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 holmes] A URJFEROUS GRA VEL MAN 1 1 1

to form definite notions of the great lapse of time and the vast transformations of nature with which we have to deal, it may be well to present briefly the main features of the later geologic history of the region. The accompanying sections, with ap- pended data, will serve to tell the story so fully that a few words only will be necessary to make it fully understood. In early Tertiary times the prototypes of the modern rivers ran out from the sierra down through the highland to the sea pretty much as they do today. The valleys were not so deep as now, as indi- cated in i, plate V, but they had strong currents and rapidly scored down the gold-bearing formations which they traversed, filling the channels with coarse, waterworn debris to the depth of hundreds of feet and depositing the freed gold along their beds. This phase of progress is indicated in 2, plate V. It is from these gravels that some of the finds of human relics are reported, and it is therefore affirmed that along the banks of these ancient rivers the first human beings of which we have a trace lived and pursued their varied avocations.

But there came over this region a momentous change. A period of great volcanic activity set in, and streams of lava and rivers of mud descended from the sierra, filling up the valleys ; new channels were eroded to be filled in their turn, one system of drainage succeeding another for a prolonged period, at the close of which the deepest valleys were filled to the brim, as shown in 3, plate V ; and when the flows of basalt — the final products of vulcanism — ceased, the waters of the high sierra began the work of laying out the drainage system that has come down to the present time.

Since that remote day the region has been elevated to greater heights ; the Merced, the Stanislaus, the Tuolumne, the American, the Yuba, and other streams have cut their channels by the slow processes of erosion down to profound depths and now run their courses in valleys two thousand feet deep and many miles in width — gorges so profound, precipitous, and vast that it is a

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