204 DYNAMICAL GEOLOGY. 



Buried river valleys. — Rivers of the Sierra Nevada, in Tuolumne County, 

 California, that had their channels buried beneath lavas in the later Tertiary, 

 afterward, in a comparatively short time, cut new channels througli the 

 thick lava stream and the underlying rocks to depths 1500 to 2000 feet 

 below the old channels. (Whitney, 1865, 1879.) They hence are strong 

 evidence of increased precipitation, as held by Whitney, and also, according 

 to LeConte (1879, 1886), of increased elevation in the mountains; and both 

 conditions characterized the Glacial period which was in progress during 

 part or nearly all of the cutting. Like evidence of elevation exists also in 

 the river channels of southern California beyond the limits of the lava-flood, 

 as observed by LeConte (1886), who thence concludes that the elevation 

 extended along the whole length of the Sierras. 



The ultimate result of denudation over a continent is, as usually stated, 

 the transfer of the mountains to the sea, bringing all to a nearly level plain. 

 But the facts from Tahiti, explained on page 182, appear to show that the 

 process would, as a general thing, first thin down the mountains to sharp 

 peaks and ridges ; and after this, the continuation of the thinning would 

 ultimate in a general level — given time suiScient. The Adirondacks ha.ve 

 stood ever since Archaean time, with the height probably never less than 

 5000 feet ; and yet they are to a large extent in the Tahitian stage. But the 

 streams of extensive drainage areas become to a greater or less extent base- 

 leveled; and through the continued leveling work along them, with that of 

 the minor tributaries, a wide region may be finally reduced approximately 

 to a plain. Such a plain has been termed by W. M. Davis a peneplarie, from 

 the Latin for almost and plain; for it may still have ledges of the harder 

 rocks and other irregularities of surface. An elevation of the land, and 

 other causes indicated above, may expose such regions to a new base- 

 leveling. 



The fluvial history of a country, it thus appears, may have great com- 

 plexity, and require a large amount of study and an experienced judgment 

 for its correct elucidation. 



Subterranean Waters. 



Water descends from the surface by gravity, filling all open subterranean 

 spaces, and also the pores of the solid rocks. Its lower limit is determined 

 by the earth's interior heat ; and the lower limit of ontward discharge, by a 

 level not much below that of the ocean's surface. At greater depths, conse- 

 quently, subterranean water may be that of early ages in geological history, 

 and in part the sea water in which the deposits were made, more or less 

 modified in its saline contents and their amount by long contact with the 

 various rocks. Not only the waters of the rains and rivers thus take a 

 downward way through the porous rocks, between their sloping layers and 

 along all crevices, but also those of lakes, which are sources of permanent 

 supply, and pre-eminently those of the ocean. 



