596 DESCRIPTIVE GEOLOGY. 



eastward to 25°, and in the ravine below the mines is found to be 65°. 

 About four miles to the northeast of the coal-mines a. prospecting shaft has 

 been sunk 100 feet, in a white, compact, fine-grained, volcanic ash, which 

 doubtless represents one of the upper beds of the Humboldt Pliocene for- 

 mation. 



The northeastern foot-hills of the range, toward the North Fork, are 

 covered, as has been seen, by flows of red and white rhyolite. On the 

 spurs to the north of the coal-mines are found a great variety of light- 

 colored earthy rhyolites, from which one passes by insensible gradations, 

 through compact, even-grained, structureless rocks, to the quartzitic sand- 

 stones, which form the main body of the hills. The rhyolites have 

 sometimes a porous earthy structure, sometimes a compact felsitic ground- 

 mass, and enclose different-colored fragments of the same material, but 

 show no distinct crystals, except very fine particles of quartz. The inter- 

 mediate member is a compact felsitic rock, of a yellowish- white color, 

 which, in the interior, is curiously striped by concentric bands of various 

 colors, almost as fine and distinct as those of an agate. The colors are 

 very brilliant and well defined, ranging from a deep purple, through brick- 

 red, into yellowish-white. Of these. Professor Zirkel remarks,^ "They are 

 perfect likenesses of the felsitic tufas or clay-stones of the Lower Permian 

 (BotJiUegendes) in Germany, originating in the old felsite-porphyries ; in 

 every respect, the true precursors of the Tertiary rhyolites." In close con- 

 nection with these curiously-striped rocks are found the fine-grained quartz- 

 itic sandstones, full of grains and crypto -crystalline fragments of limpid 

 quartz and fine angular fragments of black and green chalcedony. It 

 would seem therefore that these striped, compact rocks may represent here 

 a tufa, which is contemporaneous with the Weber Quartzite ; and the quartz- 

 itic sandstones, in contact with it, have been more or less altered, while 

 the rhyolitic tufas, which eo closely resemble it, and which are undoubtedly 

 of recent eruption, have followed the same channels, and carried with them 

 fragments of the older adjoining rocks. On the western slopes of the 

 northern point of the River Range are bodies of westerly-dipping limestones, 

 overlying the quartzites, which, from their stratigraphical position, have been 



'Microscopical Petrography, 2G2. 



