238 QUICKSILVEE DEPOSITS OF THE PACIFIC SLOPE. 



are sufficient to suggest a noii- conformity. When this relation has been 

 shown to exist elsewhere it is manifest that it affords a satisfactory explana- 

 tion of the facts at Clear Lake. 



No Miocene strata have been detected with certainty in this part of 

 the country. It is possible that such were deposited and have since been 

 completely removed by erosion ; but this appears to me very unlikely. 

 Remnants of them would almost inevitably have been preserved, if not 

 elsewhere, at least beneath the fresh-water Pliocene. I believe it much 

 more probable that a giadual rise of this region took place in the early 

 Tertiary, such as has occurred in recent times througliout the State, and 

 that during the Miocene this was a land area. No violent uplift can have 

 intervened between the Tejon (Eocene) and the Miocene, however; for, 

 wherever the two come in contact, as is frequently the case to the soutli, 

 they almost alwa3's appear entirely conformable. 



First andesitic eruption. — After tlic depositiou of the Cliico-Tejou rocks the 

 first geological event traced was the eruption of Chalk Mountain. This 

 was probably coeval with the ejection of some of the rock near Thurston 

 Lake. These lavas are dense pyroxene-andesites, which have been described 

 in Chapter IV. Chalk Mountain lies upon the north fork of Cache Creek, 

 about half a mile above the higliest point of the creek shown on the map. 

 It is a small conical hill, from a part of which the heavy bases have been 

 extracted by sulphur springs, still feebl}^ flowing. Portions of the mass 

 are fresh, however. Chalk Mountain rests upon crumpled, metaraorphic 

 strata, which were deeply eroded before the ejection of the rock. The 

 outflow of this rock certainly preceded the Cache Lake period, for the lake 

 beds are found upon its sides, and fragments, either from Chalk Mountain 

 or fi'om other unknown masses of precisely similar lithological character, 

 are abundant throughout all the lake beds shown on the map. Clialk 

 Mountain may have somewhat antedated Cache Lake, but there is as yet 

 nothing to indicate an interval, and it seems more probable that its eruption 

 accompanied the orographical changes which in the Pliocene, and probably 

 early in that period, dammed back the waters of the region. 



Cache Lake beds.- — That Caclic Lake occupied an extensive area is certain. 

 It extends to the east an unknown distance, and how great a proportion 



