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University of California. 



[Vol. i. 



the "auriferous gravels" as of Pliocene age, the tilting of the Sierra 

 Nevada peneplane was also a post-Pliocene event. The Valley of 

 California affords evidence of but very moderate uplift, as compared 

 with that of the Sierra on the one hand and the coast on the other. 

 It appears, therefore, that whatever may have been the origin of the 

 valley, whether the result of erosion or of structural depression, it 

 is, in the recent phases of its development, a synclinal trough. The 

 tendency of the coastal uplift to make the valley a closed basin has 

 been counteracted by the vigorous trenching of the mountains 

 effected by the Sacramento River at the Straits of Carquinez. It is 

 not yet known whether this trenching action has always been able 

 to keep pace with the rate of uplift so as to maintain the drainage 

 of the valley at base-level. It is also improbable that this outlet 

 antedates the Montara and Diablo upthrusts. In so far as the 

 epeirogenic uplift has accentuated the character of the Valley of 

 California, it has incidentally given us an orogenic result. 



As a consequence of the general uplift of the coast, the phys- 

 iography of the country has been radically changed in the most 

 recent geological times. During the Pliocene depression many of 

 the valleys, which had been developed in the post-Miocene interval 

 of high alti-tudes, were filled to the brim with delta deposits. Pari 

 passu with this filling process, the hills were being worn down to 

 near base level. The result at the close of the Pliocene was an approx- 

 imation to a peneplane topography — a peneplane partly of delta 

 construction and partly of mountain truncation. Remnants of this 

 peneplane are apparent in parts of the southern coast ranges in the 

 plateau-like summits of the lower ranges, although it is possible 

 that some of this peneplane effect may be ascribed to the much' 

 earlier Miocene depression. The two truncations, Pliocene and 

 Miocene, will, in the opinion of the writer, be readily discriminated 

 in detailed field work. At best, however, the Pliocene truncation 

 resulted in but an approximation to a peneplane, or series of local 

 peneplanes. Numerous peaks and ridges rose above the general 

 level. Numerous islands, large and small, fringed the coast of Cal- 

 ifornia. There were numerous submerged valleys, so that the coast 

 was well supplied with harbors. In a word, the coast of California, 

 at the close of the Pliocene, had the aspect of an archipelago. The 



