THE LAVA BEDS AND TUFFS. 267 



by the last great andesitic flows. The intervolcanic channels, being no 

 part of a permanent and established drainage, are not described or 

 further discussed in this paper. 



These last flows, coming down in rapid succession from the volcanoes 

 of Weber lake, of mount Lola, of Castle peak, and many others south- 

 ward, flooded everything and covered up the lower and middle slopes to 

 such an extent that only isolated peaks or ridges protruded above them_. 

 Their character is peculiar; they consist of a gray or brown tuffaceous 

 breccia, containing, in the foot-hills as well as in the high range, large, 

 usually angular, bowlders of andesite. They came down the slope as 

 successive mud Hows, setting soon to a hard and compact rock. Molten 

 andesitic flows are found in the lava-flooded valley between the two 

 summits, and also at some places west of the first summit, but they did 

 not extend far down the western slope. 



The thickness of these flows ranges from over a thousand feet high up' 

 to fifty or one hundred feet down toward the plains, where they are 

 nearly always underlain by volcanic sands and conglomerates washed 

 down from earlier flows winch had not reached so far down. 



The eruption of this tuffaceous breccia is assumed to close the Neocene 

 period, and its flows form an important horizon by means of which the 

 Neocene gravels may lie separated from the later Pleistocene accumula- 

 tions. The age of the flows is indicated by the numerous plant impres- 

 sions common in the clays of the rhyolitic beds, as well as in those of 

 the older underlying gravels, and which, as well known, distinctly point 

 to the Neocene period. Only rarely is there any difficulty experienced 

 in distinguishing the Tertiary from the Pleistocene gravels. 



The length of the volcanic period may be roughly measured by the 

 depths of the " cement channels " on the Forest hill divide. They have 

 cut through about one hundred and fifty feet of loose detritus and, at 

 most, one hundred feet of solid rock — not much more than a twentieth 

 of the erosion since the close of the volcanic period. 



There have been no andesitic eruptions of later date than those de- 

 scribed in the region now discussed, and the continuity of the last over- 

 whelming lava, Hoods can be traced, almost uninterruptedly in places, 

 from the plains of the great valley to the summits of the high Sierra,. 



When the volcanic activity ceased * the rivers began to seek their final 

 channels, those of to-day. The general drainage was outlined by the 



*An eruption of massive basalt occurred in some parts of the Sierra Nevada subsequently to the 

 andesitic eruptions and previously to the glaciation. In the region here considered only small 

 areas of this Pleistocene basalt are found. 



Mr H. W. Turner has recently shown (Am Jour. Sci., 3d ser., vol. xliv, 1892, p. 455) the existence 

 of ii basalt antedating the andesite in the northern part of the Sierra Nevada. This curlier basalt 

 does not occur in the area described in this paper. 



