LIFE OF THE AZOIC AGE. 147 



contrast with what exists in earlier formations, that here is the 

 period of the true beginning of the systems of life. (See further, 

 on the relations of the simple forms of life in the Azoic to those 

 of later time, the paragraph at the bottom of page 596.) 



5. Again, in America a period of folding and crystallization 

 appears to have terminated the Azoic age, making a fitting close 

 to the era of the earth's primal inorganic history. The latter part 

 of this long time of revolution (whose centuries may have been 

 counted by scores) was the epoch of the unfossiliferous Huronian 

 beds, since these terminate the Azoic, according to Logan. 



"Whenever the earliest plant, however minute, was created, then 

 the grand idea of life first had expression, and a new line of pro- 

 gress in the earth's history was announced. 



Relations of the North American Azoic to the continent. — 

 The map, fig. 139, cannot be examined without perceiving at once 

 the following striking facts : — 



That the great Azoic area of the continent has (1) its longer leg, 

 B B, parallel approximately to the Rocky Mountains and Pacific ; 

 and (2) its shorter, C C, parallel to the Appalachian Range and the 

 Atlantic ; that — 



(3) The peninsula of Florida is nearly in the course of the Pacific 

 branch, B B; and (4) the Missouri and Arkansas Azoic regions (MA) 

 are nearly in the course of the Atlantic branch, C C. 



(5.) That the northwest side of Lake Superior and the Azoic of 

 the Black Hills lie in the same line. 



Such are some of the structure-lines of the continent in this its 

 early or Azoic state. They are features that were never afterwards 

 effaced : instead of this, they were manifested in every new step 

 in the progress of the continent. 



[From this point the progress of the life of the globe is a pro- 

 minent part of geological history. A brief review of the system 

 of life is therefore here introduced, together with some of the 

 details respecting those of the subdivisions that characterize the 

 Silurian age.] 



1. ANIMAL KINGDOM. 



In the Animal Kingdom there are four Sub-Kingdoms, based on 

 four distinct types of structure, each having its system of subdi- 

 visions of several grades or ranks. These sub-kingdoms are as 

 follow, beginning with the lowest : — 



I. Radiates. — Having a radiate structure, like a flower or a star, 

 internally as well as externally. The animals have a mouth and 



