290 



PALAEOZOIC SYSTEM OF ROCKS. 



being conspicuously different from that which precedes and that which 

 follows. Whatever of life existed before, its record is too imperfect to 

 give us a clear conception of its character. But in the Palaeozoic the 

 evidences of abundant and very varied life are clear ; more than 20,000 

 species having teen described. It stands out the most distinct era in 

 the whole history of the earth. The Archaean must be regarded as the 

 mythical period. Here, with the Palaeozoic, commences the true dawn 

 of history. 



Rocks — Thickness, etc. — The rocks of this system, although less 

 powerful than the preceding, are also of enormous thickness compared 

 with those of later geological times, being in the Appalachian region 

 about 40,000 feet ; in Nevada, about 40,000 feet ; in the Wahsatch 

 Mountains, 33,000 feet (King). But these extreme thicknesses are more 

 local than in the case of the Archaean. It is believed that we are safe in 

 saying that the time represented by them is equal to all subsequent time 

 to the present. 



There is nothing very characteristic in the rocks composing Palaeo- 

 zoic strata, though the practiced eye may often distinguish them by 

 their lithological character. Though strongly folded and highly meta- 

 morphic in some regions, these characters are not so universal as in the 

 Laurentian. 



In the United States the rocks of the whole system are often con- 

 formable — for example, in New York and in Utah. In Europe, on the 

 contrary, the principal divisions are usually unconformable. In this 

 country, therefore, the subdivisions are founded almost wholly on change 

 in the life-system ; while in Europe the same subdivisions are founded 

 on unconformity of the rock-system, as well as change in the life-sys- 

 tem. Further, in this country, in passing from Pennsylvania, through 

 New York, into Canada, we pass over the outcropping edges of the 

 whole system, from the highest to the lowest, and finally into the Lau- 

 rentian (Fig. 264). This, taken in connection with the conformity of 

 the rocks, shows that during the Palaeozoic the continent in this part 

 was successively developed, from the north toward the south, by bodily 



-/ N 

 S 



Fig. 264.— Ideal Section north and south from Canada to Pennsylvania: A, Archaean; L S and 

 US, Silurian; D, Devonian; C, Carboniferous. 



upheaval of the Laurentian area and successive exposure of contiguous 

 sea-bottom. In Europe the oscillations seem to have been more fre- 

 quent and violent. 



Fig. 264 is a section from Pennsylvania to Canada, showing the 



