IGNEOUS ROCKS 463 



MIDDLE PENNSYLVANIAN 



Rieheckite-cegirite granite. — West of Diamond Hill, on Copper Mine 

 Plill, is a considerable mass of alkaline granite with which is associated 

 granite porphyry as a contact phase, A small boss of the same granite 

 outcrops 2 miles north of the main mass. Several quarries have been 

 opened in the granite, the most accessible of them being situated about a 

 mile northwest of Diamond Hill. 



The fresh granite is a medium, inclining to a fine-grained rock of a 

 light gray color, with often a slightly bluish or greenish shade. In addi- 

 tion to its granitic texture, it possesses a slightly gneissoid structure. 

 Megascopically it consists of distinct, gray or slightly bluish or greenish- 

 gray unstriated feldspar crystals which will average not far from 2 milli- 

 meters in length by somewhat less in width ; quartz, for the most part 

 finely granular (sugary) in character; abundant small, black, lustrous 

 prismoids of black hornblende, and less easily detected particles of dark- 

 green segirite. The hornblende grains are sometimes nearly as large as 

 the feldspar, though in general smaller. The dark minerals possess a 

 certain amount of alignment through the rock, which is responsible in 

 large part for the feeble gneissoid texture above noted. Grains of purple 

 fliiorite may occasionally be detected. With a pocket lens many minute 

 scales of a bright brownish yellow (dull brown in weathered specimens) 

 mineral can be seen which the microscope shows are astrophyllite.^^ The 

 smaller grain of the quartz and hornblende relative to the feldspar, as 

 the result of movement or crushing, produces the effect of a porphyritic 

 texture, and indeed some portions of the rock, probably from its marginal 

 parts, seem to have originally possessed a porphyritic texture. In the 

 writers^ opinion, however, the rock as a whole is not correctly defined as 

 having a porphyritic texture, though Emerson and Perry^^ apparently 

 call it a porphyry. 



In altered specimens the quartz is often stained yellow, as are also to 

 some extent the feldspars, while the hornblende, etcetera, have gone over 

 to magnetite and other ferruginous products. 



Thin-sections examined under the microscope show that the minerals 

 present are microcline microperthite, quartz, riebeckite, and gegirite, with 

 astrophyllite, fluorite, zircon, ilmenite ( ?), and a leucoxenic ( ?) material 

 as accessories. The microperthite is an albite-microcline intergrowth and 

 possesses much the same characteristics as that of the well known Quincy 

 granite to the north. The intergrowth appears in general, however, to be 



31 Some biotite occurs, but this is pretty certainly the remnants of small inclusions of 

 schist, diorite, or granite. 

 83 Op. cit., p. 53. 



