THE TERTIARY VOLCANICS 303 



position. The failure of a later to cover completely an earlier, or the 

 overlapping of a later over an earlier, or the failure of a later to overlie an 

 earlier at all, lead to some confusion, and make it difficult and sometimes 

 impossible to arrange them in historic sequence. However, a complete 

 series of the more important members seems to exist on the east side of 

 the mountains just north and south of the Muttleberry road. Many 

 other more or less complete occurrences have been observed, all of which 

 agree with the order here given : 



Basalt (top of series). 

 Rhyolite tuff. 

 Rhyolite lava. 

 Rhyolite tuff (base). 



Rhyolite tuffs. — The tuffs are generally white, sometimes brownish, 

 mostly fine grained, and not particular! 3^ consolidated. Some layers are 

 rich in glassy lapilli, others are highly pumiceous. No particular differ- 

 ence has been noticed between the tuffs above and those below the rhyo- 

 lite lava. The most interesting features of the tuffs for the present pur- 

 pose are those which point to their attitude of deposition. About half a 

 mile south of the Muttleberry road, on the east slope, occur very good 

 exposures of the tuff, in which were observed distinct current-bedded 

 structures. Such layers contain small rock fragments up to i inch or 

 more in diameter scattered through them. The cross-bedding plains are 

 irregular in their inclination to the true bedding and generally rather 

 curved. 



On the summit, about 2 miles north of the road summit, at the base 

 of the tuffs, occurs a conglomerate of rather coarse pebbles up to 5 or 6 

 inches across and made up of quartzite, diorite, and other bedrock 

 species. 



The summit almost due east of Lovelock is of basalt overlying tuff. 

 The contact is well exposed, showing a reddening of the topmost layers 

 of the tuff and a roughening of the basalt bottom, which has been irreg- 

 ularly forced up and invaded, apparently, by steam generated by the 

 action of heat on the moisture of the rock on which the basalt was poured. 

 The layer on which the basalt rests is distinctly waterlaid and contains 

 abundant small pebbles, chiefly of quartzite. 



Across the Humboldt valley, just north of the west end of the detailed 

 section, two layers of sandstone, one 6 inches thick, were found strati- 

 fied in the tuff. 



It may be added that all of the tuff is more or less distinctly stratified, 

 but whether by water or air action is not always evident. 



The above facts prove that at least part of the tuff series was water- 



