DIABASE HILLS. 813 



widely; the one being a true diabase, and the other, although perhaps not a 

 typical dolerite, characteristic of a large area of Western Nevada. Chemi- 

 cally the chief difference appears to be in the higher percentage of iron in 

 the basalt, at the expense of the alumina. The percentage of silica, 53.94, is 

 somewhat high for a normal dolerite, but not above the average found in the 

 glassy vaiieties of the Truckee Valley. 



Besides the Diabase Hills, there is, in this southern portion of the 

 range, but one other known outcrop which rises above the basalt. This 

 occurs at the southern point of Tutib Peak, where it forms a gray rock often 

 slightly decomposed, and is in all respects like the rock of the Diabase 

 Hills, protruding through the basalt in a dome-shaped mass, showing a 

 general rounded erosion, but no sharp ravines. The surrounding basalt 

 shows no points of difference in its field habit from that already described. 



No idea can be gotten of the extension of these diabase bodies under- 

 neath the basalts, but it would seem highly probable that they form a 

 continuous body, now appearing only as insular hill-tops above the Tertiary 

 eruptive masses. 



To the northeast, the companion summit of Tutib Peak is formed 

 of a dark-gray half-glassy rhyolite, which forms a dome-like top of about 

 a mile in diameter, and is distinctly seen to underlie the surrounding 

 basaltic field. As it is separated from the diabase by a broad sheet of 

 basalt, the relative ages of the rhyolite and diabase could not be observed. 

 In the prevailing dark-gray glass of the rhyolite are many monoclinic feld- 

 spars, a plentiful distribution of biotite, a little hornblende, but no quartz. 

 This rock, under the microscope, in thin sections, affords a most interesting 

 study of the globulitic devitrification of the hyaline varieties, and receives 

 a detailed description by Professor ZirkeV "^^o calls attention to the resem- 

 blance between this natural glass and the artificial furnace-products 

 described by H. Vogelsang. Unlike any other rhyolite observed in the 

 Truckee Range, it may be considered as an isolated outpouring, whose 

 extent is obscured by the broad fields of basalt; it is, however, very closely 

 allied to the semi-hyaline varieties near White Plains in the Montezuma 

 Range. 



' Microscopical Petrography, vol. vi, 206. 



