(iREAT WESTERN DISTRICT. 359 



distant, wliicli are known to Ijclony to tin- Knoxville series (Neocomian), 

 that tliev niiiy fairly be ascribed to that series, especially as there is notli- 

 ino- to indicate the presence of any earlier rocks in tliis part of the country. 



The nietaniorphic rocks include <;ranular, phtlianitic, and serpentinoid 

 varieties. As is usual in the Coast Ranges, no definite linos t-iin be drawn 

 between these varieties; nevertheless serpentine prevails so greatly in one 

 portion of tlie district that it seemed to me expedient to delineate it upon 

 the map (Plate VI). 'J'he outline of the serpentinized area, however, here, 

 as in the districts described in former chapters, is not to be regarded 

 as representing a shai-p and complete division, but oidy as indicating the 

 iH-evalence of one phase of metamorphism in a i)art of the district. Tiie 

 unserpentinized area is chiefly occupied l)y silicified rocks. The dip of the 

 strata is, as usual in such areas, very irregular, but tlie prevailing inclina- 

 tion is to the south, or toward Mt. St. Helena, at an angle approaching 45''. 



Lavas — Upon the sedimentary rocks lie lavas. The andesite seems to 

 have once covered a larger area than it now d(»es and to have b(;en par- 

 tially removed by erosion, leaving many patches of extremely small size. 

 The andesite is pyroxenic, and the greater part of it is glassy, though 

 asperitic modifications and tufa also occur. A portion of the asperite is 

 laminated, as is so common near Clear Lake and at Steaniljoat Springs. 

 More unusual is the occurrence of contorted beds in this asperite, as if plas- 

 ticity had been retained after the stru(;ture was established. It is possible, 

 however, that the original form of the lann'na; was an undulating one. A 

 portion of the andesite forms fine columns, which is somewhat unusual on 

 the Coast Ranges, though common enough along the Sierra. 



Basalt is also represented in this district. It crowns the highest eleva- 

 tion on the map, a hill composed of andesite, thus giving clear proof of the 

 order of succession of the lavas, which indeed would not be questionable 

 even in the absence of cases of direct superposition. A i)ortion of the 

 basalt is vesicular, and secondary crystals are sometimes found in the 

 cavitie.s. 



Ore deposits.— The oi'c dcposlt of tlic Great Western consists of tabular 

 masses of ore, situated at the contact between very slightly altered sand- 

 stone and a heavy body of serpentine. The serpentine is accompanied by 



