Page:Popular Science Monthly Volume 85.djvu/84

80 The present-day frequency with which great joint-blocks of granite give way in weathering, or from earthquake shocks, and fall to talus still suggest the subsidence hypothesis of Whitney.

A compromise, composite theory may now be advanced. Consideration of all the evidences of the structure and sculpture of the Yosemite region will justify the premise that during the upheaval of the molten granitic magma, domes swelled in rounded masses to much their present form, cooling in concentric layers, which were later exfoliated by aqueous and glacial denudation and aerial forces. Between these domes, the trough of the stone wave, faulted and cross-fractured along cleavage planes, formed the original floor of the Yosemite Valley. Subsequently, the tilting of the Sierra range caused the carving of the usual form of V-shaped valleys. Then, during the Quaternary period, down plowed the glaciers scooping out the bed-rock, sapping and scoring the walls of the valley, and leaving their moraines as mementoes to remind mankind of their ancient estate and power. But these moraines are either so wasted or considerable that much doubt is still entertained as to their extent. If erosive forces carved out the mile deep Yosemite, the question of the transportation of the detritus is still to be raised. The comparatively lightly-covered bed of the Merced River below the valley does not indicate the accumulation of any immense amount of detritus. Only by extremely fine comminution of boulders to silt could this immense mass be almost completely carried away. There is every evidence that a glacier reached the floor of the Yosemite Valley, but the extent of its quarrying is still unknown. Whether by direct grooving, or by undermining the overhanging escarpments, it unmistakably transformed the canyon's V into a broad,