REGION WEST OF PYEAMID LAKE. 847 



resembling those found in the southern end of the Montezuma Range and 

 at Mono Lake. They have the characteristic lavender and gray colors, a 

 rough, fibrous texture, with a tendency to develop half-glassy rocks. Crys- 

 tals of sanidin are abundant. Associated with these rocks are grayish- white 

 and cream-colored tufas, made up of siliceous and feldspathic material, so 

 extremely fine as to crumble into powder at the slightest blow. They are 

 of interest as showing the intimate connection between the ejection of rhyo- 

 litic products and the stratified lacustrine deposits, which, carr3^ing remains 

 of an extinct Tertiary fauna and flora, have necessarily been classed as of 

 sedimentary origin. 



There occurs here, on the north side of Mullen's Gap, a small obscure 

 mass of an uninteresting, dirty, ash-gray breccia, very little being known 

 of its mode of occurrence or immediate associations, but a specimen of it 

 was collected in the field solely as showing the great variety of rhyolitic 

 material to be found at the locality. The inclosed brecciated fragments 

 exhibit considerable variety, but are all apparently of a rhyolitic nature. 

 Small sanidins are the only crystals observed macroscopically. In thin 

 sections, under the microscope, this rock becomes one of great interest and 

 importance, the gray brecciated fragments exhibiting peculiar phenomena, 

 which Professor ZirkeP has described with considerable detail. Among the 

 most striking features of the rock is the occurrence of liquid-inclusions in 

 glass, and the presence also in the glass of minute crystals of apatite carry- 

 ing glass, which also contains a fluid-inclusion. In Plate XII, fig. 1, accom- 

 panying Professor Zirkel's report, these phenomena are beautifully shown. 



Between Mullen's Gap and the northern end of the Virginia Range, 

 all the higher portions consist of broad, massive tables of black basalt, which 

 incline gently to the eastward toward the lake or away from the granitic 

 body of the Sierras, the volcanic masses evidently breaking out along the 

 flanks of the older crystalline range. The deeply-eroded canons of the 

 basalt have never been explored, and it is possible that there may be 

 exposed in several of them some of the older volcanic rocks, especially as 

 trachytes and rhyolites occur along the shore of Pyramid Lake and to the 

 north of the basaltic table. 



' Microscopical Petrography, vol. vi, 265. 



