PAH-TSON MOUNTAINS. 781 



substance, while the microfelsitic groundmass contains microlites of an 

 uncertain nature. * 



This rock forms the main ridge, with a southwest trend, while a second 

 ridge to the west, with a north and south trend, which connects it with 

 the main mass to the north, is composed of basalt, surrounding which is a 

 gray rhyolite, very rich in black biotite, having rather a dull lustre to the 

 unaided eye, but, under the magnifying-glass, showing a pearlitic structure 

 and glassy lustre ; this may be an alteration-product of the former variety. 

 Under the microscope, the groundmass of this rock shows an abundance of 

 products of devitrification in forms which seem to be peculiar to the pearl- 

 ites ; these are globulites arranged in long needles and tendril-like forms. 

 Fine sections of brown hornblende crystals, sometimes in twins, are also 

 disclosed. These peculiarities of structure are figured in Plate IX, fig. 2, 

 of Vol. VI, which represents a section of this rock under polarized light. 



A second body of basalt caps the eastern ridge of Aloha Peak, the 

 saddle or gap between the two ridges being of the gray pearlitic rhyolite ; 

 this is probably a portion of the basalt flow, which has been separated 

 from the main body by erosion. 



These basalts belong to the class of feldspar-basalt, are dark vesicular 

 rocks, showing macroscopically only white flecks, sometimes crystals of 

 decomposed feldspar, in a homogeneous crypto-crystalline groundmass; 

 that of the western ridge is much the more vesicular. Microscopically 

 they belong to the same class as those of the Lower Truckee Valley, 

 which will be more particularly described in a later section ; they consist 

 of crystalline elements in an amorphous glassy base, and are generally poor 

 in augite. They contain more olivine than is usual in this type, and their 

 feldspars, which occur in crystals and in thin lines in the globulitic base, 

 have numerous brown glass-inclusions. 



Rhyolites apparently form the eastern foot-hills of the mountains north 

 of Aloha Peak, though they were not all examined in detail. In the long 

 straight canon, which runs eastward from Pah-keah Peak, the rocks crossed 

 in section are as follows : At the head are granites, upon which rest the 

 Archaean slates already described ; to the east of these are rhyolites forming 

 transverse ridges. These rhyolites are of two varieties, the one of a yellow- 



