GENERAL OBSERVATIONS. 377 



GENERAL OBSERVATIONS ON THE PALAEOZOIC AGES. 

 I. Rocks. 



1. Maximum thickness. — The maximum thickness of the rocks of 

 North America of the Silurian age is 22,000 feet ; of the Devonian 

 age, about 14,400 feet ; and of the Carboniferous age, nearly 15,000 

 feet. 



2. Origin. — The fragmental rocks of the series — that is, the shales, 

 sandstones, and conglomerates — were made from pre-existing rock- 

 material through the agency of water, and mainly the waters of the 

 ocean. They were formed over the continents during their more 

 or less general submergence, and mostly in shallow waters or along 

 the borders of the land left uncovered by the sea. 



The limestones were formed, without probably an exception, from 

 the calcareous relics of the living species. They were accumulated 

 generally in pure ocean-waters, like the coral limestones of the 

 present period ; and hence, while protected from the incursion of 

 detritus, perhaps, by barriers of some kind, they must still have had 

 open communication with the sea. But, as in the case of the coral 

 reefs, the waters, although sometimes deep, may generally have 

 been shallow, so that the waves could perform their part in grind- 

 ing up and compacting the rising reef. When the shells are un- 

 broken, there is sufficient evidence that the waters were too deep 

 for the heavy waves to reach them ; but this does not necessarily 

 imply more than a depth of a few fathoms. 



The hornstone, which is common in the limestones of the Palaeo- 

 zoic in some parts of the country, is proved by the observations 

 mentioned on p. 270 to be mainly of organic origin. It is probable 

 that in all cases in which the fossils of a limestone are siliceous 

 (instead of calcareous) it is owing to the same cause that has origin- 

 ated the chert, — namely, the presence, in great profusion, of the 

 siliceous remains of protophytes along perhaps with sponges. 



3. Diversities of the different Regions of the continent with regard to the 

 kinds of rocks. — The three regions into which the portion of the con- 

 tinent which has been especially considered is divided are (1) the 

 Interior Continental region, (2) the Appalachian region, and (3) the 

 Eastern border region. The rocks of the Appalachian region are 

 mainly fragmental, the limestones forming only a fourth of the whole 

 thickness. The strata of the Interior Continental basin are mostly 

 limestones, these constituting full two-thirds of the series. Although 

 New York is situated mostly within the Interior basin, it still adjoins 

 the Appalachian region, and partly lies within its border. Some idea, 



