GEASS CANON. 783 



in a compact felsitic groundmass, the whole so compact that it has a con- 

 choidal fracture through the mass. 



To the north of this peak, at the head of Grass Canon, and adjoining 

 the eastern body of granite, is a white rhyolite, full of cavities, colored in 

 the interior by iron oxide, which, by their shape, would seem to have been 

 left by feldspar crystals. In the white, felsitic groundmass can be distin- 

 guished also fresh glassy sanidins and quartz-grains. Through this rock 

 run bands of a gray compact crystalhne rhyolite similar to that just described 

 as forming the peak, and seeming to represent undecomposed portions of the 

 mass. Associated with this is a rhyolitic breccia, formed of angular frag- 

 ments ol green and brownish lithoidal rhyolites, cemented by felsitic 

 material containing free quartz, which is so compact and flinty that it has a 

 conchoidal fracture, and rings under the hammer. 



Geass Canon, which is a long, narrow ravine running out at the north- 

 ern end of the mountains, presents along its slopes the most interesting occur- 

 rences of volcanic rocks in these mountains. At its head, and along the 

 upper walls, are gray pearlites of the crystalline type. A characteristic 

 specimen (wrongly located in Vol. VI, 175, No. 375) is rich in black 

 biotite, and contains macroscopical crystals of sanidin, plagioclase, and 

 quartz. Under the microscope, the feldspar crystals are seen to contain 

 great numbers of angular bubble-bearing glass-inclusions, sometimes so 

 closely aggregated as to form entire portions of the interior of the crystals. 

 Mica is most abundant in hexagonal laminae, 0.008""" in diameter, while in 

 the colorless glass-base are feldspar-microlites and pale-green needles, to- 

 gether with gas-cavities containing magnetite. This pearlite passes into 

 one iri which the crystalline ingredients are still present, but the ground- 

 mass is a colorless glass, in which are developed concentrically-curved 

 cracks, giving a sphserulitic structure to the mass, Microlites are present 

 as products of devitrification, and, as already stated, crystalline ingredients, 

 feldspar and mica, which are difficult to detect with the unaided eye. 

 Beyond the pearlites, on the west side of the canon, about opposite the 

 North or Basalt Peak (not named on the map), is a peculiar greenish rock, , 

 having in general a granular structure, ancL showing no crystalline ingre- 

 dients, through which run many bands, alternately quite porous, and again 



