34 THE JOURNAL OF GEOLOGY. 



ranges were then inconspicuous and the slopes everywhere 

 gentle. 



It is the object of this paper to trace out this ancient topog- 

 raphy and briefly to outline the great changes by which the pres- 

 ent features were developed. Incidentally the auriferous gravels 

 will be considered, because they originated in large part at the 

 beginning of its topographic revolution, which has on this 

 account a most important economic interest. 



TOPOGRAPHY OF THE PACIFIC SLOPE. 



There are two prominent topographic belts on the Pacific 

 slope. One is the platform of the interior basin region, and the 

 other the mountain belt which lies upon the border of the conti- 

 nent. The latter embraces the Sierra Nevada, Cascade, and Coast 

 ranges, as well as the Klamath Mountains in northwestern Cali- 

 fornia and southwestern Oregon, where all the ranges meet. 

 Between the ranges to the southward of the Klamath Mountains 

 lies the Great valley of California, and to the northward the 

 Sound valley extends from central Oregon across the state of 

 Washington. The mountains are everywhere deeply cafioned by 

 the rivers, but if we take a more general view, overlooking those 

 features which are still developing, we shall discover others of 

 much greater antiquity. 



ANCIENT BASELEVEL OF EROSION. 



Upon the Jiorthwestem and northern border of the Sacramento 

 valley. — Upon the northwestern border of the Sacramento valley 

 is a well-marked plain of erosion, which extends for nearly one 

 hundred miles from about the 40th parallel around the northern 

 end of the Sacramento valley to near the Great Bend of Pit 

 river. It varies from one to fourteen miles in width, and is best 

 marked in the Greasewood and Bald hills of Tehama and Shasta 

 counties. The larger portion of the plain has been carved upon 

 the upturned edges of the Cretaceous strata, and the denudation 

 has reduced the thick, hard conglomerates and sandstones to the 

 same level as the soft shales. At a number of places the well- 

 defined plain extends for several miles into the area of harder 



