The investigations of the United States Geological Survey in the Gold belt of the Sierra Nevada, carried out under the direction of Dr G. F. Becker, with whose consent this paper is published, have included the geologic mapping of the country on topographic contour maps on the scale of 1 : 125,000, or about two miles to the inch. During the course of this mapping much information has been gained concerning the Neocene river channels, now largely covered by deep volcanic flows or cut away by. subsequent erosion. The auriferous character of the accumulated gravels gives, as is well known, great practical importance to these channels. A large number of the productive Neocene gravel deposits occur in the watersheds of the present Yuba and American rivers, which were included in the area assigned to the writer. It has been found that these deposits are parts of two river systems . . .