VOLCANIC ROCKS OF THE PINON EANGE. 561 



Range to Railroad Canon in the Diamond Range, where it overflows the 

 cream-colored Carboniferous limestone. These basalts connect the two 

 ranges, completely encircling the northern end of- Diamond Valley. It 

 would appear to be a common mode of occurrence for both rhyolites and 

 basalts, when coming to the surface along breaks in the sedimentary strata, 

 or where the beds suddenly plunge downward beneath the surface, to strike, 

 in massive eruptions, diagonally across the intervening valley to the base of 

 the next range, dividing the depression into separate basins. The basalts 

 of this long range of hills are rather uniform in character. Under the micro- 

 scope, they appear to be unusually rich in grayish-black globulitic base. 

 Augite is present in very fine grains, but the rock would appear to be remark- 

 ably poor in olivine. 



On the west side of the Pinon Range occurs a very interesting basaltic 

 table, which extends along the base of the range for 10 miles, abutting 

 against the Ogden Quartzite, and completely obscuring the Ute limestone 

 below. Along its east side, toward the range, it presents a moderately steep 

 cliff, or wall, the main body dipping very regularly to the westward at an 

 angle of 3° toward the great basaltic flows of the Cortez Range on the 

 opposite side of Garden Valley. The rock is distinctly bedded ; deep black 

 in color, and hard, but porous in texture. 



One other locality of rhyolite connected with the Pinon Range, and 

 found on the east side, some 8 miles south of the limit of the map, deserves 

 mention. It presents a nearly white groundmass studded with large irregu- 

 lar grains of quartz and broken tabular crystals of glassy feldspar. There 

 may also be detected a few small flakes of dark biotite. The groundmass 

 has a coarse crystalline structure, which at once suggests to the field- 

 observer, upon superficial investigation, a granite-porphyry, and yet a 

 closer examination, together with its geological position, shows that it 

 must be a Ter'fiary product. As rhyolite is simply the modern Tertiary 

 type of the older granite-porphyry, this structural relationship between 

 the two rocks is of interest. What adds greatly to this interest is, that 

 Professor Zirkel finds, after a microscopical analysis of the rock, the same 

 close resemblances, but concludes that the rock is undoubtedly a rhyolite. 

 Professor Zirkel also finds in this rock minute grains of garnet, an exceed- 



36 D G 



