368 THE CAMBRIAN PERIOD 



part of the modern fish-fauna, both marine and fresh-water, as 

 well as birds and mammals, are entirely absent from the Palaeozoic. 



The overwhelming majority of Palaeozoic species, and even gen- 

 era, fail to pass over into the Mesozoic, and even in the larger 

 groups which continued to flourish, almost always a more or less 

 complete change of structure occurs, so that Palaeozoic corals, Echi- 

 noderms, and fishes, for example, are very markedly distinct from 

 those which succeeded them. The difference is generally in the 

 direction of greater primitiveness of structure in the older forms, 

 Palaeozoic types standing in somewhat the same relation to subse- 

 quent types as the embryo does to the adult. 



The Palaeozoic climate appears to have been mild and equable 

 on the whole, very much the same kinds of animals and plants occur- 

 ring in high as in low latitudes. In short, we can detect no evi- 

 dence of climatic zones as being distinctly marked in those periods. 

 Certain remarkable exceptions to this rule will be noted in their 

 proper place. 



THE CAMBRIAN PERIOD 



The rocks older than the coal measures were for a long time 

 heaped indiscriminately together, under the name of Greywacke, 

 or Transition Rocks, and were little regarded by geologists. About 

 1 83 1, the problem of these ancient rocks was attacked by two 

 eminent English geologists, Sedgwick and Murchison, who soon 

 brought order out of the chaos. There was much discussion and 

 dispute as to the limits of the systems into which the Greywacke 

 should be divided, and as to the names which should be given to 

 them. The oldest fossiliferous strata were by Sedgwick called 

 Cambrian (from the Latin name for Wales), but were included 

 by Murchison in his Lower Silurian. The latter example was long 

 followed by most geologists, but the advance of knowledge has fully 

 vindicated the claim of the Cambrian to rank as a distinct system. 

 The divisions of the American Cambrian are as follows : — 



3. Upper Cambrian, Potsdam Epoch, Olenus Fauna. 



2. Middle Cambrian, Acadian Epoch, Paradoxides Fauna. 



1. Lower Cambrian, Georgian Epoch, Olenellus Fauna. 



