THE BASALT PROVINCE 325 



The idea given by the statement and its context is that the divide is 

 a line of volcanic vents or of lava flows. The fact is the ridge is essen- 

 tial!} 7 a bedrock ridge, with merely a covering of lava, just like the ridge 

 farther east already described. On this divide the lava is thin, and the 

 bedrock, where crossed by the detailed section, rises 150 feet above the 

 valley deposits to. the west. 



The basalt province. — If, now, reference is made to plate 15 of the 

 general map and the distribution of the basalt as a whole considered, 

 the argument for each of the ranges is strengthened by the consideration 

 of the whole region. The areas discussed form a moderate-sized province, 

 with the same characteristics throughout, and with sufficient outcrops in 

 the valley region to refer the different exposed masses to the same event. 

 The west base and east side of the Humboldt Lake range, the southeast 

 part of the Star Peak range, the ridge extending across the valley from 

 Star peak toward the East range, the higher valley ridge east of this, 

 and finally Table mountain, all are similarly covered with basalt, which 

 generally lies over tuffs or leveled bedrock, with no indications of vents. 

 It may be horizontal or tilted, but not folded, is affected by normal fault- 

 ing, later than the great post- Jurassic erosion period and earlier than the 

 Lahontan deposits and shorelines, never badly weathered ; in fact, al- 

 most always exceedingly fresh, notwithstanding its unprotected surface 

 position and easily alterable mineral composition. It is especially for- 

 tunate that in the region studied, which is so characteristic in its strati- 

 graphic relations, structure, and lithological nature, no other basic vol- 

 canics are found beside this one basalt. 



East side of Table mountain. — It may be added that similar basalts 

 occur in the valley on the east side of the East range, although not down 

 the east slope, as in the Humboldt Lake mountains. They were not 

 studied in the present investigations, but the lithological similarity of 

 the basalt in the divide above described and that on the east base is 

 noted in the Fortieth Parallel Survey Report.* 



Results of study of the volcanics. — The conclusion would seem to be 

 warranted that the Table Mountain region and the adjoining valleys were 

 part of the surface of low relief recognized for the Humboldt Mountain 

 area, which was produced by the long post- Jurassic erosion and on which 

 were laid the various members of the volcanic series, particularly the 

 easily traceable basalt ; that these areas were contemporaneously sub- 

 jected to orogenic deformation of the same character ; that the mass of 

 the Table Mountain range was elevated with respect to the east and west 

 bordering areas as a massive block along fault planes on both sides of the 

 range, with some shattering into secondary blocks along minor fault 



* Vol. ii, p. 704. 



