698 DESCRIPTIVE GEOLOGY. 



the Sou Spring Hills, the silica is between one and one-half and one and 

 three-quarters per cent, higher than in the other rock, at the expense of 

 the lime and alumina, while the water, which undoubtedly in some manner 

 plays an important part in the modification of the texture of volcanic rocks, 

 is here only two-tenths of one per cent, lower than in the rhyolite from ' 

 the west base. 



The analysis of the rhyolite from the Sou Spring Hills yielded : 



Silica 75.65 75.70 



Alumina 11.52 11.48 



Ferric oxide 2.37 2.42 



Lime 0.76 0.77 



Magnesia trace trace 



Soda 2.91 3.00 



Potassa -5.93 6.09 



Lithia trace trace 



Water *1.03 1.02 



100.17 100.48 

 Specific gravity, 2.44, 2.48. 



The characteristic red color of the lithoidal varieties seems to be due to 

 the quartz-grains of the microfelsitic varieties being dissolved in the feld- 

 spathic groundmass, producing a homogeneous porcelain-like base, in which 

 the ferritic needles act much more energetically as a coloring matter. In 

 general, in Central Nevada, where there is a mingling of crystalline and 

 microfelsitic rhyolites with the lithoidal forms, the latter are almost always 

 of a deeper red tint. It is certain that analysis shows but little essential 

 difference, and that the amount of silica and iron present varies only slightly, 

 although the one rock may be of a light-gray color and rich in large secre- 

 tions of quartz-grains, and the other of a deep red, and carrying no macro- 

 scopical quartz. 



Breaking through these rhyolites in a series of powerful dikes, and to 

 a great extent overflowing them, occurs a subsequent formation of a dark 

 steel-gray basalt, the greater part of which is finely cellular, though much 

 of it is entirely compact and without pores. It is closely related in petro- 



