WARREN. — ALKALI-GRANITES AND PORPHYRIES. 225 



it is a superficially altered and oxidized portion of the gray granite, 

 and always passes gradually downward at no very great depths 

 (estimated at 20 ft. in some places where particularly well exposed) 

 into the normal gray type. The original dark silicates have been 

 entirely destroyed, their places being occupied by abundant magnetite 

 crystals, quartz and feldspar and calcite. The pink or red color is 

 due to the presence of exceedingly minute hematite specks or scales 

 resulting from the oxidation of the hornblende microliths originally 

 contained in the feldspars, and in part also to a general distribution 

 of ferruginous products through the rock. 



The fourth, and perhaps the most interesting and important one, is 

 that in which a rather indistinctly marked porphyritic texture is 

 developed. This variation is found wherever the granite approaches 

 the granite-porphyry of the contact zone. Its texture, although not 

 strongly developed, is characteristic, and is due largely to the fact 

 that a part of the feldspars are grown somewhat larger than the rest. 

 This feature is well seen in the granite from the northern slopes of 

 Rattlesnake Hill, on the Great Dome, and particularly over a large 

 part of the Pine Hill area. In the latter location the passage of this 

 phase of the granite into the granite-porphyry of the contact zone is 

 perfect and gradual though always comparatively rapid. On the top 

 of Rattlesnake Hill near the southern edge of the hill, this granite is 

 found in a sharp but perfectly sealed contact with the porphyry. The 

 same phenomena may also be observed elsewhere, and while we cannot 

 doubt that the porphyry is in all cases but a more rapidly cooled phase 

 of the magma, we are forced to conclude that the magma moved to 

 some extent underneath its own cover forming sharp contacts with 

 it.^^ It is to be noted that at such contacts, so far as observed by 

 the writer, there is a more or less marked de\'elopment, along the 

 immediate contact line, of long, slender riebeckite prisms. 



Under the microscope the minerals are seen to be essentially the 

 same as in the normal granite. The texture is also much the same 

 except that a part of the feldspar has attained a larger size and, that 

 in portions of almost e\ery section examined, areas will be found that 



23 It is not unlikely that this particular contact, which is steeply inclined, 

 marks a lateral contact of a great dike which broke through or at least pushed 

 up the porphyry cover at this point. "A few hundred feet to the west and 

 a little to the north of the line of contact, rises a prominent knob of granite 

 known as the "Rattle Rock." This rises to about the same elevation as the 

 porphyry on Rattlesnake Hill and probablj^ represents the exposed stump of a 

 great dike or a cupola of granite which domed up or cut through the porphyry. 



