764 DESCRIPTIVE GEOLOGY. 



existence of still older volcanic rocks, now for the most part concealed 

 beneath immense accumulations of rhyolite. 



Following up the canon opposite White Plains to the centre of the 

 range, the main rhyolitic mass is seen to be made up of a fine-grained ground- 

 mass, in which but few large and recognizable crystals are secreted. 



The body of hard rhyolite, which is seen to break through the Ter- 

 tiary strata in the vicinity of White Plains, is of two types: first, a distinctly 

 heavily bedded gray and purplish-gray rock, whose beds stand upon end, 

 with a dip of about 80°, and strike approximately north 55° east. This 

 consists of highly-laminated rocks, whose cross-sections show distinct lines 

 of great fineness, which for the most part indicate a perfectly smooth paral- 

 lel structure, but which are occasionally crumpled up into complicated scal- 

 lops. All the darker lines are made up of what appears to be a colorless 

 glass, but which, under the microscope, is found to be composed of fine 

 polarizing g;:'ains. Rude jointings into rough columns and cuboidal masses 

 characterize the rock. Crowding through this laminated mass is a more 

 homogeneous and pearlitic variety, which consists of a gray glassy ground- 

 mass carrying abundant black mica and occasional unrecognizable feld- 

 spars. A peculiarity of this type of rhyolite is the resinous coating which 

 forms upon all the exposed surfaces. 



Pearlitic rhyolites, obsidians, and a gi'eat variety of glassy forms occur 

 over large areas in the southern end of the range. The basalts, which 

 break through and overlie these rhyolites of the southern end of the range, 

 form most of the higher summits, and cap the long ridges jutting out in 

 every direction. In general, they present a black resinous lustre, a vesic- 

 ular structure, containing amygdules of olive-colored chalcedony, mainly 

 spherical, and about the size of a small pea. They resemble the basalts 

 found on both sides of the Truckee Valley, in the Virginia and Truckee 

 Ranges, having a distinctly glassy base, rich in globulites. In texture, they 

 vary from a rather coarse crystalline variety, in which the augite and plagio- 

 clase can both be detected, down to the finer glassy types. Olivine in quite 

 large mass is found in several localities, in which, under the microscope, 

 Professor Zirkel has detected, as in many European basalts, the presence 



