ANIMALS OF LONG AGO 341 



and instances are observable where older rocks have been thrust 

 above newer ones. Needless to say, such phenomena greatly com- 

 plicate geological problems. 



THE FOUR GREAT EPOCHS.— For convenience of study, the strati- 

 fied rocks of the earth have been classified. The geologist speaks of 

 four epochs 



1. The Primary or Palseozoic (ancient-life) Epoch. 



2. The Secondary or Mesozoic (middle-life) Epoch. 



3. The Tertiary or Cainozoic (modern-life) Epoch. 



4. The Quaternary or Recent Epoch. 



The rocks of the Primary Epoch contain fossils of the oldest life- 

 forms, whilst the Quaternary deposits are quite recent. The Epochs 

 are divided into Systems, and the Systems into Strata. 



The Primary Epoch includes the strata of the following systems — 

 Archaean, Cambrian, Ordovician, Silurian, Devonian, Carboniferous 

 and Permian. Each system is remarkable for the peculiar fossils 

 which predominate in its strata. 



The Arch^an system includes strata estimated to be some 50,000 

 feet in thickness. This system is often called Eozoic (Greek, eos= 

 dawn, zoe = \ii€) because its rocks betray evidence of the beginning 

 of life ; but the organic remains they contain have been so changed 

 by the vicissitudes of time and the exigencies of change that their 

 nature cannot be definitely determined. 



The Cambrian rocks, consisting of coarse sediments which were 

 evidently laid down as shore deposits in shallow seas, contain fossils 

 known as Trilobites, or three-lobed creatures, thought by some 

 authorities to be akin to wood-lice, or shrimps. Sii*E. Ray Lankester, 

 however, considers it probable that they were a primitive marine 

 group allied to the scorpions, spiders and king-crabs.^ These Trilo- 

 bites were fairly well organized creatures, and it is remarkable that they 

 are among the earliest life-forms discoverable in a fossil state. We 

 can hardly expect to find perfectly preserved fossils of lower forms, 

 as such would not lend themselves to preservation. There can be no 

 doubt, however, that in Archfean times much lowlier forms of life 

 existed. The Cambrian rocks are estimated to be about 12,000 feet 

 thick. 



The Ordovician system gets its name from the Ordovices, a tribe 



' See Extinct Animals, 1909 Edition, p. 274. 



