PALEOZOIC ROCKS AND ERA. 2?1 



Growth of the Continent during Paleozoic Times. 



The map (Fig. 168) represents the continent at the 

 beginning of the Paleozoic. But during that era there 

 was a steady growth from this nucleus by addition south- 

 ward and westward, until, at the end, the whole of the 

 Paleozoic areas were reclaimed from the sea, and the 

 continent was nearly, though not exactly, that represented 

 on page 349. It will be seen that the continent was 

 already outlined at the beginning of the era, and was 

 steadily developed toward its present form. We shall 

 hereafter trace this development to its completion. 



Subdivisions of the Paleozoic. The Paleozoic era 

 and strata are divided into three ages, each represented by 

 corresponding rock-systems : 1. The age of Molluslcs, or of 

 Invertebrates, represented by the Cambrian and Silurian 

 system ; 2. The age of Fishes, by the Devonian ; 3. The 

 age of Acrogen Plants and Amphibian Animals, by the 

 Carboniferous. These three rock-systems, in many parts 

 of the world, are unconformable with each other ; but in 

 the United States they are usually entirely conformable. 

 Nevertheless, their life-systems (organic forms) are here, 

 as everywhere, quite different. 



All these subdivisions are well represented in the Pa- 

 leozoic basin of the United States (Fig. 169). In the fol- 

 lowing map of the main divisions of the geological strata 

 of the Eastern United States, the rocks representing these 

 three ages are all shown. It is important to study this 

 map well, for it will be referred to frequently hereafter in 

 connection with more recent strata. 



SECTION II. LOWER PALEOZOIC OR CAMBRIAN AND 

 SILURIAN SYSTEM. AGE OF INVERTEBRATES. 



Bocks ; Name. These rocks are called Cambrian and 

 Silurian, from the Eoman name for Wales and the Welsh, 

 because they were first studied in Wales, by Sedgvnck and 



