304 G. D. LOUDERBACK STRUCTURE OF THE HUMBOLDT REGION 



laid, and the evidence has been derived from the base, the top, and from 

 several intermediate parts of the series. This means that these parts 

 were deposited at a low angle, and, as the whole series shows angular 

 consonance of bedding planes, the whole series must have been formed 

 at a low angle — that is, approximately horizontal.* 



Rhyolite lava. — Somewhere in the tuff series a flow of rhyolite is 

 usualty found. It is generally underlain by a considerable thickness of 

 tuff, the greatest thickness measured in the vicinity being about 600 feet ; 

 the least, 6 or 8 feet. Where not overlain by basalt, the rhyolite is the 

 top of the series and varies up to 200 or more feet in thickness ; but 

 wherever the basalt overlies the rhyolite, it is generally separated from 

 it by tuff, from a few feet (10 or so) up to a few hundred. It is probable 

 that the tuff has been removed from above the rhyolite by erosion wher- 

 ever it was not protected by a basalt covering, and has also been removed 

 entirely where neither basalt nor rhyolite has given it protection. 



The rhyolite generally weathers buff or brown, but sometimes brick 

 red. It is distinctly porphyritic, showing much feldspar and some 

 quartz. The groundmass is generally lithoidal, but sometimes glassy, 

 with various spherulitic and other structures. A good flow structure is 

 common. In the Humboldt Lake range it is, so far as observed, con- 

 formably within the tuffs. In the Trinity range the tuffs have been 

 found disturbed by tilting before the outpouring of the rhyolite. 



Basalt. — Wherever found, the basalt is the youngest member of the 

 volcanic series. It does not occur so extensively as the rhyolite in the 

 Humboldt Lake mountains, being limited to a belt some 12 or 15 miles 

 long near the north end of that mountain range, while the rhyolites are 

 also extensively developed along the southern end of the range. It has 

 overlapped the rhyolite in several places, however, and either lies over 

 a comparatively thin tuff series, or may even lie directly on the bedrock. 



Weathering has produced a dark brown or black film on most of the 

 basalt, though a freshly broken piece is rather light colored and com- 

 monly a brownish gray. It the vicinity of the detailed section it is 

 rather crystalline, and most of the groundmass can be resolved into its 

 component minerals by the naked eye ; in fact, it is quite doleritic. Near 

 the upper surface of the flow it is commonly quite vesicular, but deeper 

 in the mass the steamholes become smaller and fewer, from which we 

 may conclude that the present surface is near the original surface of the 

 flow. The cellules near the surface are frequently coated with a botryoidal 

 opal (hyalite), and deeper in the flow they are sometimes entirely filled 

 with calcite, though, as a rule, they are empty. 



Wherever observed in these mountains, the basalt overlies the rest of 



*See also page 305. 



