The term “Northern Drift” is understood to include the masses of unstratified detrital materials and boulders which have been transported and distributed by some general cause independent, in a great degree, of the present conformation of the surface and of the direction of the existing river courses. The investigations of geologists have shown that the surface of Canada, New England, and the States north of the Ohio and north of the parallel of 39°, as far west as the Mississippi, and even for some distance beyond it in that direction, are covered by detrital materials which have been carried from the North towards the South, and often for a great distance and in immense masses. The explorations of the Geological Survey of California have demonstrated however, that there is no true Northern Drift within the limits of this State. Our detrital materials, which often form deposits of great extent and thickness, are invariably found to have been dependent for their origin and present position on causes similar to those now in action, and to have been deposited on the flanks and at the bases of the nearest mountain ranges by currents of water rushing down their slopes. While we have abundant evidence of the former existence of extensive glaciers in the Sierra Nevada, there is no reason to suppose that this ice was to any extent an effective agent in the transportation of the superficial detritus now resting on the flanks of the mountains. The glaciers were confined to the most elevated portions of the mountains, and although the moraines which they have left as evidences of their former extension are often large and conspicuous, they are insignificant in comparison with the detrital masses formed by aqueous erosion. There is nothing anywhere in California which indicates a general glacial epoch during which ice covered the whole country and moved bodies of detritus over the surface, independently of its present configuration, as is seen throughout the Northeastern States. The same condition of things prevails in Nevada and through Oregon, as far as explored by the members of the Survey. The detritus seems always to be accumulated at the base of the mountains—gravel, boulders, and sand lying below and not far distant from the beds of rock of which these materials once formed a part, and from which they appear to have been detached by weathering and aqueous erosion.