STRUCTURE OF THE EARTH'S CRUST. 



217 



which Paleontology forms an essential part. A group of 

 strata extending over a certain geographical extent, all of 

 which contain some fossils in common, no matter what may 

 be the chemical character of the rock, whether it be lime- 

 stone, sand, or clay, is termed a geological Formation. Thus, 

 the coal beds, with the intervening slates and grits, and the 

 masses of limestone, between which they often lie, constitute 

 but one formation the carboniferous formation. 



461. Among the stratified rocks we distinguish ten prin- 

 cipal Formations, each of which indicates an entirely new 

 era in the earth's history ; while each of the layers which 

 compose a formation indicates but some partial revolution. 

 Proceeding from below upwards, they are as follows, as 

 indicated in the cut, and also in the lower diagram on the 

 Frontispiece. 



Fig. 154. 



1st. The Lower Silurian. This is a most extensive for- 

 mation, no less than eight stages of which have been made 

 out by Geologists in North America, composed of various 

 limestones and sandstones.* 



* 1. Potsdam Sandstone ; 2. Calciferous Sandstone ; 3. Chazy Lime- 

 stone ; 4. Bird's-eye Limestone ; o. Black River Limestone ; 6. Trenton. 

 Limestone ; 7- Utica Slate ; 8. Hudson River Group ; being all found in 

 the western parts of the United States. 



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