242 QUICKSILVER DEPOSITS OF THE PACIFIC SLOPE. 



show tliat their history must have been contuiuous. The hater andesite, 

 represented by Mt Konocti, overhes the latest Cache Lake strata and also 

 underlies the Clear Lake sediments. It is impossible to avoid the conclu- 

 sion that the eruption of this rock accompanied the obliteration of Cache 

 Lake and the orographical changes which confined the v\aters to their pres- 

 ent bed. The vertebrate remains in the vineyard near Lower Lake thus 

 fix the geological date at which the Cache Lake period terminated and also 

 the date of the eruption of the asperites of Mt. Konocti. As was noted in 

 Chapter V, the vertebrate fossils are Pliocene, while the amount of erosion 

 and the relations to the modern lake beds show that they are Upper Plio- 

 cene. The date of the eruptions is thus fixed at about the close of the 

 Pliocene epoch. 



Later andesitic eruption. — The later andcsite is most prominently represented 

 by Konocti (or Uncle Sam) Mountain, but the same rock covers a large 

 area to the southeast and a considerable tract to the northeast of the more 

 southerly branch of the lake. It is described from a microscopical point 

 of view in Chapter IV. The prevalent variety of the rock is a coarse- 

 grained porphyry, sometimes dark and sometimes rather light colored. 

 One of its marked features is the frequency of huninated structure.^ The 

 lamina; are usually half an inch or more in thickness and not very sharply 

 divided from one another, \yeathered surfaces of such rock are corrugated, 

 and at a little distance the rock might be thought sedimentary rather than 

 volcani'c. Where heavy masses are cut through, columnar structure is 

 sometimes seen. It is particularly fine near Little Borax Lake. Between 

 Konocti and Thurston Lake there are also vast quantities of obsidian and 

 pumice, the foi-mer covering almost continuously a large tract, through 

 which the road from Kelseyville to Lower Lake passes. On this line it is 

 about four miles in width and it is said to extend a still greater distance 

 to the southwest. The best locality for the study of these forms is on 

 Thurston Creek, between one and two miles northwest of Thurston Lake. 

 Here the obsidian and pumice are interbedded with the porphyritic ande- 



' In Geol. Survey Califoruia, Geology, vol. 1, p. 96, Professor Whituey states that, as seen from the 

 opposite side of the lake, Uncle Sara appears to be made up of a closely folded, synclinal mass, prnli- 

 ably of somewliat nietamorphic Cretaceous saudstoues. This impression he certainly received from 

 the exnosert edf.'ps of these flows. In Auriferous Gravels, i>. '^3, this mountain is correctly mentioned 

 as volcanic. 



