LAVAS. 223 



now rests in places npon the older fresh-water strata. The beds immediately 

 below the andesite contain a few fossil remains which, as shown above, 

 correspond to the close of the Pliocene. The Pliocene beds near Mt. 

 Diablo also contain andesite pebbles. In addition to the relations of the 

 andesites to the sedimentary rocks at Clear Lake and Mt. Diablo, there is 

 some other evidence bearing upon their age. 



The asperites of Steamboat Springs and of Washoe show by the forms 

 of their tlows, by the slight traces of erosion, and the abundance of glassy 

 modifications that they are comparatively recent. A comparison of the 

 form of Mt. Sliasta with that of a theoretically perfect volcanic cone shows 

 that it is indeed considerably eroded, yet not so much so as to obscure its 

 derivation from a form closely resembling that deduced from theory. This 

 is also true of Mt. Konocti, on Clear Lake. The forms of these cones, as 

 well as the character of the material of which they are composed, thus show 

 that they, too, are comparatively recent. Furthermore, the amount of de- 

 parture of these cones from the theoretical form is about the same for each, 

 and so, too, are tlie other evidences of erosion. Hence they are approxi- 

 mately of the same age. From the relations of the asperite at Clear Lake 

 to the strata, this age is known to be that of the end of the Pliocene, and 

 Mt. Shasta, consequently, also dates from about the beginning of the Qua- 

 ternarv. I know of nothing tending to prove that the asperites of Washoe 

 and Steamboat are either much older or much younger than the similar 

 rocks of the Coast Ranges. 



Of the age of the rhyolite of New Almaden as compared with the 

 other lavas nothing is known. It is clear, however, that it postdates the 

 Post-Miocene uplift, for, while the Miocene of New Almaden is much dis- 

 turbed, the rhyolite dike intersects the disturbed Miocene and has itself not 

 been affected. 



The basalts are still younger than the andesites. The eruptions near 

 Clear Lake are evidently referable to a somewhat extended period, but per- 

 fect volcanic craters remain. There are also said to be among the Indians 

 of the region traditions of eruptions. In northern California there is good 

 reason for believing that there has been a small basaltic eruption within 

 forty years. 



