464 DESCEIPTIVE GEOLOGY. 



from a basalt, flowing in thin sheets, with a rudely columnar structure, and 

 becoming reddish-black upon its weathered surfaces. On the surfaces of 

 fresh fracture, it shows a dark bluish-gray, somewhat porous groundmass, 

 in which are imbedded crystals of augite and black mica, with occasional 

 large crystals of plagioclase-feldspar. Under the microscope, it is seen to 

 contain sanidin-feldspar, as well as plagioclase, some hornblende associated 

 with the augite, and pale-gray glass. It has been classed, by Zirkel, with 

 the augite-andesites, bvit, owing to the presence, unusual in these rocks, of 

 mica and hornblende, it rather approaches the hornblende-andesites. It 

 contains no olivine. An analysis made of this rock by R. W. Woodward 

 shows more resemblance to the andesites proper than to the augite-andesites. 

 The constituents are as follows: 



Silica. - . : 60.71 



Alumina 16.00 



Ferric oxide 2.09 



Ferrous oxide 3.87 



Lime _ 5.17 



Magnesia 3.07 



Soda 2.74 



Potassa 3.78 



Lithia ■. . '. trace 



Carbonic acid 1.01 



Water 1.48 



99.92 



To the north of White Rock Spring, limestones, generally lying at a 

 shallow angle, form the main ridge as far as observed. Toward the north, 

 opposite Hooper's ranch, a low ridge to the eastward of the main ridge is 

 formed of limestones and shales, having a western dip, and incrusted with 

 calcareous tufas of the lake-terraces, from one of which issues a large spring 

 of limy water. At the point where the road to Pilot Rock crosses the hills, 

 both the eastern and western slopes of the range are covered by flows of dark, 

 compact basalt, inclining with the spurs in either direction. On the crest, 

 whence the basalt has been denuded off, are exposed beds of limestone and 



