W. LINDGREN—F. H. KN OWL TON 



5. The gravels of the inter-volcanic erosion period.- — The inter- 

 val separating the rhyolitic from the andesitic outbursts appar- 

 ently differed in length at various points in the Sierra Nevada. 

 While in some places, such as along the lower courses of the Middle 

 and South Yuba, the andesitic tuffs lie almost conformably over 

 the rhyolitic tuff, there are at other points, such as on the Forest 

 Hill divide, near Placerville, and along the Mokelumne, indica- 

 tions of a relatively short period of a very active erosion, begin- 

 ning immediately after the rhyolitic flows, or in some places 

 shortly after the first flows of andesitic tuffs. This erosion was 

 of a remarkably intense character, cutting sharp V-shaped can- 

 yons in new channels through the older beds, and in some places 

 even down into the solid bed rock to a depth of about 100 feet. 

 This action is so very different from that of the ante-rhyolitic 

 and rhyolitic streams that the inference is justified that just 

 after the rhyolitic flows the tilting of the slope took place, or at 

 least began. In the bottom of these sharply cut channels a few 

 feet of gravel accumulated along stretches with less grade, while, 

 where the gorges were narrow and the grade steep no detritus is 

 found. These are the gravels of the inter-volcanic erosion period. 



6. The andesitic tuffs and tiffaceoiis breccias. — The andesitic 

 tuffs poured down the river valleys in the form of successive 

 mud flows of enormous volume, at first as sandy and clayey 

 masses, but later mixed with a great quantity of larger angular or 

 subangular fragments of hornblende and pyroxene-andesite, at last 

 covering a large part of the slope and forcing the rivers to seek 

 entirely new channels. 



Along the valley border the lone formation corresponds to 

 the ante-volcanic bench gravels along the old rivers in the 

 Sierra. The oldest or deep gravels of the river courses corre- 

 spond to the fluviatile deposits in depressions in the surface on 

 which rests the lone formation ; in other words, before the 

 lone transgression the rivers extended westward, probably to the 

 center of the valley. The volcanic period is, along the valley 

 border, represented by a series of gravels, sands, and tuffs 

 spread over the lone formation. 



