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Show ALLUVIAL CONGLOMERATES. 223 inspection of favorable exposures we may well believe; yet it is highly probable that the two kinds would be confounded on a hasty examination, and the distinction would be difficult to verify even by careful study, unless the exposures were extensive and conspicuous enough to display very fully and clearly their respective characters. These doubts generally would prevail in those cases where a decision would have to turn only upon the intimate structures of the deposits. Collateral circumstances, however, may often decide the question. Throughout the volcanic portions of the District of the High Plateaus ^he conglomerates are present in prodigious masses. They constitute a large proportion of the rock masses of the plateaus, and form many miles of escarpment more than a thousandâ€"sometimes more than 2,000â€"feet in thickness. In the central and southern portions of the plateaus they cannot fall much short of one-half of the masses now open to observation, and taking the volcanic portion of the entire district, a rough estimate would place their volume at least at a third of the whole eruptive material. They are well stratified, and though the distinctness of the bedding is somewhat variable, the stratification never becomes obscure. Indeed, on the whole, these conglomerates seem to be about as well stratified as the average of those which are attributed to sub-aqueous deposition. The individual beds are not so thick and massive and show partings more frequently or at shorter intervals. • The occurrence of large stratified accumulations of pyroclastic materials in regions or districts which have been the theaters of protracted volcanic activity is a fact of common observation. They abound throughout the State of Colorado and along the more or less volcanic ranges of Northern Wyoming, Montana, and Idaho. They excited the admiration of Scrope in Central France, and are conspicuous in Sicily and around Vesuvius. Indeed, every volcanic region will doubtless be found to display them to a greater or less extent. Where large bodies of water wash the flanks of volcanic mountains and ranges we may expect to find large bodies of sub-aqueous conglomerate formed from their debris. Volcanic tuffs are formed by the mechanical projection of dust, ash, rapilli, and small fragments from vents blowing out gases and steam, and falling |
