THE MOUNTAIN. 87 



FORMATION 10 



Is a mass of gray, brownish, coarse, and fine-grained 

 sandstones, with sometimes coarse, siliceous conglomerates. 

 It also contains blackish, carbonaceous slates and coal shales, 

 sometimes with one or two small coal-seams. It is a hard, 

 indestructible group, and consequently forms mountains. 

 It is the rock which makes up and caps the first range of 

 Alleghany spurs. It also forms many other mountains, as 

 Shickshinny, Nescopeck, Peter's, Berry's, and Mahantongo 

 mountains in Pennsylvania. It contains fossils, both ani- 

 mal and vegetable. In some localities the formation is 2000 

 feet thick; but generally, throughout Pennsylvania, it is 

 not more than 300 feet thick. 



FORMATION 11. 



On No. 10 there is a mass of red shale and sandstone, 

 also gray sandstone, sometimes compact, but generally soft 

 and argillaceous; containing also some lime layers, with 

 fossils, and a peculiar calcareous sand mass, which, from its 

 style of weathering, is easily identified, and is always a key 

 for fixing the position of masses above and below. Its 

 weathered surfaces have a remarkable oblique-lined appear- 

 ance, and the mass is full of water-worn cavities, enlarging, 

 in some places, into considerable caverns. The Formation 

 generally produces slight depressions behind the first peaks 

 or butresses of the mountains of Formation 10, its soft shales 

 being easily abraded and destroyed. 



It is sometimes, in the anthracite region, 2900 feet thick, 

 but in the bituminous measures it is not more than 250 or 

 300 feet thick. It contains an iron ore of value in some 

 places. It also contains, in certain parts of the bituminous 

 coal-region, a thin seam or two of coal, and in the south, 

 one or two large workable beds of coal, although its place 

 is under the conglomerate, the proper floor of the coal. 



