50 TRANSACTIONS OF THE [JAN. 7, 
gneiss in heavy beds. At the Schuylkill this is very narrow, but 
it forms the barrier which gave to that part of Philadelphia the 
name of The Falls. South-westward these rocks widen, until at 
_ the Pennsylvania Railroad they are about two and a half miles 
in width. 
I am inclined to regard this as an anticlinal of older rocks, 
possibly Laurentian, and the schists on each side as overlying it. 
South-east of this is another belt of the mica-schist extending to 
Fairmount, and beyond. In all this mica-schist region there are 
occasional beds of gneiss, sometimes feldspathic, sometimes horn- 
blendic, and varying in hardness between wide extremes. 
Many of these beds are quite narrow, others seem extensive, 
but the rocks vary greatly, even on the lineof strike. The Fair- 
mount gneiss, which unquestionably is identical with the gneiss 
around Chester, in Delaware County, has become well known 
through its use as a building material, for which it is very well 
suited, and also because the granite veins, or beds, in these rocks 
have afforded many varied and interesting minerals, while the 
mica-schist and the hornblendie gneisses contain almost none. 
Apparently underlying this and the other mica-schists and 
gneisses isa bed of very hard feldspathic gneiss, occasionally horn- 
blendic, but generally micaceous, which appears to be rising on 
the summit of an anticlinal, the axis of which is E. and W., while 
the general strike of the region is very constant at N.1+60° EK. 
Besides the chief outcrop at Frankford, which has been largely 
quarried, the same rock undoubtedly outcrops at Wayne Station, 
Germantown, and on the Wissahickon, near Mt. Airy. This 
has proved rich in minerals, yielding fine crystals of molybde- 
nite, epidote, stilbite, and apophyllite, besides sphene, biotite, cal- 
cite, Heulandite, Randite, Philadelphite, apatite, and fluorite. 
In considering the geology of these rocks, we have as fixed 
voints going south-eastward, the Triassic, the Potsdam (of the 
North Valley Hill), the Cambrian (of Chester Valley), and the 
Laurentian—and undetermined, the hydromica schist of the 
South Valley Hill, the rocks of Cream Valley (except the Pots- 
dam), and all the rocks south-east of the Laurentian. Mr. Hall 
suggested, contrary to the previously accepted theory, that the 
hydromica schists overlie the limestone, and are of Hudson 
River age, but the supposed absence of the Potsdam on the 
south easterly leg of the synclinal was a difficulty. As the Pots- 
dam really exists in that. position, with a limestone between it 
and the hydromica, the probability of Mr. Hall’s theory being 
correct is great. It is true that both the Potsdam and the 
limestone are very thin and out of all proportion to the out- 
crops to the north-westward, but as they lie upon the Laurentian 
