THE ART OF PREHISTORIC MEN 41 



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. 



This great basal mass of non-fossiliferous deposit is 

 called "the Archaean series." The 65,000 feet of deposit 

 above it are divided by geologists into three very unequal 

 series. The first and lowest is the Primary or Palaeozoic 

 series, occupying the enormous thickness of 52,000 feet; 

 above these we have the Secondary or Mesozoic series 

 of 10,000 feet, and lastly, bringing us to recent time, we 

 have the Tertiary or Cainozoic of only 3000 feet. These 

 three series amount in all to 65,000 feet. The Palaeozoic 

 series is more than five times as thick as the Mesozoic, and 

 these two taken together are twenty times the thickness of 

 the Tertiary. Each series is divided by geologists into a 

 series of systems, distinguished by the fossils they con- 

 tain, which, on the whole, indicate animals of a higher 

 degree of evolution as we ascend the series. 



The Palaeozoic series include the vast thicknesses of 

 the Cambrian, the Ordovician, the Silurian, Devonian, 

 Carboniferous and Permian systems. The first "trilobite" 

 is found in the lowest Cambrian rocks, and the last or 

 most recent existed in the Permian period — after 50,000 

 feet of rock had been deposited. None are known 

 of later age. The first fossil remains of a vertebrate 

 are found in the uppermost beds of the Silurian — in 

 " beds " (that is to say, stratified rocks) which are just 

 half-way in position so far as the measurable thick- 

 ness of the deposits are concerned, between the earliest 

 Cambrian fossils and the sediments of the present day. 

 To put it another way, 34,000 feet of fossiliferous rock 

 precede the stratum (upper Silurian) in which the earliest 



