WATER AS A MECHANICAL AGENT. 229 



be sure to form in favorable places. On the emergence, these deposits 

 remain to mark progress. Beach-like deposits are readily made by rivers- 

 and on lake-shores. 



Work in the Ocean's Abyssal Depths. 



The bottom of the ocean, down to about 15,000 feet, has its abundant 

 life, and besides is ever receiving relics in great profusion from the pelagic 

 life of the waters, and thus it may over large portions be making limestones 

 and flint-beds ; but it is poor in other geological work. It feels the move- 

 ment of the tidal wave, and also that of the polar flow toward the equator, 

 each under the ocean's heavy pressure. But these are infinitesimal sources 

 of force, and have, therefore, no sensible, mechanical effects, either in the 

 way of transportation or abrasion. The near convergence of ridges that 

 could bring the waters passing between them into a working condition does 

 not exist. 



There are hence no means of producing a stratified or bedded structure 

 in the abyssal deposits, excepting earthquake vibrations, the results of which 

 would be local, and variations, with the passing ages, in the pelagic or abyssal 

 life of the waters, causing variations in the showers of Diatoms or of shells 

 of Ehizopods, or in the growth of Sponges and other species over the bottom. 

 The wide-spread contributions of volcanic ashes from volcanoes, especially 

 the oceanic, drop to the bottom and rest there, undergoing only such chemi- 

 cal changes as may go on at the temperature. 



Tidal or current scour is limited to relatively shallow depths or unusual 

 conditions. Mellard Reade mentions cases of probable tidal scour at bottom 

 in channels between islands on the coast of Scotland. But the depths 

 do not exceed 800 feet. He also reports (1885) that, according to Sir 

 James Anderson, the undercurrent out of the Mediterranean near Gibraltar 

 moves the water to its bottom, and that at 500 fathoms the wire of the 

 electric cable was ground like the edge of a razor, so that they had to aban- 

 don it and lay a new cable well inshore. This is confirmed by Captain 

 Nares, who reports that he could get no specimen of the bottom probably 

 because of a " perfect swirl at that depth." 



The great oceanic currents carry on little transportation and corrosion of 

 detritus, on account of their distance from the land. The Labrador current, 

 with its westward tendency (page 46), acting against the submerged border 

 of the continent, may have produced some results in past time, if not doing 

 so now. But its chief geological work has been the transportation of ice- 

 bergs, and that has not yet ceased. It has been supposed that the course of 

 the steep outer slope of the submerged Atlantic border has been determined 

 by the oceanic currents ; but it is far more probable that the position of the 

 slope has directed the courses of the currents. The Gulf Stream along the 

 Florida Straits and toward Cape Hatteras has a velocity sufficient for abrad- 

 ing action ; but the stream does not carry its surface velocity to the bottom. 



