1914.] NATURAL SCIENCES OF PHILADELPHIA. 5 



Next comes another dike of diabase, characterized by the presence 

 of considerable pjTite, which occurs in rounded aggregates, filling 

 the interstices between crystals of augite and plagioclase. It also 

 contains vesicular cavities averaging about a millimeter in diameter, 

 filled with calcite and a little quartz, margined by acicular secondary 

 hornblende. 



Somewhat south of the beach is a large intrusion having an irregu- 

 larly rhomboidal outline, consisting of trachyte porphyry with large 

 orthoclase phenocrysts in a felsitic matrix containing some horn- 

 blende and a little quartz. 



Four sections were made from specimens of the shale associated 

 wdth dikes mentioned, one broadly striped, another showing finer 

 laminations, and the remaining two of rather uniform texture. All 

 are highly silicious, including quartz grains up to a half millimeter 

 in diameter scattered among finer grains of quartz and some secondary 

 minerals, chiefly micas, sometimes biotite, and in one sample, a bright 

 green mica. The extent of metamorphosis is indicated by apparent 

 metosomatic penetration of the secondary minerals into some of the 

 primary quartz grains. Striping, when present, is due to the concen- 

 tration of such secondary minerals in layers, which in the original 

 sediment were probably less purely silicious than the rest. 



Beyond a gully south of Lobster Point is a very noticeable dike 

 about five feet in diameter, transected at an acute angle by another 

 of same size. The first may be classed as a diabase porphyrite and 

 contains vesicles about a millimeter in diameter such as characterize 

 so many of the dikes here, but in this case there is about as much 

 quartz as calcite in the cavities, while generally the filling is entirely 

 of calcite. This rock also contains much pyrite in the form of isolated 

 grains in the interior of the dike, but in clouds of minute particles 

 several millimeters across, in the basaltic textured rock near the 

 contact. The other dike is an olivine diabase, notable for numerous 

 large idiomorphic phenocrysts of olivine now completely altered to 

 serpentine of unusually high double refraction, apparently consisting, 

 in part at least, of chrysotile, showing development along irregular 

 cracks, so characteristic of the alteration process in olivine. 



Possibly a mile further south, beyond Perkin's Cove, there is 

 exposed on the shore an extensive intrusion of diabase, under which 

 there is a water-worn cave between tide levels, locally known as the 

 Devil's Kitchen. This rock is a rather coarse-grained diabase with 

 some primary biotite, in which the augite is perfectly fresh, but 

 another ferro-magnesian constituent originally present has been 



