EAST HUMBOLDT RANGE. 535 



microscope, the quartz proves to be rich in fluid-inclusions and empty cavi- 

 ties. There are considerable zones, in which the quartz is a pure material, 

 carrying neither mica nor garnet ; but these zones rarely reach more than 

 20 or 30 feet in thickness, and are the exception rather than the rule. 

 Beneath the quartzites is the narrow zone of dolomitic limestone already 

 mentioned, and, under these, thin associated quartzites, and then the heavy 

 mass of gneiss and gneissoid schist, which form the great bulk of the range. 

 Of these, in the northern part of the range, there cannot be less than 5,500 

 or 6,000 feet. 



In the region of Clover Peak, the gneisses are usually composed of 

 quartz, orthoclase, brownish mica, and hornblende. They are varied by 

 limited beds of dioritic schist, which are interesting from the number of 

 minerals they contain. In them, plagioclase predominates over orthoclase, 

 but both are always present. The mica is of a dark earthy brown, while 

 the hornblende, always more abundant than the mica, is of a dark-green 

 color. Titanic iron and apatite in unusually large quantities are present. 

 Inclosed between the beds of gneiss, rich in orthoclase and poor in mica, 

 are some sheets of pure amphibolite, noticeable as containing no foreign 

 minerals, not even quartz. An unusually interesting gneiss is found under- 

 neath the quartzite on the west slope of the range below Clover Peak. It 

 is a fine-grained, brilliantly-colored gray rock, in which the white particles 

 of quartz and black mica form a decidedly granitoid arrangement, but the 

 rock at large has a ver}^ distinctly fissile structure, and cleaves easily in 

 fine sheets of an inch or more in thickness. Besides the black mica, there 

 is a large proportion of a brilliant coppery-bronze-colored variety; the rock 

 is further distinguished by containing, as shown by the microscope, a con- 

 siderable amount of fine zircon, and the rock is considerably stained by 

 infiltrated oxide of iron. The gneisses in the region of the dolomitic lime- 

 stones are more or less enriched by crystals of carbonate of lime. Besides 

 these, in the remoter gneisses, there are calcareous passages, where the crys- 

 tals are irregularly distributed, never enough to give a distinct character to 

 the beds, but still enough to cause it to effervesce under treatment with 

 acids. Garnet is confined to the quartzites and the lowest members of the 

 gneiss. Quartz, with the exception of the dioritic beds, invariably predom- 



