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Show 214 GEOLOGY OF THE HIGH PLATEAUS. on the east and the Tushar on the west. Farther northward to the Juab Valley a similar relation prevails. So far is the entire trough of the Sevier, except at the barriers, from being due to erosion, that its floor has been built up by the growth of alluvial formations of considerable magnitude. They are of special interest because of the light they throw upon an interesting problem in dynamical geology. THE FORMATION OF CONGLOMERATES. There are several kinds of conglomerate, formed by processes which, though they may have some features in common, are on the whole strikingly different. Glacial drift, though it undoubtedly falls within the usual conception of a conglomerate, has an origin wholly different from that of a littoral or alluvial conglomerate. Yet in respect to the source from which its materials are derivedâ€"the disintegration of the harder rocks by water and frostâ€"the distinction is not well marked. The great difference is in the methods and agents of transportation and final distribution. Alluvial conglomerates agree with the littoral in having the same origin for their materials, and the same transporting agent, moving water, but the two differ in respect to the conditions under which the transporting power is exercised and the materials distributed. Thus these three kinds have something in common and each has some features peculiar to itself. Sources of materials.â€"The stones and pebbles included in these formations are derived from the break-up of the hardest classes of rocks, which are usually metamorphic or volcanic. Ordinary sandstones, limestones, and clays, and shaly rocks in general seldom contribute to the mass of fragments found in conglomerates. Attrition, weathering, and solution utterly destroy them before they reach a resting-place. A few remnants of rock not usually reckoned as metamorphic nor volcanic are sometimes inclosed, but they come from sedimentary strata as hard and enduring as the others, and such strata are rare. Hard masses, originally contained in softer beds, are sometimes found, but they owe their preservation to their excessive durability, such as the flints of chalk, the chert, and many forms of amorphous silica occurring in limestones. The localities from which the stones come are no doubt very near those where they are |
