1922] Vaughan: Geology of San Bernardino Mountains 371 



This granite mass stretches to the front of the range on the north, 

 Rattlesnake Canon on the west, and beyond the quadrangle to the 

 east. For the most part this is a rather heterogeneous mass in which 

 the order of intrusions is hard to decipher, but clearly cutting this 

 mass are relatively smaller areas of uniform coarse-grained granite 

 with orthoclase crystals up to two centimeters across. The quartz and 

 orthoclase are about equal in amount and the principal accessory is 

 biotite, which is rather prominent. The largest of these areas extends 

 two miles to the south from Saddlerock Spring. The older granite 

 is crossed by many aplite and pegmatite dikes. One of these, right 

 at Saddlerock Spring, is eight feet thick and near the center are indi- 

 vidual masses of orthoclase more than two feet across, but near the 

 sides the texture is fine like that of an aplite. The quartz in the coarser 

 portions occurs as small stringers parallel to the trend of the dike. 



Farther north is a broad hummocky plateau extending nearly to 

 Rock Corral. On the south side of the hill two miles north of Saddle- 

 rock Spring the older heterogeneous granite cut by numerous peg- 

 matite dikes clearly forms the roof of an intrusive mass of grey 

 granite similar to the large mass farther south. Numerous examples 

 of this relationship are found to the west, but because of the small 

 size of the greater part of these intrusions only the larger ones were 

 mapped. 



Two and a quarter miles southeast of Rock Corral there is an 

 interesting dike of pegmatite striking N 60° E and dipping nearly 

 vertically. It is about two feet thick and for the most part is rather 

 fine-grained, but it contains lenticular patches of coarser rock up to 

 six inches thick and two feet long. These consist mostly of quartz, 

 some pieces of which are more than four inches wide by eight long. 

 The peculiar thing about the rock is that this quartz possesses a cleav- 

 age so pronounced that fragments glistening in the sun might easily 

 be mistaken for amblygonite. The surrounding heterogeneous rock is 

 traversed by numerous smaller dikes, but this one cuts straight across 

 them all. 



Along Rattlesnake Creek just below Mound Spring a very complex 

 mass has been formed by various igneous rocks intruding the 

 Arrastre quartzite. To add to the complexity, some of the intrusives 

 themselves resemble the quartzite at first sight because of their light 

 color and imposed schistosity parallel to the quartzite which they have 

 invaded. A specimen of this intrusive, which is whitish grey when 

 fresh but weathers yellowish, was examined in thin section and found 



