MONTEZUMA EANGE. 757 



as an aid in determining doubtful rocks is well shown in this instance, as, 

 in thin sections, both the quartz and feldspar are characterized by large 

 liquid-inclusions, while glass-inclusions are entirely wanting, which settles 

 the question definitely that the rock belongs to the older series of felsite- 

 porphyries. At the mouth of French Canon, a mile to the eastward of the 

 porphyry, occurs an outburst of normal rhyolite, having a gray microfelsitic 

 groundmass, rich in quartz-grains, which breaks out near the line of contact 

 between the granite and slates. 



Along the front edge of the granite, bordering the Humboldt Valley, 

 are a number of outbreaks of fine-grained, black basalt, none of which, how- 

 ever, attain any great elevation above the Quaternary plain or present any 

 features of mineralogical interest. 



In the low depression of Indian Valley, and in the re-entering bay 

 north of Black Cafion, the Truckee Miocene beds lie inclined at an angle 

 of 1^ to 2^ degrees, with a thickness of over 700 feet. They cover a large 

 superficial area, resting upon the granites to the south, the slates to the 

 north, and extending eastward until concealed by the Quaternary deposits. 

 Owing to their soft, porous, friable nature, they have suffered greatly from 

 erosion, the streams having cut broad, deep channels through strata made 

 up almost exclusively of volcanic material, consisting of coarse frag- 

 ments of quartz and sanidin, imbedded in a fine comminuted ash. Many 

 of the layers are quite compact and rock-like, while interstratified between 

 them are others of nearly pure, lavender-colored pumice and red earthy 

 beds, crumbling in pieces at the slightest touch, the entire exposures show- 

 ing the greatest variety in composition, texture, and stratification. That 

 the formation is a subaqueous deposit there is abundant evidence, but the 

 mode of occurrence would indicate marked changes in the rapidity with 

 which the beds have been laid down, the depth and agitation of the super- 

 natant water causing more or less sorting of the lighter and heavier parti- 

 cles, and the pressure of the overlying water and ash compacting the finely- 

 comminuted feldspathic paste into thin, shaly beds. The color of the 

 formation is very varied, with many shades of orange, red, yellow, purple, 

 lavender, gray, and white. Diiectly overlying these beds are a number of 

 horizontal tables of black, fine-grained dolerite, the longest of which, Table 



