Page:An Illustrated History of the State of Oregon.djvu/23

 which are found silicified stumps with roots extending twenty feet and penetrating into the boulder material beneath evidently in situ. Resting directly on this forest ground-surface, and therefore inclosing the erect stumps, is a layer of stratified sandstone, two or three feet thick, filled with beautiful and perfect impressions of leaves of several kinds of forest trees, possibly of the very trees about whose silicified bases they are found. Above this leaf bearing stratum rests a coarse conglomerate [sic] to that beneath at the water level. Scattered about in the lower part of this upper conglomerate, and in the stratified sandstone, and sometimes lying in the dirt beneath it, fragments of silicified driftwood are found. Above this last conglomerate, and resting upon it, rise the layers of lava, mostly columnar basalt, one above another to a height of 3,000 feet. From these facts the following order of events are deduced: The region of the Columbia river was a forest, probably a valley, overgrown by conifers and oaks. The subsoil was a coarse boulder drift produced by erosion of some older rock. An excess of water came on, either by floods or changes of level, and the trees were killed, their leaves shed and buried in mud, and their trunks. rotted to stumps. Then came on a tumultuous and rapid deposit of course drift, containing driftwood, which covered up the ground and the still remaining stumps to a depth of several hundred feet. The surface thus formed was eroded into hills and dales, and then followed the outburst of lava in successive flows, and the silification of the wood and fermentation of the drift by the percolation of the hot alkaline waters containing silica. Finally followed the process of erosion by which the present streams, channels and valleys, whether main or tributary, are cut to their enormous depth. The great masses of sediment sent down to sea by the erosion of the primary Cascade range, forming a thick offshore deposit, gave rise in turn at the end of the Miocene to the upheaval of the Coast Range, the Cascade mountains being at the same time rent along the axis into enormous fissures from which out poured the grand lava floods, building the mountains higher and covering the country for great distances. This is probably the grandest lava flow known to geology, covering as it does an area of not less than 200,000 square miles. It covers the greater portion of northern California and northwestern Nevada, nearly the whole of Oregon, Washington and Idaho, and runs far into British Columbia on the north. Its average thickness is 2,000 feet and the greatest (shown where the Columbia, Des Chutes, Snake and other rivers cut through it) 4,000 feet. To produce this, many successive flows took place, and great periods of time elapsed during which this volcanic action continued. During the period of these Cascade eruptions, the Coast range was being slowly elevated, and became in turn the scene of local volcanic action, though not very severe. At last the great fissure eruptions drew to a close. The fissures became blocked up. The volcanic action became confined to a few localities. The period of crater eruptions followed. This continued for a long time-almost to our own day. These crater eruptions built up the great snowy peaks.

By the formation of the Cascade a great interior basin was made, the waters of which collected into secondary reservoirs, some of very large extent, and which were at length carried off by the rivers which have cut their way from the interior to the sea. The Columbia and its tributaries drained the northern part of this immense basin, and at this period doubtless the great Salt Lake of Utah found its outlet to the sea by the Snake and Columbia rivers. Thence came the lava floods, whose great flows have since been worn [sic] in places, exposing the tertiary and cretaceous beds, and revealing the former conditions of the region by the fossils found therein. At the end of the Miocene the lava flows from the Cascade fissures commenced, but it was long before they reached the entire extent of the basins of Oregon, which continued to exist and be endowed with life well into the Pliocene.