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THE JOURNAL OF GEOLOGY.

Mr. Turner, to protruding hard rocks, it is possible that a considerable portion resulted from deformation when the Sierra Nevada was upheaved. For it will be shown later on that since this peneplain was formed by erosion, the Sierra Nevada has been greatly uplifted, and it would be very remarkable indeed if in the upheaval of such an enormous mass as the Sierra Nevada the original plain of its western slope were not warped and broken.

Platform of the interior region.—The fact that the baselevel plain passes to the eastward from the northern end of the Sacramento valley beneath the lavas of the Lassen Peak district, suggests that it may reach the platform of the interior region, which is now covered by volcanic material. Within northeastern California and the adjacent portion of Oregon there are vast stretches of level plains which are nearly of the same altitude above the sea. As far as known, all the surrounding hills and mountains are of lava. There are no projecting peaks of older rocks, and their absence from wide stretches of plateau country tends to show a general level of the subjacent surface analogous to that of the interior plateu in British Columbia described by Dr. G. M. Dawson.

The erosion plains we have traced upon the borders of the Sacramento valley, in the Klamath Mountains, upon the western slope of the Sierra Nevada, and probably also in the interior region of northeastern California, join one another in such a way as to show that they are simply different portions of one extensive baselevel of erosion which formerly spread over a large part, if not the whole, of middle and northern California and the adjacent portion of Oregon. What is the geological age of this plain of erosion?

Deposits upon the border of the ancient baselevel.

General statement.—In order to determine the conditions under which the baselevel was developed, and its age, it is necessary to study the formations deposited during its development. At the eastern edge of the baselevel, in the Sacramento valley, there are three formations, all of which were more or less

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