The amount of the sedimentary deposits of the earth's crust belonging to the Pleistocene or Quaternary Period–about 250 feet in thickness–is exceedingly small, and represents a surprisingly short space of time as compared with that indicated by the vast thickness of underlying deposits. It has nevertheless been possible to study and classify the "horizons" of this latest very short period minutely because the deposits are easily excavated, and having been more recently "laid down" have not suffered so much subsequent breaking up and destruction as have the older strata; and further, because they embed at certain levels and in favourable situations an abundance of well-preserved bones and teeth of animals and the implements and carvings in stone and bone made by man. It is worth while to look at this matter a little more exactly. The total thickness of sedimentary deposits–that is, deposit laid down by the action of water on the earth's surface, and now estimated by the measurement of strata lying one over the other in various parts of the globe–tilted and exposed to view so that we can trace out their order of super-position–is about 130,000 feet. The lower half of this huge deposit contains no fossilized remains of the living things which were present in the waters which laid it down; they were soft, probably shell-less and boneless, and so no fossilized trace of them is preserved. Thus we divide the sedimentary crust into 65,000 feet of "archaic" non-fossiliferous deposit, and an overlying 65,000 feet of fossil-containing deposits. The earliest remains of living things known are not very different from marine creatures of to-day; they are the strange shrimp-like Trilobites and the Lingula-shells found in the lower Cambrian rocks of Wales. Over them lie 65,000 feet of sedimentary deposit teaming with fossils–the petrified remains of animals and plants. The Trilobites and the Lingulas must have had a long series of ancestors leading up to them from the simplest beginnings of life–for they are highly organized creatures. But no trace of those ancestors is preserved in the 65,000 feet of sedimentary rock underlying the earliest fossils.