the spring floods will be at some future time removed. After the flood season when the river subsides within its channel it acts with destructive force upon the alluvial banks softened and diluted by the recent overflow Several acres at a time, thickly covered with wood are precipitated into the stream and large portions of the islands are frequently swept away. Some years ago, observes Captain Hall when the Mississippi was regularly surveyed all its islands were numbered from the confluence of the Missouri to the sea but every season makes such revolutions not only in the number but in the magnitude and situation of these islands that this enemeration is now almost obsolete. Sometimes large islands are entirely melted away at other places they have attached themselves to the main shore or which is the more correct statement the interval has been filled up by myriads of logs cemented together by Mount Andrabish. Rafts One of the most interesting features in the Great Rivers of this part of America is the frequent accumulation of what are termed rafts or masses of floating trees which have been arrested in their progress by snags, islands, shoals, or other obstructions and made to accumulate so as to form natural bridges reaching entirely across the stream. One of the largest of these was called the Raft of the Achalphalia an arm of the Mississippi which was certainly at some former time the channel of the Red River when the latter found its way to the Gulf of Mexico by a separate course. The Achalphalia being in a direct line with the general direction of the Mississippi catches a large portion of the timber annually brought down from the north and the drift trees collected in about 38 years previous to 1816 formed a continuous raft no less than 10 miles in length 220 yards wide and 8 feet deep the hole rose and fell with the water yet was covered with green bushes and trees and its surface enlivened in the autumn by a variety of beautiful flowers it went on increasing till about 1835 when some of the trees upon it had grown to the height of about 60 feet steps were taken then by the state of Louisiana to clear away the whole raft and open the navigation which was affected not without great labor in the space of four years. The rafts on Red River are equally remarkable in some parts of its course cedar trees are heaped up by themselves and in other places pines. On the rise of the waters in summer hundreds of these are seen some with their green leaves still upon them just as they have fallen from a neighboring bank others leafless broken and worn in their passage from a far distant tributary wherever they accumulate on the edge of a sandbar they arrest the current and soon become covered with sediment. On this mud the young willows and the poplars called cottonwood spring up their boughs still farther returning the stream and as the inundation rises accelerating the deposition of new soil the bank continuing to enlarge the channel at length becomes so narrow that a single long tree might reach from side to side and the remaining space is then soon choked up by a quantity of other timber. Unfortunately for the navigation of the Mississippi observes Captain Hall some of the largest trunks after being cut down from the position on which they grew get their roots entangled with the bottom of the river where they remain anchored as it were in the mud the force of the current naturally gives their tops a tendency downwards and by its flowing past soon strips them of their leaves and branches. These fixtures called snags or planters are extremely dangerous to the steam vessels proceeding up the stream in which they lie like a lance in rest concealed beneath the water with their sharp ends pointed directly against the boughs of the vessels coming up. For the most part these formidable snags remain so still that they can be detected only by a slight ripple above them not perceptible to inexperienced eyes. Sometimes however they vibrate up and down alternately showing their heads above the surface and basing them beneath it so imminent until lately was the danger caused by these obstructions that almost all the boats on the Mississippi were constructed on a particular plan to guard against fatal accidents but in the last 10 years by the aid of the power of steam and the machinery of the snag boat as it is called the greater number of these trunks of trees have been drawn out of the mud. The prodigious quantity of wood annually drifted down by the Mississippi and its tributaries is a subject of geological interest not merely as illustrating the manner in which abundance of vegetable matter becomes in the ordinary course of nature embedded in submarine and estuary deposits but as a testing the constant destruction of soil and transportation of matter to lower levels by the tendency of rivers to shift their courses. Each of these trees must have required many years some of them centuries to attain their full size. The soil therefore whereon they grew after remaining undisturbed for long periods is ultimately torn up and swept away. It is also found in excavating at New Orleans even at the depths of several yards below the level of the sea that the soil of the delta contains innumerable trunks of trees layer above layer some prostrate as if drifted others broken off near the bottom but remaining still erect and with their roots spreading on all sides as if in their natural position. In such situations they appeared to me to indicate a sinking of the ground as the trees must formally have grown in marshes above the sea level. In the higher parts of the Alluvial Plain for many hundred miles above the head of the delta similar stools and roots of trees are also seen buried in stiff clay at different levels one above the other and exposed to view in the banks at low water. They point clearly to the successive growth of forests in the extensive swamps of the plain where the ground was slowly raised year after year by the mud thrown down during inundations. These roots and stools belong chiefly to the deciduous Cypress, Tuxodium disticum and other swamp trees and they bear testimony to the constant shifting of the course of the Great River which is always excavating land originally formed at some distance from its banks. Formation of lakes in Louisiana Another striking feature in the basin of the Mississippi illustrative of the changes now in progress is the formation by natural causes of Great Lakes and the drainage of others. These are especially frequent in the basin of the Red River in Louisiana where the largest of them called Bistinol is more than 30 miles long and has a medium depth of from 15 to 20 feet. In the deepest parts are seen numerous Cypress trees of all sizes, now dead and most of them with their tops broken by the wind yet standing erect underwater. This tree resists the action of air and water longer than any other and if not submerged throughout the whole year will retain life for an extraordinary period. Lake Bistinol as well as Black Lake, Caddo Lake, Spanish Lake, Natchitohe's Lake and many others have been formed according to Darby by the gradual elevation of the Bad of Red River in which the alluvial accumulations have been so great as to raise its channel and cause its waters during the flood season to flow up the mouth of many tributaries and to convert parts of their courses into lakes. In the autumn when the level of Red River is again depressed waters rush back and some lakes become grassy meadows with streams meandering through them. Thus there is a periodical flux and reflux between Red River and some of these basins which are merely reservoirs alternately emptied and filled like our tide estuaries with this difference that in the one case the land is submerged for several months continuously and in the other twice in every 24 hours. It has happened in several cases that a raft of timber or a bar has been thrown by Red River across some of the openings of these channels and then the lakes become like Bistinol constant repositories of water. But even in these cases their level is liable to annual elevation and depression because the flood of the main river when at its height passes over the bar just as where sandhills close the entrance of an estuary on the Norfolk or Suffolk coast. The sea, during some high tide or storm has often breached the barrier and inundated again the interior. I am informed by Mr. Featherstone Hall that the plains of the Red River and the Arkansas are so low and flat that whenever the Mississippi rises over 30 feet above its ordinary level those great tributaries are made to flow back and inundate a region of vast extent. Both the streams alluded to contain red sediment derived from the decomposition of red porphyry and since 1833 when there was a great inundation in the Arkansas an immense swamp has been formed near the Mamell Mountain comprising 30,000 acres with here and there large lagoons where the old bed of the river was situated in which innumerable trees for the most part dead are seen standing of Cyprus cottonwood of poplar the triple-thord acacia and others which are of great size. Their trunks appear as if painted red for about 15 feet from the ground at which height and currently level line extends through the whole forest marking the rise of the waters during the last flood but most probably the causes above us signed for the recent origin of these lakes are not the only ones. Subterranean movements have altered so lately as the years 1811 and 1812 the relative levels of various parts of the basin of the Mississippi situated 300 miles northeast of Lake Bistino in those years the Great Wally from the mouth of the Ohio to that of the St. Francis including attract 300 miles in length and exceeding in area the whole basin of the Thames was convulsed to such a degree as to create new islands in the river and lakes in the Alluvial Plain. Some of these were on the left or east bank of the Mississippi and were 20 miles in extent as for example those named real food and opion in Tennessee formed in the channels or wallies of small streams bearing the same names. But the largest area affected by the Great Convulsion lies 8 or 10 miles to the westward of the Mississippi and inland from the town of New Madrid in Missouri. It is called the sunk country and is said to extend along the course of the white water and distributaries for a distance of between 70 and 80 miles north and south and 30 miles or more east and west. Throughout this area innumerable submerged trees some standing leafless others prostrate are seen and so great is the extent of lake and marsh that an active trade in the skins of muskrats mink, otters and other wild animals is now carried on there. In March 1846 I skirted the borders of the sunk country nearest to New Madrid passing along the bio St. John and Little Prairie where dead trees of various kinds some erect in the water others fallen are strewed in dense masses over the bottom in the shallows and near the shore I also beheld countless rents in the adjoining dry alluvial plains caused by the movements of the soil in 1811-1812 and still open though the rains, frost and river inundations have greatly diminished their original depth. I observed moreover numerous circular cavities called sunkholes from 10 to 30 yards wide and 20 feet or more in depth which interrupt the general level of the plain these were formed by the spouting out of large quantities of sand and mud during the earthquakes that the prevailing changes of level in the delta and alluvial plain of the Mississippi have been caused by the subsidence rather by the upheaval of land appears to me established by the fact that there are no protuberances of appraised alluvial soil projecting above the level surface of the great plain it is true that the gradual elevation of that plain by newer sessions of matter would tend to efface every inequality derived from this source but we might certainly have expected to find more broken ground between the opposite loves had local upthrows of alluvial strata being of repeated occurrence End of Chapter 18 Part 1 Chapter 18 Part 2 of Principles of Geology This is a LibriVox recording all LibriVox recordings are in the public domain For more information or to volunteer please visit LibriVox.org Principles of Geology by Charles Lyall Chapter 18 Part 2 Antiquity of the Delta The vast size of the alluvial plain both above and below the head of the delta or the branching off of the uppermost arm of the achet falaya has been already alluded to its superficial dimensions according to Mr. Forsche exceed 30,000 square miles nearly half of which belong to the true delta The deposits consist partly of sand originally formed upon near the banks of the river and its tributaries partly of gravel swept down from the main channel of which the position has continually shifted and partly of fine mud slowly accumulated in the swamps The further we descend the river towards its mouth the finer becomes the texture of the sediment The whole alluvial formation from the base of the delta upwards slopes with a very gentle inclination rising about 3 inches in a mile from the level of the sea at the balleys to the height of about 200 feet in a distance of about 800 miles That a large portion of this fluvia tile deposit together with the fluvia marine strata now in progress near the balleys consists of mud and sand with much vegetable matter intermixed may be inferred from what has been said in the trees, flouted down every summer These are seen matted together into a network around the extensive mud banks at the extreme mouth of the river Everyone acquainted with the geography of Louisiana is aware that the most southern part of the delta forms a long narrow tongue of land protruding for 50 miles into the Gulf of Mexico at the end of which are numerous channels of discharge This singular promontory consists simply of the river and its two low flat banks covered with reeds, young willows and poplars Its appearance answers precisely to that of the banks far in the interior when nothing appears above water during inundations but the higher part of the sloping glacis or bank In the one case we have the swamps or an expanse of fresh water with the tops of trees appearing above In the other the bluish green surface of the Gulf of Mexico An opinion has very commonly prevailed that this narrow promontory, the newest product of the river has gained very rapidly upon the sea since the foundation of New Orleans But after visiting the balleys in 1846 in company with Dr. Carpenter and making many inquiries of the pilots of the present outline of the coast with the excellent Spanish chart published by Charle Waugh 120 years before we came to a different conclusion The rate of permanent advance of the new land has been very slow not exceeding perhaps one mile in a century The gain may have been somewhat more rapid in former years when the new strip of soil projected less far into the Gulf since it is now much more exposed to the action of a strong marine current The tides also, when the waters of the river are low enter into each opening and scour them out destroying the banks of mud and the sandbars newly formed during the flood season An observation of Darby in regard to the strata composing part of this delta deserves attention In the steep banks of the Achafalaya before alluded to the following section he says is observable of low water First an upper stratum consisting invariably of bluish clay common to the banks of the Mississippi Below this a stratum of red ochreous earth peculiar to Red River under which the blue clay of the Mississippi again appears and this arrangement is constant moving as that geographer remarks that the waters of the Mississippi and the Red River occupied alternately at some former periods considerable tracts below their present point of union Such alternations are probably common in submarine spaces situated between two converging deltas For before the two rivers unite there must almost always be a certain period when an intermediate tract will by turns be occupied and abandoned by the waters of each stream Since it can rarely happen that the season of highest flood will precisely correspond in each In the case of the Red River and Mississippi which carry off the waters from countries placed under widely distant latitudes An exact coincidence in the time of greatest inundation is very improbable The antiquity of the delta or length of the period which has been occupied in the deposition of so vast a mass of alluvial matter is a question which may well excite the curiosity of every geologist Sufficient data have not yet been obtained to afford a full and satisfactory answer to the inquiry but some approximation may already be made to the minimum of time required When I visited New Orleans in February 1846 I found that Dr. Riddell had made numerous experiments to ascertain the proportion of sediment contained in the waters of the Mississippi and he concluded that the mean annual amount of solid matter was to the water as one to 1245 in weight or about one to three thousand in volume From the observations of the same gentleman and those of Dr. Carpenter and Mr. Forshee an eminent engineer to whom I have before alluded the average width, depth and velocity of the Mississippi and then the mean annual discharge of water were deduced I assumed 528 feet or the tens of a mile as the probable thickness of the deposit of mud and sand in the delta founding my conjecture chiefly on the depth of the Gulf of Mexico between the southern point of Florida and the Belize which equals on an average one hundred phasms and partly on some borings six hundred feet deep in the delta near Lake Pontchartain, north of New Orleans in which the bottom of the alluvial matter is said not to have been reached the area of the delta being about thirteen thousand six hundred square statute miles and the quantity of solid matter annually brought down by the river three billion seven hundred two million seven hundred fifty eight thousand four hundred cubic feet it must have taken sixty seven thousand years for the formation of the whole and if the alluvial matter of the plane above be 264 feet deep or half that of the delta it must have required thirty three thousand and five hundred more years for its accumulation even if its area be estimated as only equal to that of the delta whereas it is in fact larger if some deduction be made from the time here stated in consequence of the effect of the driftwood must have aided in filling up more rapidly the space above alluded to a far more important allowance must be made on the other hand for the loss of matter owing to the finer particles of mud not settling at the mouth of the river but being swept out far to sea during the predominant action of the tides and the waves in the winter months when the current of fresh water is feeble yet however, whilst the time during which the Mississippi has been transporting its earthy burden to the ocean the whole period, though far exceeding perhaps one hundred thousand years must be insignificant in a geological point of view since the bluffs or cliffs bounding the Great Wall-E and therefore older in date and which are from fifty to two hundred and fifty feet in perpendicular height placed in great part of loam containing land fluvia tile and lacustrine shells of species still inhabiting the same country before we take leave of the Great Delta we may derive an instructive lesson from the reflection that the new deposits already formed or now accumulating whether marine or fresh water must greatly resemble in composition the general character of their organic remains many ancient strata which enter largely into the earth's structure yet there is no sudden revolution in progress whether on the land or in the waters whether in the animate or in the inanimate world notwithstanding the excessive destruction of soil and uprooting of trees the region which yields a never-failing supply of driftwood is densely closed with noble forests and is almost unrivaled in its power of supporting animal and vegetable life in spite of the undermining of many alofty bluffs and the encroachments of the delta on the sea in spite of the earthquake which rands and fissures the soil or causes areas more than sixty miles in length to sink down several yards in a few months the general features of the district remain unaltered or are merely undergoing a slow and insensible change herds of wild deer graze on the pastures or browse upon the streets and if they diminish in number it is only worth they give way to man and the domestic animals which hollow in his train the bear, the wolf, the fox, the panther and the wildcat still maintain themselves in the fastnesses of the forests of Cyprus and Gumtree the raccoon and the opusum are everywhere abundant while the muskrat, otter and mink still frequents the rivers and lakes and a few beavers and buffaloes have not yet been driven from their ancient homes the waters steam with alligators, tortoises and fish and their surface is covered with millions of migratory waterfowl which performs their annual voyage between the Canadian lakes and the shores of the Mexican Gulf the power of man begins to be sensibly felt and many parts of the wilderness to be replaced by towns, orchards and gardens the gilded steamboats, like moving palaces stems a force of the current or shoot rapidly down the descending stream through the solitudes of the forests and prairies already does the flourishing population of the Great Wally far exceed that of the 13 United States when first they declared their independence such is the state of a continent where trees and stones are harried annually by a thousand torrents from the mountains to the plains and where sand and finer matter are swept down by a vast current to the sea together with the wreck of countless forests and the bones of animals which perish in the inundations when these materials reach the Gulf they do not render the waters unfit for aquatic animals but on the contrary the ocean here swarms with life as it generally does where the influx of a great river furnishes a copious supply of organic and mineral matter yet many geologists when they behold the spoils of the land heaped in successive strata and landed confusedly with the remains of fishes or interspersed with broken shells and corals when they see portions of erect trunks of trees with their roots still retaining their natural position and one tier of these preserved in a fossil state above another imagine that they are viewing the signs of a turbulent instead of a tranquil and settled state of the planet they read in such phenomena the proof of chaotic disorder and reiterated catastrophes instead of indications of a surface as habitable as the most delicious and fertile districts now tenanted by man as an example of still larger delta advancing upon the sea in opposition to more powerful tides I shall next describe that of the Ganges or Buranputa these, the two principal rivers of India descend from the highest mountains in the world and partially mingle their waters in the low plains of Hindustan before reaching the head of the Bay of Bengal the Burakhma Putra, somewhat the larger of the two formerly passed to the east of Dhaka even so lately as the beginning of the present century pouring most of its waters into one of the numerous channels in the delta called the Meghna by that name the main stream was always spoken of by Ranael and others in their memoirs of this region but the main trunk now unites with an arm of the Ganges considerably higher up at a point about 100 miles distant from the sea and it is constantly, according to Dr. Hooker working its way westward having formerly, as may be seen by ancient maps moved eastward for a long period the area of the delta of the combined rivers for it is impossible now to distinguish what belongs to each is considerably more than double that of the Nile even if we exclude from the delta a large extent of low flat alluvial plain doubless of fluviatile origin which stretches more than 100 miles to the hills west of Calcutta sea map at figure 25 and much further in the northerly direction beyond the head of the great delta the head of the delta is that point where the first arm is given off above that point a river receives the waters of tributaries flowing from higher levels below it on the contrary it gives out portions of its waters to lower levels through channels which flow into adjoining swamps or which run directly to the sea the Mississippi as before described has a single head which originated at an unknown period when the Red River joined it in the great delta of Bengal there may be said to be two heads nearly equidistant from the sea that of the Ganges G map figure 25 about 30 miles below Rajmohal or 216 statute miles in a direct line from the sea and that of the Brahmaputra B below Chirapungi where the river issues from the Cassia mountains a distance of 224 miles from the bay of Bengal it will appear by reference to the map that the great body of fresh water derived from the two rivers enters the bay on its eastern side and that a large part of the delta bordering on the sea is composed of a labyrinth of rivers and creeks all filled with saltwater except those immediately communicating with the Hougli or principal arm of the Ganges this tract alone known by the name of the woods or Sunderbands more properly Sundarbunds a wilderness infested by tigers and crocodiles is, according to Renel equal in extent to the whole principality of whales on the sea coast there are eight great openings each of which has evidently at some ancient period served in its turn as the principal channel of discharge although the flux and reflux of the tide extend even to the heads of the delta when the rivers are low yet when they are periodically swollen by tropical rains their volume and velocity counteract the tidal current so that, except very near the sea the ebb and flow become insensible during the flood season therefore the Ganges and Brahmaputra almost assume in their delta the character of rivers entering an inland sea the movements of the ocean being then subordinate to the force of the rivers and only slightly disturbing their operations the great gain of the delta in height and area takes place during the inundations and during other seasons of the year the ocean makes reprisals scoring out the channels and sometimes devouring rich alluvial plains islands formed and destroyed Major Air Age Cullibrook in his account of the course of the Ganges relates examples of the rapid filling up of some of its branches and the excavation of new channels where the number of square miles of soil removed in a short time the column of the earth being 114 feet high was truly astonishing 40 square miles or 25,600 acres are mentioned as having been carried away in one place in the course of a few years the immense transportation of earthy matter by the Ganges and Brahmaputra is proved by the great magnitude of the islands formed in their channels during a period far short of that of a man's life some of these many miles in extent have originated in large sandbanks thrown up around the points at the angular turning of the rivers and afterwards insulated by bridges of the streams others formed in the main channel are caused by some obstruction at the bottom a large tree or a sunken boat is sometimes sufficient to check the current and cause a deposit of sand which accumulates till it usurps a considerable portion of the channel the river then undermines its banks on each side to supply the deficiency of its bed and the island is afterwards raised by fresh deposits during every flood in the great gulf below Lakhipur formed by the united waters of the Ganges and Magna some of the islands, Cézrenel arrival in size and fertility the Isle of White whilst the river is forming new islands in one part it is sweeping away old ones in others those newly formed are soon overrun with reeds long grass, the Tamarix Indica and other shrubs and other impenetrable thickets where the tiger, the rhinoceros, the buffalo, deer and other wild animals take shelter it is easy therefore to perceive that both animal and vegetable remains may occasionally be precipitated into the flood and become embedded in the sediment which subsides in the delta three or four species of crocodile in the area, abound in the Ganges and its tributary and contiguous waters and Mr. H. T. Coulibourg informed me that he had seen both forms in places far inland many hundred miles from the sea the genetic crocodile or gavel in correct orthography, Gariel is confined to the freshwater living exclusively on fish but the commoner kinds called umiach and mugar frequent both fresh and salt being much larger and fiercer in salt and brackish water these animals swarm in the brackish water along the line of sandbanks where the advance of the delta is most rapid hundreds of them are seen together in the creeks of the delta or basking in the sun on the shoals without they will attack men and cattle destroying the natives when bathing and tame and wild animals which come to drink I have not unfrequently says Mr. Coulibourg been witness to the horrid spectacle of a floating corpse seized by a crocodile with such avidity that he half emerged above the water with his prey in his mouth the geologist will not fail to observe how peculiarly the habits and distribution of these sorens expose them to become embedded in the horizontal strata of fine mud which are annually deposited over many hundred square miles in the Bay of Bengal the inhabitants of the land which happen to be drowned or thrown into the water are usually devoured by these voracious reptiles but we may suppose the remains of the Zorians themselves to be continually and tuned in the new formations the number also of bodies but the poorer class of Hindus thrown annually into the Ganges is so great that some of their bones or skeletons can hardly fail to be occasionally enveloped in a fluvial mud it sometimes happens at the season when the periodical flood is at its height that a strong gale of wind conspiring with a high spring tide checks the descending current of the river and gives rise to most destructive inundations from this cause in 1763 the waters at Luckipur rose six feet above their ordinary level and the inhabitants of a considerable district with their houses and cattle were totally swept away the population of all oceanic deltas are particularly exposed to suffer by such catastrophes recurring at considerable intervals of time and we may safely assume that such tragical events have happened again and again since the kinetic delta was inhabited by man if human experience and forethought cannot always guard against these calamities still less can the inferior animals avoid them and the monuments of such disastrous inundations must be looked for in great abundance in strata of all ages if the surface of our planet has always been governed by the same laws when we reflect on the general order of tranquility the trains in the rich and populous delta of Bengal notwithstanding the havoc occasionally committed by the depredations of the ocean we perceive how unnecessary it is to attribute the embedding of successive races of animals in older strata to extraordinary energy in the causes of decay and reproduction in the infancy of our planet or to those general catastrophes and sudden revolutions so often resorted to End of Chapter 18 Part 2 Chapter 18 Part 3 of Principles of Geology This is a LibriVox recording All LibriVox recordings are in the public domain For more information or to volunteer please visit LibriVox.org Principles of Geology by Charles Lyle Chapter 18 Part 3 Deposits in the delta The quantity of mud held in suspension by the waters of the Ganges and Drachma Putra is found, as might be expected, to exceed that of any of the rivers alluded to in this or the preceding chapters For in the first place their feeders flow from mountains of unrivaled altitude and do not clear themselves in any lakes as does the Rhine in the Lake of Constance or the Rhone in that of Geneva And secondly, their whole course is nearer the equator than that of the Mississippi or any great river respecting which careful experiments have been made to determine the quantity of its water and earthy contents The fall of rain, moreover, as we have before seen is excessive on the southern planks of the first range of mountains which rise from the plains of Hindustan and still more remarkable is the quantity sometimes poured down in one day see above page 200 The sea where the Ganges and Drachma Putra discharge their mainstream at the flood season only recovers its transparency at the distance of from 60 to 100 miles from the delta And we may take for granted that the current continues to transport the finer particles much farther south than where the surface water first becomes clear The general slope therefore of the new strata must be extremely gentle According to the best charts there is a gradual deepening from 4 to about 60 phasms as we proceed from the base of the delta to the distance of about 100 miles into the bay of Bengal At some few points 70 or even 100 phasms are obtained at that distance One remarkable exception however occurs to the regularity of the shape of the bottom Opposite the middle of the delta at the distance of 30 or 40 miles from the coast a deep submarine valley occurs called the Swatch of No Ground about 15 miles in diameter where soundings of 180 and even 300 phasms fail to reach the bottom This phenomenon is the more extraordinary since the depression runs north to within 5 miles of the line of shoals and not only do the waters charged with sediment pass over it continually but during the moonsoons the sea, loaded with mud and sand is beaten back in that direction towards the delta As the mud is known to extend for 80 miles further into the Gulf an enormous thickness of matter must have been deposited in the Swatch We may conclude therefore either that the original depths of this part of the bay of Bengal was excessive or that substances have occurred in modern times The latter conjecture is the less improbable as the whole area of the delta has been convulsed in the historical era by earthquakes and actual substances have taken place in the neighboring coast of Chittagong while the Swatch lies not far from the volcanic band which connects Sumatra, Barron Island and Ramri Opposite the mouth of the Hogley River an immediate south to Sogor Island 4 miles from the nearest land of the delta a new islet was formed about 20 years ago called Edmontstone Island on the center of which a beacon was erected as a landmark in 1817 In 1818 the island had become 2 miles long and half a mile broad and was covered with vegetation and shrubs Some houses were then built upon it and in 1820 it was used as a pilot station The severe gale of 1823 divided it into two parts and so reduced its size as to leave the beacon standing out in the sea where after remaining 7 years it was washed away The islet in 1836 had been converted by successive storms into a sand bank half a mile long on which a sea mark was placed Although there is evidence of gain at some points the general progress of the coast is very slow for the tides when the river water is low are actively employed in removing alluvial matter In the Sunderbans the usual rise and fall of the tides is no more than 8 feet but on the east side of the delta Dr. Hooker observed in the winter of 1851 a rise of from 60 to 80 feet producing among the islands at the mouth of the Meghna and Fenni rivers a lofty wave or war as they ascend and causing the river water to be ponded back and then to sweep down with great violence when the tide ebbs The bay for 40 miles south of Chittagong is so fresh that neither algae nor mangroves will grow in it We may therefore conceive how effective may be the current formed by so greater volume of water in dispersing fine mud over a wide area Its power is sometimes augmented by the agitation of the bay during hurricanes in the months of May The new superficial strata consists entirely of fine sand and mud such at least are the only materials which are exposed to view in regular beds on the banks of the numerous creeks Neither here or higher up the Ganges could Dr. Hooker discover any land or freshwater shells in sections of the banks which in the plains higher up sometimes form cliffs 80 feet in height at low water In like manner I have stated that I was unable to find any buried shells in the delta of modern river cliffs of the Mississippi No substance so coarse as gravel as in any part of the delta of the Ganges and Brahmaputra Nor nearer the sea than 400 miles Yet it is remarkable than the boring of an artesian well at Fort William near Calcutta in the years 1835-1840 displayed at the depths of 120 feet clay and sand with pebbles This boring was carried to a depth of 481 feet below the level of Calcutta and the geological section obtained in the operation has been recorded with great care Under the surface soil at a depth of about 10 feet they came to a stiff blue clay about 40 feet in thickness below which was sandy clay containing in its lower portion abundance of decayed vegetable matter which at the bottom assumed the character of a stratum of black peat 2 feet thick This peaty mass was considered as a clear indication like the dirtbed of Portland of an ancient terrestrial surface with a forest or a sundra-bound vegetation Logs and branches of a red-colored wood occur both above and immediately below the peat So little altered that Dr. Vali was able to identify them with this hondry tree Heritiera littoralis one of the most prevalent forms at the base of the delta Dr. Falconer tells me that similar peat has been met with at other points around Calcutta at the depths of 9 feet and 25 feet It appears therefore that there has been a sinking down of what was originally land in this region to the amount of 70 feet or more perpendicular For Calcutta is only a few feet above the level of the sea and the successive peatbeds seem to imply that the subsidence of the ground was gradual or interrupted by several pauses Below the vegetable mass they entered upon a stratum of yellowish clay about 10 feet thick containing horizontal layers of kunkar or kankar anodular concretionary argillaceous limestone met with abundantly at greater or less depth in all parts of the valley of the Ganges over many thousand square miles and always presenting the same characters even at a distance of 1,000 miles north of Calcutta Some of this kunkar is said to be of very recent origin in deposits formed by river inundations near Saharan poor After penetrating 120 feet they found loam containing water-worn fragments of mika slate and other kinds of rock which the current of the Ganges can no longer transport to this region In the various beds pierced through below consisting of clay, marl and friable sunstone with kunkar here and there internext no organic remains of decidedly marine origin were met with Too positive a conclusion ought not it is true to be drawn from such a fact when we consider the narrow bore of the auger in fact in crushing shells and bones Nevertheless it is worthy of remark that the only fossils obtained in a recognizable state were of a fluvia tile or terrestrial character Thus at the depths of 350 feet the bony shell of a tortoise a trionix, a freshwater genus was found in sand resembling the living species of Bengal From the same stratum also they drew up the lower half of the humerus of a raminant at first referred to a hyena It was the size and shape, says Dr. Falconer of the shoulder bone of the caribous porcanus or common hog deer of India At the depths of 380 feet clay with fragments of lacustrine shells was incumbent on what appears clearly to have been another dirt bed or stratum of decayed wood implying a period of repose of some duration and a forest covered land which must have subsided 300 feet to admit of the subsequent superposition of the overlying deposits It has been conjectured that at the time when this area supported trees the land extended much further out into the Bay of Bengal than now but in later times the Ganges while enlarging its delta has been only recovering lost ground from the sea At the depths of about 400 feet below the surface an abrupt change was observed in the character of the strata which were composed in great part of sand, shingle and boulders The only fossils observed being the vertebrae of a crocodile shell of a trionix and fragments of wood very little altered and similar to that buried in beds far above These gravelly beds constituted the bottom of the section at the depths of 481 feet when the operations were discontinued in consequence of an accident which happened to the ogre The occurrence of pebbles at the depths of 120 and 400 feet implies an important change in the geographical condition of the region around or near Calcutta The fall of the river or the general slope of the alluvial plain may have been formerly greater or before a general and perhaps unequal subsidence hills once nearer the present base of the delta may have risen several hundred feet forming islands in the bay which may have sunk gradually and become buried under floviatile sediment Antiquity of the delta It would be a matter of no small and scientific interest if experiments were made to enable us to determine with some degree of accuracy the mean quantity of earthy matter discharged annually into the sea by the united waters of the Ganges and Brahmaputra The Reverend Mr. Everest instituted in 1831-32 a series of observations on the earthy matter brought down by the Ganges at the Gazapur, 500 miles from the sea He found that in 1831 the number of cubic feet of water discharged by the river per second at that place was during the rains, four months 494,208 during the winter, five months 71,200 during the hot weather, three months 36,330 so that we may state in round numbers that 500,000 cubic feet per second flow down during the four months of the flood season from June to September and less than 60,000 per second during the remaining eight months The average quantity of solid matter suspended in the water during the rains was by weight 1,428 part but as the water is about one-half the specific gravity of the dried mud the solid matter discharged is 1,856 part in bulk or 577 cubic feet per second this gives a total of 6,082,041,600 cubic feet for the discharge in the 122 days of the rain the proportion of sediment in the waters at other seasons was comparatively insignificant the total amount during the five winter months being only 247,881,600 cubic feet and during the three months of hot weather 38,154,240 cubic feet the total annual discharge then would be 6,368,077,440 cubic feet this quantity of mud would in one year raise a surface of 228,5 square miles or a square space each side of which should measure 15 miles a height of one foot to give some idea of the magnitude of this result we will assume that the specific gravity of the dried mud is only one half set of granite it would however be more in that case the earthy matter discharged in a year would equal 3,184,338,720 cubic feet of granite now about 12,5 cubic feet of granite weight one ton and it is computed that the great pyramid of Egypt if it were of solid mass of granite would weigh about 600 million tons the mass of matter therefore carried down annually would according to this estimate more than equal in weight and bulk 42 of the great pyramids of Egypt and that born down in the four months of the rains would equal 40 pyramids but if without any conjecture as to what may have been the specific gravity of the mud we attend merely to the weight of solid matter actually proved by Mr. Everest to have been contained in the water we find that the number of tons weight which passed down in the 122 days of the rainy season was 339,413,760 which would give the weight 56 pyramids and a half and in the whole year 355,361,464 tons or nearly the weight of 60 pyramids the base of the great pyramid of Egypt covers 11 acres and its perpendicular height is about 500 feet it is scarcely possible to present any picture to the mind which will convey an adequate conception of the mighty scale of this operation so tranquilly and almost insensibly carried on by the Ganges as it glides through its alluvial plane even at a distance of 500 miles from the sea it may have ever be stated that if a fleet of more than 80 India men each freighted with about 1400 tons weight of mud were to sail down the river every hour of every day and night for 4 months continuously they would only transport from the higher country to the sea a mass of solid metro equal to that born down by the Ganges even in this part of its course in the 4 months of the flood season or the exertions of a fleet of about 2000 such ships going down daily with the same burden charging it into the Gulf would be no more than equivalent to the operations of the Great River the most voluminous current of Lava which was flowed from Etna within historical times was that of 1669 Ferrara after correcting Borelli's estimate calculated the quantity of cubic yards of Lava in this current at 140 million now this would not equal in bulk one-fifth of the sedimentary matter which is carried down in a single year by the Ganges past Ghazipur according to the estimate above explained so that it would require 5 grand eruptions of Etna to transfer a mass of Lava from the subterranean regions to the surface equal in volume to the mud carried down in one year to that place Captain Air Stashe of the Bengal engineers has remarked to me not only it's at Ghazipur where Mr. Everest's observations were made is 500 miles from the sea but that the Ganges has not been joined there by its most important feeders these drain upon the whole 750 miles of the Himalaya and no more than 150 miles that mountain chain have sent their contributions to the main trunk at Ghazipur below that place the Ganges is joined by the Goghra, Gunduk Khosi and Tista from the north to say nothing of the zone flowing from the south one of the largest of the rivers which rise in the table land of Central India Seamap figure 25 moreover the remaining 600 miles of the Himalaya comprise that eastern portion of the basin where the rains are heaviest the quantity of water therefore carried down to the sea may probably be four or five times as much as that which passes Ghazipur the Brahmaputra according to Major Wilcox in the months of January when it is near its minimum discharges 150,000 cubic feet of water per second at the Alpara not many miles above the head of its delta taking the proportions observed at Ghazipur at the different seasons as a guide the probable average discharge of the Brahmaputra for the whole year may be estimated at about the same as that of the Ganges assuming this and secondly in order to avoid the risk of exaggeration that the proportion of sediment in their waters is about a third less than Mr. Everest's estimate the mud born down to the Bay of Bengal in one year would equal 40,000 millions of cubic feet or between six and seven times as much as that brought down to Ghazipur according to Mr. Everest's calculations in 1831 and ten times as much as that conveyed annually by the Mississippi to the Gulf of Mexico Captain Strahi estimates the annually inundated portion of the delta at 250 miles in length by 80 in breadth making an area of 20,000 square miles the space south of this in the bay where sediment is thrown down maybe 300 miles from east to west by 150 north to south or 45,000 square miles which added to the former gives a surface of 65,000 square miles over which the sediment is spread out by the two rivers suppose then the solid matter to amount to 40,000 millions of cubic feet per annum the deposit he observes must be continued for 45 years and three tenths to raise the whole area a height of one foot 13,600 years to raise it 300 feet and this as we have seen is much less than the thickness of the fluvia tail strata actually penetrated and the bottom not reached by the auger at Calcutta nevertheless we can by no means deduce from these data alone what will be the future rate of advance of the delta nor even predict whether the land will gain on the sea or remain stationary at the end of 13,000 years the bay may be less shallow than now provided a moderate depression corresponding to that experienced in part of Greenland for many centuries shall take place c.13 a subsidence quite insensible to the inhabitants of Bengal not exceeding two feet three inches in a century would be more than sufficient to counterbalance all the efforts of the two mighty rivers to extend the limits of their delta we have seen that the artesian boring at Calcutta attest what the vast depths of the swatch may also in all likelihood indicate that the antagonist force of subsidence has pre-dominated for ages over the influx of fluvia tail mud preventing it from racing the plains of Bengal or from filling up a larger portion of the bay end of c.18 c.18 c.18 c.18 c.18 c.18 c.18 c.18 c.18 c.18 c.18 c.18 c.18 c.18 c.18 c.18 c.18 c.18 c.18 c.18 c.18 c.18 c.18 c.18 c.18 in number. The deltas of the pole and the adage, for instance, would separate themselves within the recent era, as in all probability with those of the isonzo and the tor. If, on the other hand, we speculate on future changes, we may anticipate the period when the number of deltas will greatly diminish, for the pole cannot continue to encroach at the rate of a mile in a hundred years, and other rivers to gain as much in six or seven centuries upon the shallow gulf, without new junctions occurring from time to time, so that Eridanus, the king of rivers, will continually boast a greater number of tributaries. The Ganges and the Brahmaputra have perhaps become partially confluent in the same delta within the historical, or at least within the human era, and the date of the junction of the Red River and the Mississippi would, in all likelihood, have been known, if America had not been so recently discovered. The union of the Tigris and the Euphrates must undoubtedly have been one of the modern geographical changes of our Earth. For Colner Rowlingson informs me that the delta of those rivers has advanced two miles in the last 60 years, and is supposed to have encroached about 40 miles upon the gulf of Persia in the course of the last 25 centuries. When the deltas of rivers, having many mouths, converge, a partial union at first takes place by the confluence of some one or more of their arms, but it is not until the main trunks are connected above the head of the common delta that a complete intermixture of their joint waters and sediment takes place. The union, therefore, of the Poe and Adige, and of the Ganges and Brahmaputra is still incomplete. If we reflect on the geographical extent of surface drained by rivers, such as now enters the Bay of Bengal, and then consider how complete the blending together of the greater part of their transported matter has already become, and throughout how vast a delta it is spread by numerous arms, we no longer feel so much surprise at the area occupied by some ancient formations of homogeneous mineral composition. But our surprise will be still farther lessened when we afterwards inquire, Chapter 21, into the action of tides and currents in disseminating sediment. Age of existing deltas. If we could take for granted that the relative level of land and sea had remained stationary ever since all the existing deltas began to be formed, could we assume that their growth commenced one on the same instant when the present continents acquired their actual shape? We might understand the language of geologists who speak of the epoch of existing continents. They endeavour to calculate the age of deltas from this imaginary fixed period, and they calculate the gain of new land upon the sea at the mouth of the rivers as having begun everywhere simultaneously. But the more we study the history of deltas, the more we become convinced that upward and downward movements of the land and contiguous bed of the sea have exerted, and continue to exert, an influence on the physical geography of many hydrographical basins, on a scale comparable in magnitude or importance, to the amount of flowy attire deposition affected in an equal lapse of time. In the basin of the Mississippi, for example, proves both of descending and ascending movements to a vertical amount of several hundred feet can be shown to have taken place since the existing species of land and freshwater shells lived in that region. The deltas also of the Poe and Ganges have each, as we have seen, when probed by the artesian auger, born testimony to a gradual subsidence of land to the extent of several hundred feet, old terrestrial surfaces, tarf, peat, forest land and dirt beds, having been pierced at various depths. The changes of level at the mouth of the Indus in Couch, and those of New Madrid in the valley of the Mississippi, are equally instructive as demonstrating unceasing fluctuations in the levels of those areas into which running water is transporting sediment. If, therefore, the exact age of all modern deltas could be known, it is scarcely probable that we should find any two of them in the world to have coincided in date, or in the time when their earliest deposits originated. Grouping of Strata in deltas. The changes which have taken place in deltas, even within the times of history, may suggest many important considerations in regard to the manner in which subaqua sediment is distributed. With the exception of some cases hereafter to be noticed, there are some general laws of arrangement, which must evidently hold good in almost all the lakes and seas now filling up. If a lake, for example, be encircled on two sides by lofty mountains, receiving from them many rivers and torrents of different sizes, and if it be bounded on the other sides, where the surplus waters issue by a comparatively low country, it is not difficult to define some of the leading geological features which must characterize the lack of stream formation when this basin shall have been gradually converted into dry land by the influx of sediment. The strata would be divisible into two principal groups, the older comprising those deposits which originated on the side adjoining the mountains, where numerous deltas first began to form, and the newer group consisting of beds deposited in the more central parts of the basin, and towards the side farthest from the mountains. The following characters would form the principal marks of distinction between the strata in each series. The more ancient system would be composed for the most part of coarser materials, containing many beds of pebbles and sand, often of great thickness, and sometimes dipping at a considerable angle. These, with associated beds of finer ingredients, would, if traced round the borders of the basin, be seen to vary greatly in colour and mineral composition, and would also be very irregular in thickness. The beds on the contrary in the newer group would consist of finer particles, and would be horizontal, or very slightly inclined. Their colour and mineral composition would be very homogeneous throughout large areas, and would differ from almost all the separate beds in the older series. The following causes would produce the diversity here alluded to between the two great members of such lacustrine formations. When the rivers and torrents first reach the edge of the lake, the detritus washed down by them from the adjoining heights sinks at once into deep water, all the heavier pebbles and sand subsiding near the shore. The finer mud is carried somewhat farther out, but not to the distance of many miles, for the greater part may be seen, as for example, where the ron enters the lake of Geneva, to fall down in clouds to the bottom, not far from the river's mouth. Thus, alluvial tracts are soon formed at the mouth of every torrent and river, and many of these in the course of ages become of considerable extent. Pebbles and sand are then transported farther from the mountains, but in their passage they decrease in size by attrition, and are in part converted into mud and sand. At length some of the numerous deltas, which are all directed towards a common centre, approach near to each other, those of adjoining torrents become united, and each is merged in stern, in the delta of the largest river, which advances most rapidly into the lake, and renders all the minor streams, one after the other, its tributaries. The various mineral ingredients of all are thus blended together into one homogeneous mixture, and the sediment is poured out from a common channel into the lake. As the average size of the transported particles decreases, while the force and volume of the main river augments, the newer deposits are diffused continually over a wider area, and are consequently more horizontal than the older. When at first there were many independent deltas near the borders of the basin, their separate deposits differed entirely from each other. One may have been charged, like the arve, when it joins their own, with white sand and sediment derived from granite. Another may have been black, like many streams in the Tyrol, flowing from the waste of the composting rocks of dark slate. A third may have been colored by ochreous sediment, like the Red River in Louisiana. A fourth, like the Elsa in Tuscany, may have held much carbonite of lime in solution. At first they would each form distinct deposits of sand, gravel, limestone, marl, and other materials, but after their junction, new chemical combinations and a distinct color would be the result, and the particles having been conveyed 10, 20, or a greater number of miles over alluvial plains would become finer. In those deltas where the tides and strong marine currents interfere, the above description would only be applicable with certain modifications. If a series of earthquakes accompanies the growth of a delta and change the levels of the land from time to time, as in the region where the Indus now enters the sea, the phenomena will depart still more widely from the ordinary type. If, after a protracted period of rest, a delta sinks down, pebbles may be born along in shallow water near the foot of the boundary hills, so as to form conglomerates, overlying the fine mud previously thrown into deeper water in the same area. Causes of stratification in deltas. The stratified arrangement, which is observed to prevail so generally in aquas deposits, is most frequently due to variations in the velocity of running water, which cannot sweep along particles of more than a certain size and weight when moving at a given rate. Hence, as the force of the stream augments or decreases, the materials thrown down in successive layers at particular places are rudely sorted, according to their dimensions, form and specific gravity. Where this cause has not operated, as where sand, mud and fragments of rock are conveyed by glacier, a confused heap of rubbish devoid of all stratification is produced. Natural divisions are only occasioned in deltas by the interval of time, which separates annually the deposition of matter during the periodical rains, or melting of snow upon the mountains. The deposit of each year may acquire some degree of consistency before that of the succeeding year is superimposed. A variety of circumstances also give rise annually, or sometimes from day to day, to slight variations in color, fineness of the particles and other characters, by which alternations of strata distinct in texture and mineral ingredients must be produced. Thus, for example, at one period of the year driftwood may be carried down, and at another mud, as was before stated to be the case in the delta of the Mississippi, or at one time, when the volume and velocity of the stream are greatest, pebbles and sand may be spread over a certain area, over which, when the waters are low, fine matter or chemical precipitates are formed. During inundations, the turbid current of fresh water often repels the sea for many miles, but when the river is low, saltwater again occupies the same space. When two deltas are converging, the intermediate space is often for reasons before explained. Alternately, the receptacle of different sediments derived from the converging streams. The one is, perhaps, charged with calcerius, the other was argilaceous matter, or one sweeps down sand and pebbles, the other impalpable mud. These differences may be repeated with considerable regularity, until a thickness of hundreds of feet of alternating beds is accumulated. The multiplication also of shells and corals in particular spots and for limited periods gives rise occasionally to lines of separation and divides a mass which might otherwise be homogeneous into distinct strata. An examination of the shell marl now forming in the Scotch lakes, or the sediment termed warp, which subsides from the muddy water of the Humber and other rivers, shows that recent deposits are often composed of a great number of extremely thin layers, either even or slightly undulating, and preserving a general parallelism to the planes of stratification. Sometimes, however, the laminae in modern strata are disposed diagonally at a considerable angle, which appears to take place where there are conflicting movements in the waters. In January 1829, I visited in company with Professor L. A. Necker of Geneva, the confluence of the Rhine and Arve, where those rivers were very low and were cutting channels through the vast heaps of debris thrown down from the waters of the Arve in the preceding spring. One of the sandbanks which had formed in the spring of 1828 were the opposing currents of the two rivers which neutralized each other and caused a retardation in the motion, had been undermined. And the following is an exact representation of the arrangement of laminae exposed into a vertical section. The length of the portion here seen is about 12 feet, and the height 5. The strata A. A. consists of irregular alternations of pebbles and sand in undulating beds. Below these are seams of very fine sand, B. B., some as thin as paper, others about the quarter of an inch thick. The strata C. C. are composed of layers of fine greenish gray sand as thin as paper. Some of the implying beds will be seen to be thicker at their upper, others at their lower extremity, the inclination of some being very considerable. These layers must have accumulated one on the other by lateral opposition, probably when one of the rivers was very gradually increasing or diminishing in velocity, so that the point of great retardation caused by their conflicting currents shifted slowly, allowing the sediment to be thrown down in successive layers on a sloping bank. The same phenomenon is exhibited in all their strata of all ages. If the bed of a lake or of the sea be sinking, whether at a uniform or an unequal rate, or oscillating in level during the deposition of sediment, these movements will give rise to a different class of phenomena, as for example, to repeated alternations of shallow water and deep water deposits, each with peculiar organic remains, or to frequent repetitions of similar beds formed at a uniform depth, and enclosing the same organic remains and other results too complicated and varied to admit of enumeration here. Formation of conglomerates. Along the base of the meritium alps, between Tolon and Genoa, the rivers, with few exceptions, are now forming strata of conglomerate and sand. Their channels are often several miles in breadth, some of them being dry, and the rest easily forted for nearly eight months in the year. Whereas during the melting of the snow, they are swollen, and a great transportation of mud and pebbles takes place. In order to keep open the main road from France to Italy, now carried along the seacoast, it is necessary to remove annually great masses of shingle brought down during the flood season. A portion of the pebbles are seen in some localities as nearness to form beds of shingle along the shore, and the greater part are swept into a deep sea. The small progress made by the deltas of minor rivers on this coast need not surprise us when we recollect that there is sometimes a depth of 2,000 feet at a few hundred yards from the beach as nearness. Similar observations might be made respecting a large proportion of the rivers in Sicily and among others respecting that which, immediately north of the port of mestina, hurries annually vast masses of granitic pebbles into the sea. Constant interchange of land and sea. I may here conclude my remarks on deltas, observing that, imperfect as is our information of the changes which they have undergone within the last 3,000 years, they are sufficient to show how constant an interchange of sea and land is taking place on the face of our globe. In the Mediterranean alone, many flourishing inland towns and the still greater number of ports now stand where the sea rolled its waves since the era of the early civilization of Europe. If we could compare with equal accuracy the ancient and actual state of all the islands and continents, we should probably discover that millions of our race are now supported by lands, situated where deep seas prevailed in earlier ages. In many districts not yet occupied by men, land animals and forests now abound where ships once sailed. And, on the other hand, we shall find on inquiry that inroads of the ocean have been no less considerable. When to these revolutions produced by aqueous causes, we add analogous changes wrought by igneous agency. We shall perhaps acknowledge the justice of the conclusion of ours total, who declared that the whole land and sea on our globe periodically changed places. End of chapter 18 part 4