The whole region west of the Rocky Mountains is comparatively recent. The Coast Range which now fronts the Pacific is composed entirely of marine Tertiary strata, and when they were deposited, the waves of the Pacific beat against the flanks of the Sierra Nevada. At length the Coast Range was upheaved and a wide valley left between it and the Sierra of over 400 miles in length, and with an average breadth of seventy-five miles. The Sierra itself is old land, the lower hills consisting of Triassic slates and the higher ranges of granite, and it has never been under water since the Secondary Age though doubtless it stood much higher before it was so greatly denuded. All along its western flank and far down into the great valley is an enormous bed of auriferous gravel, doubtless derived from the waste of the rocks of the Sierra during an immense time by old rivers now buried under their own deposits. While these deposits were going on a great outburst of volcanoes occurred on the western slope of the Sierra, and successive sheets of tuffs, ashes, and lavas are interstratified with the gravels, while finally an immense flow of basalt covered up everything. The country then presented the appearance of a great plain, sloping gradually downwards from the Sierra according to the flow of the basalt and lavas. This plain was in its turn attacked by denudation and worn down by the existing main rivers into valleys and gorges, and by their tributary streams into a series of flat-topped hills, capped by basalt and divided from one another by deep and narrow cañons. The immense time required for this latest erosion may be inferred when it is stated that where the Columbia river cuts through the axis of the Cascade Mountains, the precipitous rocks on either side, to a height of from 3000 to 4000 feet, consist of this late Tertiary or Post-Tertiary basalt, and that the Deschutes river has been cut into the great basaltic plain for 140 miles to a depth of from 1000 to 2500 feet, without reaching the bottom of the lava. The American and Yuba valleys have been lowered from 800 to 1500 feet, and the gorge of the Stanislas river has cut through one of these basalt-covered hills to the depth of 1500 feet. [Illustration: SECTION OF GREAT CALIFORNIAN LAVA STREAM, CUT THROUGH BY RIVERS. _a_, _a_, basalt; _b_, _b_, volcanic ashes; _c_, _c_, tertiary; _d_, _d_, cretaceous rocks; _R_, _R_, direction of the old river-bed; _R´_, _R´_, sections of the present river-beds. (Le Conte, from Whitney.)] The enormous gorge of the Colorado has cut its cañons for hundreds of miles from 3000 to 6000 feet deep through all the orders of sedimentary rocks from the Tertiaries down, and from 600 to 800 feet into the primordial granite below, thus draining the great lakes which in Tertiary times occupied a vast space in the interior of America which is now an arid desert.