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TOPOGRAPHY OF THE PACIFIC COAST.
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If, when thus mantled with residuary material, the Sierra Nevada region were affected by a change of level in such a way as to slightly increase the fall of the streams upon its slopes, it is believed, as already suggested, that during a comparatively brief period owing to overloading they would be forced to deposit and fill their channels. A portion of the process is, in a measure, illustrated by what has taken place along some of the present streams of the Sierra Nevada where hydraulic mining has been extensively carried on. The streams are overloaded by the debris forced into them from the mines and their channels are at least temporarily filled with gravel.

After the deposition of the earlier gravels the declivity of some of the streams at certain points appears to have been so decreased that they deposited finer material and covered the gravel with sand and clay. This may have resulted from differential elevation, differential subsidence, or both, and there is evidence that both occurred within the gravel period. At Cherokee Flat upon the eastern border of the Sacramento valley the finer, essentially estuarine deposits, over 300 feet in thickness, lap over to the eastward upon the ancient river and shore gravels mined at that place. This overlapping evidently resulted from a subsidence of that region.

Summary.

A study of the ancient topographic features upon the borders of the Sacramento valley, in the Klamath Mountains, and upon the western slope of the Sierra Nevada, shows that during the earlier portion of the auriferous gravel period, southern California, by long continued degradation, was finally reduced approximately to baselevel conditions. The mountain ranges were low, and the scenery was everywhere characterized by gently flowing slopes.

The distribution of the Ione formation and the early auriferous gravels, as well as the plant remains which they contain, point clearly to the same conclusion.

The topographic revolution consisted in developing out of such conditions the conspicuous mountain ranges of to-day.

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