CARROLLTON PORPHYRY AND QUANA GRANITE 137 



quartz. The latter is especially common in the porphyry of the Carroll- 

 ton mountains. In general, the porphyry does not take on a granitic 

 phase, though a few such instances were noted. In Blue Creek canyon 

 there are associated with the porphyry certain eoryolites, showing 

 lithophysse and other rocks apparently rhyolitic and tuffaceous. It is 

 this pink porphyry which forms the common type of the mountains 

 and is seen south and east of mount Scott, in the Carrollton mountains, 

 in Signal mountain, and in the limestone hills north of the main range, 

 where it forms the platform upon which the Cambrian conglomerate 

 rests and has been brought up by faulting. The second type of porphyry 

 was noted only along the north flank of the main granite range west of 

 Saddle mountain. It differs from the usual type in having a much 

 darker ground-mass, which may be a wholly inconsequential difference. 

 Wherever the porphyry outcrops it shows a characteristic topography, 

 forming rounded hills with smooth flowing contours. It is often much 

 shattered, and the surface of such hills usually shows man y .small sharp- 

 edged fragments of rock, as distinguished from the granite and gabbro 

 outcrops, which are characterized by rounded boulders of disintegration. 



QUANA GRANITE 



This is by far the most common crystalline rock in the region, and 

 shows but little variation in character. The rock is predominantly 

 feldspathic, with a subordinate amount of quartz and a still smaller 

 amount of a green mineral, presumably hornblende. So far as was ob- 

 served, the granite is wholly free from mica, and in fact mica was only 

 found in two localities, where it occurs in small yellow flakes in connec- 

 tion with certain dikes of granular white quartz radiating from the granite 

 mass and cutting the gabbro. Granite forms the bulk of mounts Scott, 

 Sheridan, Baker, Haystack, and Saddle mountain, and in fact all the 

 more prominent peaks. It weathers characteristically into large boulders 

 marked off by joint cracks. On the top of a large tabular mountain just 

 east of the Mesquite valley one such boulder of disintegration was seen, 

 measuring 60 by 40 by 30 feet. Figure 1 of plate 16 shows a character- 

 istic view on the side of mount Scott, and figure 2 of the same plate 

 gives one a correct impression of the size of the blocks on the lowland. 

 These boulders are so common that over much of the granite area it is 

 difficult to find exposures of the solid rock. 



The granite has suffered much deformation. Shear zones are not un- 

 common, and faulting with well developed slickensides was repeatedly 

 observed. The rock is eruptive through the gabbro, as already shown, 

 and its relations to the porphyry are quite as clear. At contacts ob- 

 served on the southwest flank of mount Scott the granite was seen to 



