40 SECRETS OF EARTH AND SEA 



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 



