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Show 218 GEOLOGY OF THE HIGH PLATEAUS. fragments, shingle, and gravel. The latter may build up a conglomerate at its outlet; the former never. The action of the sea upon coarse materials has a very close analogy to that of rivers. Currents are generated by the tides and winds along coasts. The surface-waters are rolled in waves upon the shore and flow outwards along the bottom. But their directions are frequently vacillating, trending both ways along the coast with varying obliquity. These currents are usually fast enough to move gravel, shingle, and pebbles as large as those ordinarily seen in marine conglomerates, and may transport them several miles. The general effect of the agitation produced in littoral waters by tides and winds is to seize upon the loose materials of the shore within reach and distribute them over the bottom with an approach to uniformity, and this distributive action prevails wherever the influence of that disturbance exists. The distribution of the materials.â€"It is sometimes a little difficult to realize the agency which has, in the stratification of conglomerates, scattered the fragments over considerable areas and arranged them harmoniously in beds. The stratification of conglomerates is often as conspicuous as that of finer strata, though in general it is less so In the case of marine conglomerates, which are usually formed in the vicinity of the shores, and at no great distance from the sources of their materials, the problem is not difficult. Currents of no mean intensity are perpetually generated along the bottom, near the coast, by tides and the outward flow of water, which has been blown landwards at the surface by winds. These currents, though having at any given locality an average direction, in the long run are never constant in direction from hour to hour, nor from day to day, but sweep hither and thither. But the average flow at the surface is generally landwards, while at the bottom it is seawards. In any case, however, the general trend is oblique, with reference to any given portion of a coast, and never, or at least very seldom, normal to it. These vacillating movements are highly conducive to a harmonious and definite arrangement of the materials upon which the currents act, ever tending to sift and to sort them, and finally to stratify them. The power of these currents to transport is perhaps greater than we are apt to imagine. The drift of sand along coasts |
