STRUCTURE OF THE EARTH S CRUST. 



185 



to Geology, of which it 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 limestone, 

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

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

 masses of limestone in which they are often imbedded, 

 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 in- 

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

 frontispiece. 



Fig. 154. 



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

 formation, 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. Chrzy Lime- 

 stone; 4. Bird's-eye Limestone ; 5. Black River Limestone ; 6. Trentoii 

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

 the western parts of the United States. 



16* 



