ASPERITE. 243 



site, all being intenningled, often with the accompaniment of trati.sitional 

 forms. In some cases notUiles of obsidian are immediately inclosed in con- 

 centric layers of pumice and vesicular obsidian, while in other instances 

 angular fragments of obsidian are directly embedded in structureless pumice. 

 In this locality the stream has cut through solid obsidian, leaving sheer 

 walls ten or more feet in height. Elsewhere in the district this glass is 

 rarely found exposed in place, owing to its tendency to break up into small 

 fragments which cover the surflice. The andesitic obsidian is usually dis- 

 tinguisliable with ease from basaltic glass by its higher and more resinous 

 luster and its greater opacity. The andesitic origin of this glass is demon- 

 strated by its manner of occurrence. The microscopic character agrees with 

 this reference. 



About three miles from Kelseyville, on the Lower Lake road, and again 

 a little northwest of Thurston Lake, stratified, andesitic tufa is found The 

 former occurrence is very considerable and of course indicates the presence 

 of water during the eruption, though Cache Lake beds have not been rec- 

 ognized in the neighborhood. The presence of ponds or lakes near volca- 

 noes is of course a usual phenomenon, due to the damming back of streams 

 by ejecta or to more or less important orographical changes. 



Age of the younger andesite — Exceptiug the bcd of Clear Lake, the whole 

 region has been undergoing erosion ever since the andesitic eruption, and 

 the surfaces of the flanks and peaks of Konocti show that the degradation 

 has been considerable. There is no recognizable trace of a crater on the 

 peak; on the contrary, the bedded flows of rock near the summit are 

 shown in cross section. Sufficient time has elapsed since the eruption to 

 permit considerable decomposition in exposed masses of rock. The sum- 

 mit, however, is more exposed to degradation than the general surface of 

 the country, which has been little lowered in recent times, the underlyino- 

 metamorphic Cretaceous and Pliocene rocks, where they have been pro- 

 tected by the andesite, not lying perceptibly above the ordinary level. The 

 main area of andesite southwest of the lake probably overlies metamorphic 

 hills, for altered Cretaceous strata appear under the volcanic rock at the ex- 

 tremity of Elgin Point (" Snake Rocks") and at Bailey Point. The pres- 

 ent topography of the country between Konocti and Thurston Lake is 



