quiescent volcanic forces exerted themselves. The valleys filled with rhyolitic flows which covered the auriferous gravels, and caused new stream courses to be outlined in the old valleys. Renewed disturbance resulting in a westward tilting of the main blocks, probably combined with normal faulting, began along the previous eastern breaks. This strongly emphasized the monoclinal nature of the range, and the streams immediately began to cut their beds deeper; they repeatedly crossed their old courses and the concentration of gold in the new canyons proceeded under less favorable torrential conditions. Eruptions of andesitic tuffs and mud flows then began in enormous volume, filling the valleys of a large number of the streams to the rims and effectually burying them, (see illustration on page 20). The close of the Tertiary period, in startling contrast to the vegetation clad hills of the Miocene, saw almost the whole of the northern Sierras covered with a desolate, steaming expanse of mud and other volcanic flows. The torrential storm waters, flowing down the slopes rapidly excavated channels and rills in the soft tuffs. The rills became progressively gullies, ravines, creeks, and new master streams. These new streams, their erosive power enlarged by the torrential grades, began the canyon-cutting epoch of the late Pliocene and early Quaternary, the results of which may be seen today in the V-shaped trenches excavated in the hard rock to a depth of 1000 to 4000 feet below the surface of the flows. The old Tertiary river beds were exposed in many places and cross-sections of their valleys are now seen on the steep slopes of the canyons high above the river beds. This is vividly illustrated in the above photograph. Long stretches of the old channels remained secure below their blanket of 1000 feet of hardened volcanic mud. Wherever these new, deep-cutting streams destroyed the old channels the gold in those channels became concentrated in the canyons, while thousands of disintegrated quartz veins added to the previous concentrates. Owing to the steep grades of the Quaternary rivers, much of the detrital material and the fine gold was swept out into the valley at the western foot of the range over alternately advancing and retreating flood plains. The bare lava flows and slopes again became covered with vegetation, this time of a type belonging to a cooler but still temperate climate. Later in the Quaternary different developments came. The range became covered with persistent snows which consolidated in the upper valleys and formed glaciers. These filled the valleys for a comparatively brief time only, disappearing rapidly before the drying winds of a warmer or dryer climate, and leaving the summit regions a dreary expanse of bare, polished granite rocks. During the last few thousand years, a relatively brief span of geologic time, the Sierra Nevada has remained as we know see it, with the oak groves in the foothills, the pines on the middle slopes, and the weather-beaten species of the highest ridges. Topography The foothills of the Sierra rise rather abruptly from the plains of the Sacramento and San Joaquin valleys. slope of the foothills, which border the valleys in a series of ridges parallel with the range, is decidedly more pronounced than that of the range as a whole. At an average The |