178 PROCEEDINGS OF THE ACADEMY OF [1892. 



decomposition was kuown. At this point halotrichite was at times 

 very abundant and in quite large masses, due to the fact that the 

 rock in question seems to lie in a synclinal, the axis of which rises 

 rapidly southwestward, forming on the northeast side a series of 

 nooks protected by overhanging ledges, so that the halotrichite 

 formed was protected from the Aveather. Associated with this is a 

 subsulphate of iron, or iron sinter, probably glockerite, also 

 alunogen. 



Pebbles of many varieties of quartz, jasper, basanite, etc., have 

 been found in the gravel which overspreads a large part of Phila- 

 delphia. 



All the rocks mentioned above have a general northeast and 

 southwest strike, but there is a belt of very hard gneiss extending 

 from Frankford to the Wissahickon, with a strike nearly west, 

 probably rising on the crest of an anticlinal Avave or possibly an in- 

 trusive mass. This gneiss being hard and quite uniform, with a 

 straight fracture, makes a valuable building stone, and it has been 

 quarried at several points, at some quite largely. It varies in text- 

 ure from a rather coarse granitic or syenitic gneiss to an almost 

 cryptocrystalline felsite or granulite. Granite segregations are not 

 common, but they are sometimes large and chiefly of orthoclase. The 

 largest quarries of this are at Frankford. Here the minerals were 

 finest and most abundant. Chief among these was stilbite, rarely 

 in crystalline forms, usually in stellate radiations on the surface of 

 what the quarrymen call " heads " in the gneiss, that is joint jilanes 

 at right angles, or nearly so, to the bedding. This was very abun- 

 dant, many tons of rock covered with it being exposed at a single 

 blast, and some of it was beautiful, but, occurring as it did on this 

 hard rock, and on faces at right angles to the cleavage, it Avas'often 

 difficult to procure specimens in the midst of great abundance. 

 Fortunately, at times, there were subordinate joints close to the 

 main one; these, too, would be filled with stilbite and along such 

 line the rock would split easily. Usually the coating was very thin, 

 almost immeasurable, but occasionally the joint would widen, and 

 the stilbite would occur, half an inch or more in thickness. AVith 

 the stilbite is associated, rarely, apophyllite in fine crystals some of 

 them half an inch across, usually opaque glassy-white but some- 

 times colorless and transparent. 



Molybdenite occurs imbedded in the gneiss, sometimes in large 

 masses (one of over a pound weight of pure molybdenite) and also 



