In the late Tertiary period a chapter of very remarkable events occurred. For a second time the evenly laid beds of the sea-bottom were crumpled by the shrinking of the earth. The ocean flowed back into deeper and narrower limits, and, fronting the Sierra Nevada, appeared the present system of Coast Ranges. The intermediate depression, or sea-trough as I like to call it, is the valley of California, and is therefore a more recent continental feature than the Sierra Nevada. At once then from the folded rocks of the Coast Ranges, from the Sierra summits and the inland plateaus, and from numberless vents caused by the fierce dynamical action, there poured out a general deluge of melted rock. From the bottom of the sea sprang up those fountains of lava whose cooled material forms many of the islands of the Pacific, and all along the coast of America, like a system of answering beacons, blazed up volcanic chimneys. The rent mountains glowed with outpourings of molten stone. Sheets of lava poured down the slopes of the Sierra, covering an immense proportion of its surface, only the high granite and metamorphic peaks reaching above the deluge. Rivers and lakes floated up in a cloud of steam and were gone forever. The misty sky of these volcanic days glowed with innumerable lurid reflections, and at intervals along the crest of the range great cones arose, blackening the sky with their plumes of mineral smoke. At length, having exhausted themselves, the volcanoes burned lower and lower, and at last by far the greater number went out altogether. With a tendency to extremes which “development” geologists would hesitate to admit, nature passed under the dominion of ice and snow.
The vast amount of ocean water which had been vaporized floated over the land, condensed upon hill-tops, chilled the lavas, and finally buried beneath an icy covering all the higher parts of the mountain system. According to well-known laws, the overburdened summits unloaded themselves by a system of glaciers. The whole Sierra crest was one pile of snow, from whose base crawled out the ice-rivers, wearing their bodies into the rock, sculpturing as they went the forms of valleys, and brightening the surface of their tracks by the friction of stones and sand which were bedded, armor-like, in their nether surface. Having made their way down the slope of the Sierra, they met a lowland temperature of sufficient warmth to arrest and waste them. At last, from causes which are too intricate to be discussed at present, they shrank slowly back into the higher summit fastnesses, and there gradually perished, leaving only a crest of snow. The ice melted, and upon the whole plateau, little by little, a thin layer of soil accumulated, and, replacing the snow, there sprang up a forest of pines, whose shadows fall pleasantly to-day over rocks which were once torrents of lava and across the burnished pathways of ice. Rivers, pure and sparkling, thread the bottom of these gigantic glacier valleys. The volcanoes are extinct, and the whole theatre of this impressive geological drama is now the most glorious and beautiful region of America.
