 I mentioned to you earlier that before we could add any absolute dates to this geological timescale of ours, we actually had a relative timescale. We had rocks from all these different time periods and we had them in order. So then how exactly did we then go about adding absolute ages to this relative timescale? It started in the late 1800s and early 1900s when radioactivity was first discovered and the discovery of the uranium-led decay scheme. The first person to ever date a rock using the uranium-led system was someone called Bertram Bebaltwood and this happened in 1907 and then shortly afterwards this other person called Arthur Holmes. He was the first person to actually create a relative like a timescale with absolute dates added into it. So what he did was he collected rocks from different time periods like you know the Cambrian, the Devonian and he dated them using the uranium-led system and what was really cool about what he did was like he was doing all of this before the discovery of isotopes, before neutrons had even been discovered. He did all of this just by measuring the ratio of uranium to lead in rocks from these different time periods and like that's not the way that we really do it nowadays because now we know that there are isotopes and we measure isotopes when we're doing our dating and we have really different rigorous and more complex ways of doing it nowadays but the answers for the ages of the different time periods that he got are actually not that far off the times that we have right now.