



522 



PECULIARITIES OF THE VOLCANIC DEPOSITS. 



were entirely independent of eacli other as to the times when accumulation ceased and erosion 

 began, I do not know. 



The character as well as the extent of the denudation which has taken place on the western slope 

 of the Sierra, since the close of the volcanic era, lias evidently varied considerably at different locali- 

 ties, and probably owing often, to special causes which we do not yet understand. For example, 

 in the basin of the Middle Fork of the American the denudation seems to have been chiefly lim- 

 ited to the excavation of the modern canons. At least, I do not know at present of any evidence 

 to prove that the crests of the broader ridges and volcanic-capped areas here have been perceptibly 

 lowered since that time. But in the country adjacent to the Tuolumne County Table Mountain 

 it appears that broad areas have been lowered for hundreds of feet at least, or sufficient to leave 

 that Table Mountain prominent above the surrounding region. 



In the eastern portion of the valley the volcanic materials, in the shape of pebbles, sand, etc., 

 are scattered many miles beyond the foot-hills of the mountains proper, and indeed it seems 

 probable that they may be found almost anywhere, to a greater or less extent, until we reach the 

 eastern margin of the true adobe soil which covers all the central portion of the valley ; and one 

 point of interest in connection with the borings from that artesian well at Stockton would be to 

 discover whether there is any volcanic matter hero beneath the adobe soil, and if so, to how great 

 a depth it may be found, as this would give interesting information as to how great a depth of 

 filling has accumulated in the central portion of the valley since the commencement of the vol- 

 canic era. 



It may be noticed as an interesting fact that in the vicinity of Placerville wherever volcanic 

 gravel and breccia occur together, the breccia always overlies the gravel, while in the region about 

 Sutter Creek and below there the reverse is generally the fact. 



One point of no little general interest and importance in connection with the distribution of the 

 volcanic matter over the western slope of the Sierra is the feet that the earliest volcanic beds con- 

 sist in general of the finest materials. Indeed, at many localities the first warning of the approach 

 of the volcanic era seems to have been a sudden thickening of the waters of the streams with vol- 

 umes of finest mud, of which the so-called " chocolate" at Dead wood and elsewhere is an example. 



Moreover, the white lava, enormous as its quantity is at many localities, whenever it occurs in 

 close association with other and coarser volcanic materials (which is oftener than otherwise the 

 ease), always underlies them, forming the lowest volcanic stratum. The same rule holds with 

 other varieties of fine volcanic sediment and ash. Wherever they occur, their position is low 

 down, at or near the bottom of the volcanic matter. 



This fact is especially conspicuous with the white lava, as its quantity is so great. These finer 

 sediments are generally entirely free from boulders, and almost entirely free from pebbles of any 

 kind. They do indeed contain occasionally small pebbles, which, however, are just as often of 

 quartz or metamorphic rock as they are of anything volcanic. It is by no means easy to under- 

 stand exactly how such enormous masses of this white lava have been so perfectly segregated as 

 they are from all other kinds of material. It has certainly, I think, been done by water. But 

 why should not the water which transported such vast quantities of this material, line and light as 

 it doubtless was, have brought more frequently little pebbles or pellets of some kind with it ?° Was 

 it indeed brought here as sediment, freely suspended in and deposited by running water at all ? 

 Or did it come in the form of semi-fluid mud-flows ? (It could hardly have been viscid at all. It is 

 too sharply granular for that.) Or was it first distributed over the country in the form of showers 

 of ashes through the air, and afterwards collected in the valleys by the rains] The last, indeed, 

 seems hardly likely ; for in that case it is difficult to see how so few pebbles could have been 

 mixed with, it in the process. But there are some difficulties, also, in either of the first two hy- 

 potheses, and nothing seems altogether satisfactory in explanation of it. I feel sure, however, that 

 it came here in some way through the agency of water, and in the condition of volcanic ash, and not 

 in a state of igneous fusion; for not only does the whole texture and appearance of the'material 

 speak strongly against the last idea, but I consider the presence of even the few little pebbles and 





