398 A. C. LANE POKPPIYRITIC APPEARANCE OF ROCKS 



Mr Crosby states on page 74 that 



"The granite of this belt is normally very closely and profusely porphyritic, 

 being in large part crowded with feldspar plienocrysts from 1 to 3 inches long ; but 

 tow^ard the margins of the belt the plienocrysts become elongated" [?], ** broken, 

 and indistinct, and the porphyritic structure gradually fades out, with or without " 

 [note well] '* the development of a more or less pronounced laminated or gneissic 

 structure." 



On page 75 he says : 



" Everywhere near its contact . . . the granite of this belt is non-porphyritic 

 and distinctly gneissoid, . . . but within a few^ yards of the contact, as a rule, 

 \vepassgradually to the normally coarsely porphyritic granite. . . . The gneissic 

 structure is plainly marked throughout the entire breadth of the granite, especially 

 by the lenticular forms of the quartz and the usually parallel orientation of the great 

 feldspar phenocrysts. Through the western middle portion of the belt the normal 

 granitic structure is best preserved, but advancing toward the schist and more 

 especially toward the diorite the feldspars show distortion by stretching and 

 cracking, w^hich increases slowly at first and then more rapidly. . . ." 



This marginal gneissic structure of the granite Mr Crosby regards 

 (page 95) as " an original structure due to the drag of the stiffly viscous 

 granite magma along its walls during its intrusion." It may be inter- 

 esting to remark that I arrived at the same conclusion from the grain 

 study that I made, independently, though later. 



Crosby gives on page 87 a detailed section of the aqueduct tunnel 

 with the fault on the contact of the granite and schist at 758 feet, a 

 large inclosure from 827 to 845 feet. From 845 on the laminated 

 gneissoid structure becomes less distinct. At 900 feet, as I measured 

 them, the large feldspar phenocrysts were about an inch long, at 1,000 

 feet they were about 2 inches long, and thereafter, as far as I went, 

 nearly uniform in size. Crosby says that at 1,625 feet there is a large 

 irregular pegmatitic development blending with the inclosing granite. 

 Is not this due to the aqueous residue near the center of the dike? At 

 1,755 feet there are inclusions and other disturbances, but, says Crosby, 

 the coarsely porphyritic, massive, and indistinctly gneissoid characters 

 are " uninterrupted for over 1,600 feet, but east of 2,600 feet the phe- 

 nomena of the western border are repeated," the contact with the diorite 

 being at 2,690 feet. 



The faults and inclosures forbid us to expect any great accuracy, and 

 yet we may obtain some interesting results. The whole zone affected by 

 injection can not be less than 563 meters, the least width of the granite 

 dike, and may well be much more. The grain of the feldspar at the 

 middle and in the belt of constant grain is about 51 millimeters, the in- 

 crease from 1 inch at 900 feet to 2 inches at 1,000 will give a minimum 



