176 PROCEEDINGS OF THE ACADEMY OF [1893. 



show much difference in thickness. Extinction invariably parallel 

 to the longer axis. 



Sericite-schist, also called hydromica schist, has been observed 

 on several outcrops about one mile north of Berwyn, Pa. It is a 

 glistening soft schistose rock much interspersed with quartz. 

 Parallel to the layers it is smooth and fatty to the touch ; at right 

 angle to it, rather sharp and rough. It is so brittle that trans- 

 parent sections are difficult to make. 



With the microscope the section gave indications of the fragmental 

 nature of its constituents. These are the peculiar leaflets of sericite, 

 very irregular in outline and in distribution, and fragments of a feld- 

 spar, which is probably orthoclase, and quartz. As accessories I found 

 some chlorite having faint dichroic properties playing between 

 green and brown. A multitude of acicular crystals is strewn 

 through the mass which seem to be apatite. Phosphoric acid and 

 lime were found by chemical tests. 



Amphibolite. This is a perfectly black rock found at Swarth- 

 more, Delaware Co., Penna. There are lines which indicate strati- 

 fication and it may be therefore of metamorphic origin. The 

 fracture is curved. The hand specimen is of an even, crystalline 

 structure throughout the mass, but the crystals are not lying all 

 in the same direction. It is very compact and sound without any 

 sign of weathering or decomposition. It is apparently nothing 

 but amphibole, so far as the ordinary vision goes. In thin section, 

 however, there are, beside the amphibole, small irregular shaped frag- 

 ments of feldspar, mica and quartz, and magnetite in fair quantity. 



Gabbro-Phonolite. Mr. Theo. D. Rand showed me in a not 

 very deep ravine near Radnor Station, Delaware Co., Penna., a rock 

 of very dark color, fine-grained and tough which had not been 

 determined. He is of the opinion that the rock is in its original 

 position, which may be correct, although I have not been able to 

 see the proofs of it. It seems to me to be an isolated boulder. 



If struck with the hammer it does not give a metallic sound, but 

 its mineralogical composition seems to indicate a phonolite. The 

 thin section magnified 100 diameters gave the following result: 

 Figure 1, sanidine, plagioclastic feldspar, diallage and magnetite. 



Phonolite. About a mile east of French Creek Falls Station 

 Chester Co., Penna., occurs a dyke whose rock splits in slabs after 

 heating it and marking with the chisel and hammer the direction in 

 which the division shall take place. I have seen plates of I inch in 



