1880.] NATURAL SCIENCES OF PHILADELPHIA. 329 



A Potsdam Sandstone Outcrop on the S. Valley Hill of Chester 

 Valley. — Mr. H. C. Lewis remarked that an occurrence of Potsdam 

 sandstone on the farm of Mr. S. Tyson, near King of Prussia, 

 Montgomery Co., to which Mr. T. D. Rand had directed attention 

 last May, was of considerable interest. A recent examination of 

 the locality with Mr. Rand, had shown that the blocks of sand- 

 stone there found were not, as had been supposed, out of place, 

 but belonged to a narrow outcrop of the sandstone on the South 

 Valley Hill. It had a strike, and apparently a dip, nearly iden- 

 tical with that of the limestone in the valley below. In one place 

 the decomposed rock is quarried for white sand. Pits for iron 

 ore have been sunk in a very ferruginous variet}' of the same 

 rock. The exposure, which can be traced by blocks upon the 

 surface, suddenlj' comes to an end in a ravine, as though by a fault. 

 A tongue of sandstone blocks extends three hundred 3'ards or 

 more down this ravine, towards the valley, in a line at right angles 

 to the line of strike. On the farther side of the ravine, to the 

 east, no sandstone has been found, its place being filled by the 

 usual damourite slate of the South Valley Hill. The blocks of 

 sandstone therefore make an " L," the shorter arm of which 

 extends down the ravine. There is here an interesting example 

 of the work of erosion in carrying down these blocks to a lower 

 level. Whether or not the existence of a fault can be proved, the 

 occurrence of Potsdam sandstone at a new locality on the South 

 Valle}^ Hill is well worth}' of studv. This formation forms the 

 North Valle}^ Hill, but is almost totally absent on the South 

 Valley Hill. It is found here only in a few isolated patches. Its 

 place is supplied by a greenish damourite slate. If, as is sup- 

 posed, the North and South Valley Hills are opposite sides of a 

 synclinal trough which dips beneath the limestone of Chester 

 Vallej', it is curious that the rocks of each hill are so very dis- 

 similar. It is important that each one of the rare exposures of 

 sandstone on the South Valley Hill should be made known, and it 

 is thought that a determination of their relations to the adjoining 

 slates will greatly help to elucidate the geology of that region. 



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