1>ALK()Z<)K1 ROCKS AND ERA, #73 



Murchison. But they arc fur more perfectly represented 

 in the United States. 



Area. It will be seen., by reference to the map, Fig. 

 .1(5!), that in the great Paleozoic basin these rocks form 

 an irregular border to the Canadian and Blue Ridge 

 Archasan areas. These borders were marginal sea-bot- 

 toms at the beginning of the Silurian times, and were 

 elevated and reclaimed during and at the end of that 

 time. There are many other smaller areas in the West, 

 but these can not be defined. 



Physical Geography. We have already given this 

 for the beginning of the age in the map, Fig. 168. For 

 the end of the age, as just stated, we must add the Silu- 

 rian area to the Archaean area. There was also at the 

 end added a large island of Silurian sea-bottom in Ohio 

 and Tennessee (see map, Fig. 169). 



Subdivisions. The Lower Paleozoic rocks are sub- 

 divided into 1. Primordial, or Cambrian ; 2. Lower 

 Silurian ; 3. Upper Silurian ; and these, again subdivided, 

 as shown in the following schedule. We simply give these 

 by name for reference, if necessary, but will treat of the 

 whole Cambrian and Silurian together : 



| Helderberg period 

 3. Upper Silurian. \ Salina 



'^Niagara " 



2. Lower Silurian. J Trenton 



I Canada " 



1. Cambrian, or | 



Primordial. f Primordial 



Life- System. 



We have already spoken of the apparent suddenness of 

 the appearance of a somewhat diversified fauna in the 

 Primordial, and accounted for it by the existence of a lost 

 interval. Immediately after the Primordial the fullness 

 of Paleozoic life became really wonderful. These early 



LE CONTE, GEOL. 18 



