The forces within the earth became active again. Apparently Nature did not intend that the gold should be forever buried, or that the country should always appear so uninteresting. Internal forces raised the mountain block for a second time, tilting it still more to the westward. Volcanoes broke forth along the summit of the range near the line of fracture, and floods of lava and volcanic mud ran down the slopes, completely filling the broad valleys of the northern Sierras and burying a great part of the gold-bearing gravel. The eruptions turned the streams from their channels, but on the steeper slope of the mountains the rivers went energetically to work making new beds. They cut down through the lava and the buried gravel until they finally reached the solid rock underneath. Into this rock, which we call "bed-rock," they have now worn canons two thousand feet deep. The beds of gravel that lay under the old streams frequently form the tops of the hills between these deep canons. Here they are easily accessible to the miners, who by tunnels or surface workings have taken out many millions of dollars' worth of gold. The important canons of the northern Sierras, where the gold is found, have been made by the American and Feather rivers. Farther south are the deeper and more rugged canons of the Tuolumne, Merced, King's, and Kern rivers, which open to us inviting pathways into the mountains. It might be supposed that the mantle of snow and ice which at that time covered most of the surface of the earth would have protected it from further erosion, but this was not the case. In the basin at the head of each stream the snow accumulated year after year until it was more than a thousand feet deep. Under the influence of the warm days and cold nights the snow slowly turned to ice, and moved by its own weight, crept down into the canons. The solid rock walls were ground and polished, and even now, so long a time after the glaciers have melted, some of these polished surfaces still glisten in the sunlight. The glaciers deepened and enlarged the canons, but running water was the most important agent in their making. Upon the disappearance of the glaciers, the streams went to work again deepening their canons. From their starting-points, under the lofty crags, they first ran through broad upland valleys, then tumbled into the canons; but until they had reached the lower mountain slopes, to which the glaciers had not extended, they passed through a dreary and desolate region devoid of almost every sign of life. The glaciers had swept away all the loose rock and soil, and it was many long years before the surface again crumbled so that forest trees could spread over it once more.