358 F. L. Eansome — Lava Flows of California. 



the foot-hill region of closely compressed clay-slates, schists, 

 limestones, quartzites, and various igneous rocks, with a pre- 

 vailing northwest and southeast strike, and nearly vertical 

 northeast dip, and in the higher portions of the range of gneis- 

 soidal and granitic rocks. 



Toward the end of the baselevelling process, which, as 

 shown by Diller* and Lindgrenf for other portions of the 

 Sierra, lasted at least into the Miocene, the series of volcanic 

 eruptions began which continued through the Pliocene, and 

 ended by covering the old surface far and wide with a blanket 

 of clastic andesitic material in the form of breccias, rolled 

 gravels, and fine tuffs. At the same time began the tilting 

 which has given the peneplain its present slope of about two 

 degrees to the southwest. The old surface revealed to-day by 

 the stripping off of a great part of its andesitic cover, is by no 

 means a perfectly even one. If the sharp Y-shaped canons of 

 post-Tertiary date were all filled up, and the remnants of the 

 Tertiary volcanic cover quite removed, there would still remain 

 a surface of gentle relief surmounted by occasional monad- 

 nocks. On the whole though, it would be a topography of 

 greatly subdued relief when contrasted with the possibilities 

 for alpine types of form suggested by the nearly vertical schis- 

 tose rocks and batholithic plutonic masses out of which it had 

 been carved. 



The old peneplain character is not everywhere equally well 

 preserved. In the foot-hill region, embracing roughly the 

 southwestern third of the map, the existence of long belts of 

 highly inclined rocks of varying resistance to erosion, has led 

 to the rapid development of subsequent streams and a general 

 degradation of the surface of the country. The least deface- 

 ment of the Neocene peneplain is found in the region lying 

 between Murphy and Clover Meadow. Here the homogeneity 

 of the underlying rocks has tended toward the perpetuation of 

 a general consequent character of drainage, the dominant type 

 of topography being that impressed upon the region by the 

 prevalence of long, narrow, and often flat-topped ridges, sepa- 

 rated by sharp Y-shaped canons. In the shelter of the latter 

 flourish some of the noblest of forest trees, including the 

 Sequoia gigantea, and the beautiful sugar pine, Pinus lamber- 

 tiana. From Clover Meadow northeastward, the Neocene 

 accumulations have been more thoroughly eroded away, leav- 

 ing a bare hummocky granitic slope, deeply trenched by the 

 gorges of the larger streams, and surmounted by residual 



* Topographic revolutions on the Pacific Coast, 14th Ann. Rept. U. S. G-. S., pp. 

 419-421. 



f Age of Auriferous Gravels of the Sierra Nevada, Jour, of Geol., vol. iv, pp. 

 893 and 898. 



