82 THE MOUNTAIN. 



to a proper understanding of the exact geological position 

 of the Mountain. 



The whole mass of rocks has been separated into nine 

 groups, by some observers ; again, from careful .analysis, into 

 forty-eight formations, none of which are coextensive with 

 the great palaeozoic or fossil-bearing basin in which they are 

 found, and many of them of quite circumscribed range. 



The old number of thirteen formations, of the geological 

 reports of Pennsylvania, will suit all purposes here, the nu- 

 merical series commencing below and counting upward. 



In contact with the lower crystalline rocks, and to a cer- 

 tain extent mingled therewith, is the formation at the base of 

 the great sedimentary pile, designated in the annual reports 

 of the Pennsylvania survey as 



No. 1. 



It is a sand-rock formation, composed of fine-grained, 

 white quartzose strata, sometimes of a gray color, especially 

 dark, loose-grained, and full of cherty concretions where it 

 passes into the formation above. 



In other regions this is called Potsdam and Calciferous 

 sand-rock. Thickness sometimes 1000 feet. 



No. 2. 



Reposing on No. 1 is a large mass of limestone, generally 

 of a blue color, with layers of chert. Some parts of the 

 formation are a light blue, or dove-color, even of a fawn 

 tint, containing a quantity of magnesia. Where it passes 

 into the formation above the layers are argillaceous and 

 nearly black. When it mingles with the formation below it 

 presents siliceous limestones and calcareous sandstones. The 

 group is replete with fossil corals, encrinites, trilobites, nu- 

 merous bivalves, and orthocerata, etc., of the Silurian rocks 

 of England. This is the Trenton limestone of some geolo- 

 gists, and forms the long, boat-shaped, limestone valleys of 

 Pennsylvania, called also "coves." The largest caves or 

 caverns of the State are in this formation. 



