370 University of California Publications in Geology [Vol. 13 



these, along Pipes Creek, the granites form a heterogeneous mass 

 intruded by a few pegmatite and aplite dikes, which are particularly 

 prominent in some places where the intruded granite is badly decom- 

 posed, in fact just north of The Pipes they at first appear to be the 

 principal country rock. In the upper part of Burns Canon most of 

 them dip 20° to 50° east and are so numerous that from a distance 

 they look like sedimentary strata. They vary in thickness from less 

 than an inch to more than three feet and in texture from fine aplite 

 to very coarse pegmatite, with masses of orthoclase and quartz over 

 a foot long. Some of the larger ones are fine near the margins and 

 increase in coarseness to the center. Many rather peculiar features 

 were observed and an occurrence a quarter of a mile west of Burns 

 Spring is particularly interesting. On the north side of the canon is 

 a pegmatite dike 3.5 feet thick, 2 feet being nearly solid orthoclase 

 with a little quartz and the other 1.5 feet being nearly solid quartz 

 with a little orthoclase. It has many branches ramifying into the 

 granite below in contrast to the general tendency of the dikes to be 

 clean cut. The granite varies a great deal as to its biotite content and 

 hornblende granite is comparatively rare. 



Four miles north of Burns Spring there is a great area of fine- 

 grained granitic gneiss light yellow in color somewhat resembling 

 an aplite. Quartz and orthoclase are in equal amounts and are inter- 

 locked and elongated in the same general direction and show undu- 

 latory extinction, these characteristics no doubt being due to flow 

 under great stress. The rock contains a little oligoclase and only a 

 very small quantity of pale green biotite. Magnetite is rare. One 

 peculiarity is that some of the titanite crystals are as large as those 

 of feldspar. This rock intrudes a schist which at first might be takeu 

 for altered sediments, but under the microscope it is seen to contain 

 as much orthoclase as quartz. The other constituents are a little 

 oligoclase-andesine, considerable biotite, scattered prisms of apatite, 

 and a few grains of magnetite and titanite. In some places this rock 

 is cut by lamprophyre dikes, and a specimen from one of these was 

 examined in thin section. It consists essentially of deep green horn- 

 blende and less than half as much plagioelase, oligoclase-albite showing 

 albite and pericline twinning. This rock, therefore, is camptonite. A 

 large part of the hornblende has been altered to a colorless amphibole. 

 A few grains of titanite are associated with the hornblende and the 

 feldspar includes rods of apatite. Magnetite is sparsely distributed, 

 usually with the hornblende. 



