Page:The Journal of geology (IA journalofgeology21894univers).pdf/48

ranges were then inconspicuous and the slopes everywhere gentle.

It is the object of this paper to trace out this ancient topography and briefly to outline the great changes by which the present 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.

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 continent. The latter embraces the Sierra Nevada, Cascade, and Coast ranges, as well as the Klamath Mountains in northwestern California 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 cañoned 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.

Upon the northwestern 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