PENNSYLVANIA HILL. 145 



beds can only be observed along the cliff faces. Above the porphyry is a 

 body of purple silicious shales, succeeded by white saudstone, with an occa- 

 sioual ba.nd of black sandstone similar to that already described. Prom- 

 inent among these sandstones is a very coarse conglomerate, with large 

 pebbles of quartz and fragments of Archean schists and granite. As one 

 proceeds east the dip of the beds steepens slightly, perhaps to about 25°, 

 till, on approaching within two hundred yards of the fault, it changes — 

 apparently with great suddenness — to a practically vertical angle. At the 

 same time the beds are found to be greatly decomposed and stained a red- 

 dish-yellow color. These beds being much more readily disintegrated, the 

 structure lines, when seen close to, become indistinct, being masked by 

 debris. They consist, as well as can be determined, of shales and sand- 

 stones, with one belt of blue limestone, immediately adjoining the dark 

 knob on the west, whi,ch has a thickness of about eight feet, adjoining which 

 is a bed of White Porphyry. The dark knob, which forms so prominent a 

 feature on the north wall of the hill, is white quartzite, 50 feet or more 

 in thickness, which on its eastern side is singularly altered. It has here 

 become a light frothy mass of cavernous quartz. Careful examination shows 

 that this quartzite, though the main mass stands vertical, probably urched 

 over to the eastward, and therefore forms a part of the anticlinal fold which 

 adjoins the fault on this side. The flat summit of the hill east of this point 

 is made up of beds of White Limestone, included in which is a reddish 

 decomposed porphyry. The actual curving of the White Limestone can 

 scarcely be distinguished, inasmuch as decomposition has proceeded so far 

 in the crest of the fold that a shallow ravine scores off the face of the hill 

 adjoining the quartzite knob, in which all structural lines are obliterated by 

 the sand resulting from that disintegration. Steep as are the north slopes 

 here, it is useless to search for the actual fault line or the structure lines on 

 either side of it. East of the fault there is no difficulty, and the Cambrian 

 and Silurian beds overlying the Archean can be traced continuously along 

 the wall of Mosquito gulch. Aside from the fact that the curve in the beds 

 of this mass of white quartzite can be distinguished, its position adjoining 

 the White Limestone would be sufficient tu determine it as the Parting 

 Quartzite, which forms the summit of the Silurian formation ; but in the 



MON XII 10 



