1890.] NATURAL SCIENCES OF PHILADELPHIA. 81 



that the hornblende rocks are more abundant in the deeper valleys 

 and upon the remarkable alternation of hornblende schist and mica 

 schist? 



At the Falls of Schuylkill we find the porphyritic gneiss before 

 spoken of. This porphyritic gneiss Mr. Hall does not mention. 

 Prof. Rogei's regards it as a mere alternation, but its great difference 

 from any other of the rocks of the region, its surprising uniformity 

 over long distances, both with and across the strike, in a region in 

 which the alternations are as numerous as in this, as well as the 

 fact that it widens rajndly southwestward, preserving all its charac- 

 ters, points to it as more than a mere stratum. 



On the east side of the Schuylkill its visible width is not over two 

 or three hundred feet. On Cobb's Creek it is nearly four miles. Its 

 visible length is over eight miles ; the southwesterly part much cov- 

 ered with soil. 



On Cobb's Creek it is ver}' well exposed. The southerly part is 

 most porphyritic ; in the central parts, and north of the centre, 

 schists appear with chai'acters of the Manayunk, and also of the 

 Chestnut Hill series, while to the northwest the true porphyritic hard 

 gneiss is well exposed about a quarter of a mile from the Chestnut 

 Hill schists, near St. Denis Church, with no ]\Ianayunk schists visi- 

 ble intervening. 



The hardness of this rock makes it everywhere prominent, and 

 next to the Laurentian ridge, it is by far the most distinct formation 

 of the region. While at the Schuylkill apparently monoclinal, and 

 interstratified in the mica schists, I am disposed to regard it as an an- 

 ticlinal, and at Cobb's Creek as two anticlinals, with an included 

 synclinal of the schists. On each side of it, the schists exhibit cer- 

 tain peculiarities alike on both sides, and on the northwest side near 

 Wynnewood, and on Cobb's Creek, it appears to dip beneath the 

 schists. 



In this porphyritic belt there is granite, unlike any other granite 

 of the region. This granite is usually very coarse, composed of a 

 flesh colored orthoclase and a peculiar chalk-white feldspar appar- 

 ently not at all decomposed, but with very little lustre. It contains 

 but little mica and quartz, much the larger part being the two 

 feldspars, and it seems to be in small segregated masses in the schists 

 rather than in beds or veins. Part of it is schistose. Sometimes it 

 is a graphic granite, the structure developed by weathering and not 

 conspicuous otherwise. Rarely it contains tourmaline, and no 



