1893.] NATURAL SCIENCES OF PHILADELPHIA. 179 



Haycock Mountain except that the sound emitted is not so highly 

 metallic as in the case of the latter. There are, however, twins of 

 diallage in the section which will easily be recognized by those inter- 

 ested in the subject. The sanidine is present in less quantity than 

 the plagioclastic feldspar which seems to be labradorite. Huge 

 blocks, rounded by weathering, occur in the neighboring fields in 

 great abundance. Specimens collected and studied in the same way, 

 gave the same general results, and have therefore not travelled any 

 great distance. 



Mr. Theo. D. Rand collected two specimens of rock which he sub- 

 mitted to me for determination. The one was found at Buck Run, 

 2 miles southwest of Mortonville, Chester County, Pa. ; and the other 

 from 2 miles south-southeast of Thorndale, Chester County, Pa. 

 Both specimens proved to be the same kind of gabbro-phonolite. 

 Slight differences are noticed in the Mortonville rock ; beside the 

 sanidine, plagioclastic feldspar, the diallage and augite, there is some 

 hematite beside the magnetite. 



The Thorndale rock is the same in composition except that 

 some chlorite was recognized in the mixture. 



Granulite. The specimen was collected in the quarry at Pigeon 

 Cove, Mass., by Mr. Theo. D. Rand who told me that the quarrymen 

 complained of the extreme hardness of the rock, which they could 

 not account for. I was requested to investigate it and to ascertaiu 

 what the cause of the extreme hardness might be. 



The hand specimen is a coarse grained, mostly light colored rock 

 intermixed irregularly with large black patches having a metallic 

 lustre. The rock contains no mica. 



The thin section (PL II, fig. 4) as observed beneath the microscope 

 showed the constituent minerals to be orthoclastic feldspar, quartz 

 and magnetite. It is essentially the same rock which is so exten- 

 sively used in Philadelphia for Belgian block pavements. 



The feldspar and the quartz are interpenetrated and can be best 

 recognized by reflected light ; when so examined the quartz appears 

 dark and the feldspar light. With the aid of the polarized ray the 

 interference phenomena of the two mineral species are clearly shown ; 

 the reflected ray however, shows it to the best advantage especially 

 for the shading in the illustration. The contour was drawn with 

 the pen and the aid of a rectangular prism (Dr. Piffards) which 

 projects the picture of the slide upon the paper. Under these con- 

 ditions the contour can be drawn with accuracy and ease, provided 



