604 DESCRIPTIVE GEOLOGY. 



In some places, where the granite splits into thin slabs only about an 

 inch tliick, a little microscopical hornblende is found, and the plagioclase 

 seems' to predominate over the orthoclase. 



On the southwestern point of the peak, at the contact with the overl}^- 

 ing limestones, is a small development of felsitic porphyry, enclosing a vein 

 of red oxide of copper. It has a fine-grained grayish groundmass, enclos- 

 ing crystals of feldspar and large proportion of hexagonal plates of black 

 mica and crystals of quartz. Under the microscope, the larger quartz crys- 

 tals are seen to contain only liquid-inclusions, in some of which are cubes 

 of salt. In the groundmass, which is generally micro-crystalline, there is a 

 tendency toward a sphserulitic formation, which is particularly observable 

 around some of the small quartz crystals, which are surrounded by radiating 

 felsitic fibres. 



On the western flanks of the peak, which are very steep, but well 

 covered by detrital material, the limestones have a north and south 

 strike, with a dip of 70° to 80° to the westward. Through them runs a 

 curious dike or bed of rhyolite, which, on its weathered surface, at first 

 glance has considerable resemblance to a felsite-porphyry. It is gen- 

 erally of a yellowish-gray color, showing large crystals of mica, horn- 

 blende, and quartz porphyritically imbedded in a dull, opaque, felsitic 

 groundmass, whose fracture is very uneven and granitoid. In some 

 cases, the groundmass has a greenish color, and is quite glassy, passing 

 into a pearlite, and shows crystals of glassy sanidin. Under the micro- 

 scope, the groundmass is seen to be distinctly sphserulitic, but without axial 

 fibration. 



The limestones seem to close completely around the granite body, and 

 are best seen in section on Coal Creek, where they have an east and west 

 strike, with a dip of about 45° to the southward. A thickness of about 2,000 

 feet of limestones is here observed, the upper member of which is a con- 

 glomerate, which may probably represent the lower bed of the Weber 

 Quartzite. In the canon of Coal Creek, about a mile from its mouth, is a 

 considerable development of black shales, in which in the creek bottom 

 was found an exposure of about 15 feet of black anthracitic material, 



