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baselevel. The removal of material was chiefly by solution, and the insoluble residuary material thus set free by the disintegration of the rocks accumulated to considerable depths upon the land.

The long period during which the land of northern California remained comparatively stationary, and which enabled the streams in many parts of that region to practically complete their cycles of erosion from youth to old age, was brought to a close by the initiation of an orogenic movement which generally increased the grade of the streams upon the western slope of the Sierra Nevada. At first the differential change of level was very moderate and increased the declivity of the streams but little, but being long continued it became in time revolutionary in its effects, and finally, accompanied by extensive volcanic eruptions, gave birth to the High Sierra of to-day with the deep cañons upon its western slope.

The first result of this change of slope was to rejuvenate the streams and invigorate erosion. On account of surface deformation which must have accompanied the upheaval of such a large mass as the Sierra, the stream grades would be differently affected even along the same channel, and in fact, as Mr. Lindgren has pointed out, in at least one case, owing to direction of flow, the stream grade has been not only diminished but reversed.^[1]

The country being covered by a thick coating of soft residuary material, of which the great mass was fine particles, erosion was easy. There were coarser fragments of quartz, largely vein matter, as well as boulders of disintegration which had withstood the chemical changes. The streams readily became loaded not only to their full capacity but overloaded with the mass of fine material, and were thus forced to deposit the coarser particles. The grains and fragments not quite suspendable under the conditions of load were rolled along the bottom and rounded by attrition.

In this way the old channels of the baselevel period became filled with gravel of which by far the larger part is quartz. In