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In General it may be said that the mountain ranges of Idaho are volcanic upheavals, the mighty bending upward of the crust of the earth’s surface when its inborn fires were lashed to unwonted fury in some stormy age of old eternity. The valleys were doubtless formed by this upheaval of its enclosing ranges, leaving the floor of the surface here comparatively undisturbed. This really rests on a foundation of aqueous rock of unmeasured thickness, on which the alluvial matter that forms its soils has been deposited. With this there are, in many places, deep deposits of water-worn pebbles and stratified sand, which were made at an era much more modern than that of the underlying sandstone. It is useless to endeavor to identify these changes chronologically, as creation in its being and in its mutations writes its historic days in millennials of age, and thus puts our conception of time, drawn as it is from human experience and human history, entirely at fault.
Of course, in indicating the forces that formed the now verdant valleys, glacial action must not be forgotten. Far extending moraines and wide glaciated surfaces tell the story of the far-away eras when these mighty ice-plows furrowed and planed down the broken face of the earth’s crust, and smoothed it into its now beauteous vales.
Enough has already been said to indicate to the reader that the Mountains of Idaho are of volcanic formation. The great snow peaks are all volcanoes. They are called extinct, though some of them still give distinct evidence of an internal unrest born of pentup fires. Buffalo Hump has been in active eruption within the memory of the present generation. The great summit intervals between these peaks are generally granitic rock, covered with a deep vegetable soil, intermixed with decayed granite. In fact, there were many successive overflows, as on the broken faces of the cliffs clearly defined lines of stratification are presented more numerous as we approach the great summits that were their fountain. The molten iron sea rolled onward, over-lying the whole country, drinking up the Rivers, shearing off the forests, and seizing a nightly holocaust of animal life in its devouring maw. For ages, how long no one can know, this great lava plain, first red and hot and simmering, then black and cold, and rending itself into deep chasms in its slow cooling, lay out under the stars without vegetable or animal life, almost without springlet or dewdrop, to cool or soften its black and rugged face. The fires of the volcanoes at length burned low. The mountain summits cooled. A few stray clouds floated over the tortured earth. A few drops of rain touched its iron surface with their imprisoned might. Showers followed. The springs that fountain Rivers began to bubble from beneath the cloven lava beds, searching out an open way seaward through their broken chasms. And thus the changes of the ages went on. The basalts were ground to powder in the mills of the streams. The old surfaces over which the lava had once spread were cut into valleys, hundreds of feet deep. Fecund soils were deposited. Vegetation sprang forth again. Animal life found food and drink and shelter, and still the changes went on. Frost and snow and raindrop and stormy winds and burning suns wrought the miracle of a new genesis, leaving a field in which nature has written the most legible and astonishing records of her processes and her powers.
The mountain ranges present a wonderful conglomeration of basalts, granite, slate, sandstone, with vast beds of stratified sand and water-worn gravel. In places one formation predominates, in other places some other formation, and then again several of them appear intermixed, or over-lying one another. It is evident that the heat attending the volcanic action that lifted the vast ridges to their present position was great enough to cause perfect fusion in only a few places; while yet the forces below were mighty enough to cause the wonderful and weird displacements of the primitive rocks so often arresting the observant eye. One hour the traveler among these Mountains will be passing over scoriated basalt, or along cliffs of basaltic columns, the next among great granite boulders or over gray granite pinnacles, then over miles of aqueous deposits in the form of stratified sandstone or stratified beds of sand and gravel intermixed; or again slate slopes and hillsides will arrest his eye, until he is lost in the wilderment of his strange surroundings.
The Blue Mountains margin on the west the great lava plains of Snake River valley. The volcanic conditions, so plainly marked in the Cascade and Blue Mountains, and the valley intervening between them, continue and are intensified as we enter the great upper valley of Snake River, which lies mostly in the state of Idaho, which was once the mightiest scene of volcanic action on the American continent, if not in the world.
We should not dismiss the whole subject of the geology of this most interesting region, with these general statements for the lay reader without some more distinctly scientific record for the benefit of the more technical reader and student. For him geology would write about the following history of the conditions and changes of untold ages and marvelous processes through which this wonderful Idaho world was being formed.
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For an immense period before the existence of the Coast and Cascade ranges of Mountains, the primeval ocean washed the western shores of the great Rock mountain chain, and throughout the palaeozoic era and the whole Triassic and Jurassic periods of the Mesozoic era numerous Rivers kept bringing down debris until an enormously thick mass of off-shore deposits had accumulated. This marginal sea-bottom became the scene of intense aqueousigneous action in its deeply buried strata, producing a line of wrackness, which, yielding to the horizontal thrust produced by the secular contraction of the interior of the earth, was crushed together and swollen upward into the Cascade and Sierra Nevada range at the close of the Jurassic period. The range thus produced was not of very great height. It existed for unknown centuries, the scene of erosion and plant growth, roamed over by the now extinct fauna of the Cretaceous and Tertiary periods. It was combed by forests of conifers and oaks. Then followed the great lava-flow and uplift of the mountain range of the modern Cascades. Beneath the overlying lava, where the Columbia breaks through the barriers of this great range, there is found along the water’s edge, and for nearly twenty feet upward, a coarse conglomerate of rounded porphyritic pebbles and boulders of all sizes up to six feet in diameter, held together by an imperfectly lithified earthy paste. Above the conglomerate is a very distinct, though irregular ground surface bed, in 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 similar 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 three thousand 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 rocks. 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 coarse 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 the 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 outpoured 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 two hundred thousand 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 two thousand feet, and the greatest (shown where the Columbia, Des Chutes, Snake and other Rivers cut through it) four thousand 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 tins 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 away 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 great basins, which continued to exist and be endowed with life well into the Pliocene.