OCR Text |
Show 78 GEOLOGY OF THE HIGH PLATEAUS. alluvial cones, so flat or gently sloped that the conical form is not at first recognized by the eye. Each cone has its apex at the gateway of some mountain gorge, wThile its base is several miles out in the middle of the valley. These cones are so broad and numerous, that they are confluent at their bases and give the general impression of a very gently undulated surface of alluvium covering the entire expanse of the valley. Could we see them in vertical cross-section, we should find them to possess a well-marked stratification agreeing with the stratification of the older conglomerates. A few fortunate exposures have here and there revealed their internal structure, and. a careful comparison leaves little doubt that the valley alluvium and the ancient conglomerates were formed in substantially the same manner and by the same process. If it be true that these conglomerates have been derived from the secular decay of massive eruptive rocks, of which the debris have been carried down the old mountain slopes by running water and stratified in great beds of alluvia, then we may expect to find certain correlated facts, of which the following are examples: (1.) We should expect to find these conglomerates grouped around ancient eruptive centers still preserving remnants of the massive rocks which are presumed to have furnished the material of the conglomerates. (2.) We should also expect to find that these remnants consist of rocks of exactly the same varieties as we find in the fragments of the conglomerates; provided, however, that eruptions from these centers subsequent to the formation of the conglomerates have not completely overflowed and hidden the older outbreaks. (3.) We should expect to find the loftiest portions or crowning summits of the plateaus to consist not of conglomerates, but of massive rocks; unless, indeed, the relative altitudes of the two classes of rocks has been reversed or modified by subsequent upheavals or sinkages. The general idea here conveyed is that the process which formed the conglomerates consisted in the transportation of fragmental matter from high-standing ancient volcanic piles to low-lying plains and valleys around their bases or along their flanks. These relations, I think, are very satisfactorily shown after a careful analysis of the facts. We may still discern the mo*re important ancient eruptive centers with the conglomerates grouped |