650 DESCKIPTIVE GEOLOGY. 



and even to the naked eye are seen to be very much broken, as if by «i 

 movement in the groundmass after their formation. 



The rhyolites which immediately adjoin the andesite at the liead of 

 the canon belong to the porphyritic variety, being- of a pinkish-red color, 

 rich in crystals of sanidin, mica, and quartz, and splitting readily into thin, 

 sherdy fragments. Under the microscope, the groundmass of these rhyo- 

 lites, which is of a brownish-yellow color, is seen to be fibrous, but is 

 unaifected by polarized light. To the east of the summit is a compact 

 porphyritic rhyolite, having almost the texture of an older porphyry, and 

 containing, with the smoky quartz and sanidin-feldspars, dull, opaque, ortho- 

 clastic feldspars, and also a few plagioclase crystals. 



Near the mouth of the canon, the rhyolites are generally of white color, 

 carrying no mica, but a great deal of snioky quartz and somewhat decom- 

 posed feldspar. With them are associated breccias made up of the same 

 material, portions of whose groundmass seem to be rather crystalline, while 

 other portions have a distinctly spheerulitic structure. Associated with these 

 white rhyolites, on the western foot-hills, is a red porphyritic rhyolite, 

 which contains angular fragments of diabase. It is rich in crystals of 

 quartz, mica, and sanidin, and contains also some plagioclase. Under the 

 microscope, the quartz crystals are seen to contain inclusions of pure glass, 

 and also hexagonal zones of the half-fibrous groundmass. Among the white 

 rhyolites, near the mouth of the canon, is one which on the surface is 

 tinged with a delicate rose color, and full of small cavities lined with minute 

 crystals of quartz, upon which are deposited white, opaque, thin, hexagonal 

 plates, resembling tridymite. 



About 3 miles above the mouth of the canon, a dike of dark, compact 

 feldspar-basalt traverses the rhyolites, spreading out upon the spurs in flat, 

 tabular masses. On the divide, at the head of Clan Alpine Canon, and 

 spreading out over the spurs to the west, is a dark, compact basalt, which 

 belongs to that type which has a, globulitic, glassy base. It contains some 

 sanidin, with predominating plagioclase-feldspar ; also augite and olivine. 

 Under the microscope, the feldspars are seen to have a schistose structure, 

 and abound in half-glassy included particles, and the olivines to have glass- 



