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Show 222 GEOLOGY OF THE HIGH PLATEAUS. bare its anatomy. Our surprise is often great at finding the cone wonderfully well stratified, but in a peculiar way. The most perfect stratification is presented when the dissecting cut is made radially. But when a cut transverse to the radius is made by excavations of another stream, the stratification, though still conspicuous, is much less uniform and harmonious. The cone appears to be built up of long radial or sectoral slabs superposed like a series of shingles or thatches. There are marked differences between the cones formed by streams which have their entire descent within unaltered sedimentary strata and those running among volcanic and metamorphic rocks. The fragments resulting from the decay of sandstones, limestones, and shales are much more susceptible to the influence of weathering and are more readily worn-out by the abrasion of travel. Even when they escape destruction by the wear of the torrent and reach a resting-place upon the surface of the cone, the gentler but more insidious action of meteoric forces gradually crumbles them to sand or dissolves them, and they at length disappear. But the compact volcanic and metamorphic rocks are much more durable and do not yield so readily either to mechanical or chemical forces; more of them reach the cones, where they survive long enough to be buried beneath later accumulations and thus receive final protection from dissolution. Hence the cones derived from the waste of sedimentary strata seldom contain much coarse debris, while those from harder rocks are largely composed of it. This difference in texture in turn produces some difference in the proportions of cones. The sedimentary cones are usually very slightly flatter and broader. The difference in this respect is on the whole quite small, but the measurement of a considerable number of both kinds seems to indicate that it really exists. In consequence of the flatness of the cones and their lateral confluence, the general result of their serial aggregation is a long and thick stratum made up of many subordinate folia. In process of time it may also become consolidated and hardened into a rock mass resembling in all essential respects the stratified conglomerates usually reckoned among the members of a stratigraphic series. That distinctions between such a conglomerate and one deposited littorally would be readily detected after close |
