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 analogv 

 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 cur- 

 rents 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 disturb- 

 ance exists. 



The distribution of the materials. — It is sometimes a little difficult to real- 

 ize 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 con- 

 glomerates, 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 g-iven 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 land- 

 wards, while at the bottom it is seawards. In any case, however, the gen- 

 eral trend is oblique, with reference to any given portion of a coast, and 

 never, or at least veiy 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 



