STRUCTURE OF THE VOLCANICS ' 305 



the volcanic series without any apparent angular unconformity. As 

 might be expected, evidences of erosion between the rhyolite and basalt 

 outpourings have been found. 



The basalt offers us additional evidence of the low angle of the slope 

 at which the series was formed. Following it north and south, or from 

 crest to valley, we find great uniformity of thickness of the sheet. This 

 could hardly have been the case if it had flowed down into the valley 

 from the crest at its present altitude, or if it had encountered canyons or 

 fair sized gullies on the mountain side. 



Structure and distribution of the volcanics. — The volcanic series over- 

 lies unconformably the bedrock series. At several places angles of 

 20 degrees or less have been observed in the overlying basalt, where the 

 bedrock slates dip at 88 to 90 degrees. 



Nowhere are the volcanics folded or overthrust. They are affected by 

 simple tilting, which may reach 45 degrees. The tuffs are not indurated, 

 and none of the members are at all metamorphosed.* Faulting occurs, 

 but it is always normal faulting, generally parallel to the range trend, 

 more rarely transverse. Wherever it occurs the effect on the topography 

 is direct, a more or less perfect scarp is always present, and the upthrown 

 side forms the scarp. The only effect of erosion has been to change the 

 slope or displace the front slightly or furrow the scarp with gullies. The 

 flow of basalt on the top makes the recognition of the faults by strati- 

 graphic discordance very simple, for the layer is broken along the fault 

 plane, and, l} 7 ing on the surface, abuts upon the scarp on the down- 

 thrown side and caps it on the upthrown block. When the whole vol- 

 canic series is present, the effect is more striking. 



Examples of such faults showing simple fault scarps f or fault scarps 

 only slightly affected by erosion may be seen in the detailed section 

 carefully drawn to scale. Several can be seen in plate 17, figure 2, and 

 a small but distinct one in plate 17, figure 1. 



The distribution of the volcanic series is an important consideration 

 in the present inquiry. Along the north-central part of the Humboldt 

 Lake mountains — in other words, the highest part of the southern range — 

 the volcanic series is entirely on the east side of the range. It reaches 

 from the crest line, which it forms for a considerable distance, to the 

 east base, where it dips down under the alluvium of the valley of Carson 



* This is not meant to include weathering, yet the tuff and lavas are not badly weathered. The 

 feldspars are generally glassy, the pyroxenes and micas lustrous. 



t" 1. Where the fault has displaced the surface and the break remains undefaced or only slightly 

 obscured, we may call the resulting cliff a simple fault scarp. 



"2. Where erosion has acted unequally along a fault on account of the difference as to hard- 

 ness between two rocks forced into juxtaposition or between a crushed zone and an intact one 

 the resultant cliff may be termed an erosion fault scarp." Spurr : Bull. Geol. Soc. Am., vol. 12, pp. 

 258-259. 



XL— Boll. Geol. Soc. Am., Vol. 15; 1903 



