GENERAL DESCRIPTION. 293 



continent. This is shown by the white dotted line. It possibly con- 

 nected in North Atlantic region with mass No. 1. (3.) A large West- 

 ern land-mass of unknown shape and size in the Basin region. (4.) A 

 large land-mass in the region of the Colorado and Park ranges. There 

 are still other small Archaean areas, but these were probably not land 

 at that time, but have been exposed by erosion. Between these land- 

 masses on the north, the east, and the west, there was an immense sea 

 — the great interior Palaeozoic Sea. 



The reason for thinking that the Eastern land-mass was an extensive 

 one is the immense thickness of Palaeozoic sediments which accumu- 

 lated in the sea along its western border. The reason for thinking 

 that there was a large strip of land in the Basin region is, because in 

 all that region the Mesozoic rocks rest directly and unconformably on 

 Archaean, the whole Palaeozoic being wanting. 



This was the continent at the beginning of the Palaeozoic era. 

 From this as a nucleus the continent somewhat steadily developed until 

 the whole of the Palaeozoic area was added to it, and the continent be- 

 came perhaps somewhat like that represented on page 470, as the con- 

 tinent of Cretaceous times. 



If we compare the Palaeozoic rocks of the Appalachian region 

 with the same in the central portion of the Mississippi basin, we ob- 

 serve the following changes as we go westward : (a.) The rocks in the 

 Appalachian region are highly met amor phic ; as we go westward, they 

 become less and less so, until in the region about the Mississippi River 

 they are wholly unchanged, (b.) In the Appalachian region they are 

 strongly and complexly folded ; as we go west, these folds pass into 

 gentle undulations, which die away into horizontality (see section, Fig. 

 225, on p. 253). (c.) In the Appalachian region they are about 40,000 

 feet thick ; as we go west, they thin out until the whole series is only 

 4,000 feet at the Mississippi, (d.) In the Appalachian region grits and 

 sandstones and shales predominate greatly over limestones ; as we go 

 west, the proportion of limestones increases, until these are the predomi- 

 nating rocks. These four changes are closely connected with each other, 

 and all with the formation of the Appalachian chain, as we have already 

 explained in the chapter on Mountain-Formation (p. 258). 



Subdivisions. — The Palaeozoic era is divided into three ages, which 

 are embodied in three distinct subordinate rock-systems. These ages 

 are each characterized by the dominance of a great class of organisms. 

 They are : 1. The Silurian System, or Age of Invertebrates, or some- 

 times called Age of MollusJcs ; 2. The Devonian System, or Age of 

 Fishes ; and, 3. The Carboniferous System, or Age of Acrogens and 

 Amphibians. These are three chapters in the Palaeozoic volume. 



These three systems are generally conformable with each other in 

 the Palaeozoics of the United States, as we have already shown, but 



