WATER. 659 



ing great transporting effects lies in its incessant action. The waves 

 thus beat back the detritus thrown out by rivers, and cause them 

 to be deposited mainly over the bottom in the shallower waters, and 

 against the shores, and so prevent their being lost to the land by 

 sinking in the depths of the ocean. 



In the passage of the great wave of the eagre on the Tsien-tang (p. 653), the 

 boats floating in the middle of the stream rise and fall on the tumultuous 

 waters, but are carried only a very short distance forward. Yet, along the sides 

 of the river, the wave tears away the banks, and in places sends a deluging flood 

 over the shores, a true tidal current, which devastates the country. 



It follows, from the facts stated, that no continent can contribute 

 to the detrital accumulations of another continent except through 

 the aid of icebergs. Had there formerly existed a continent in the 

 midst of the present North Atlantic, America would have received 

 from it little or no rock-material. The tides and waves, and tidal 

 and wave currents, all work shoreward. 



3. Distribution of material, and the formation of marine and 

 fluvio-marine deposits. 



1. Oceanic Formations. 



The deposits of oceanic currents consist only of fine detritus : no 

 conglomerates or coarse sandstones can, therefore, be made from 

 them. The Gulf Stream has little power in making such deposits, 

 as it carries along scarcely any detritus. The bottom of the Atlan- 

 tic between Ireland and Newfoundland consists almost solely of 

 the shells of microscopic organisms (p. 612). 



By means of icebergs, the currents of the ocean may distribute 

 widely the coarsest of rock-material; but nearly all the icebergs 

 of the North Atlantic drop their loads of gravel and stone in the 

 vicinity of the American continent, and not in mid-ocean. The 

 deposits made by icebergs consist of gravel, sand, and stones of all 

 sizes, up to many tons in weight, promiscuously mingled, without 

 stratification. They are thus unlike all the rock-formations over 

 the continent preceding the period of the Post-tertiary. 



Mr. Babbage has shown that, taking four kinds of detritus, of 

 such a size, shape, and density that they would sink — the first kind 10 

 feet an hour, the second 8, the third 6, the fourth 4, then if a stream 

 containing this detritus were 100 feet deep at mouth, and entered 

 a sea having a uniform depth of 1000 feet, and a rate of motion of 

 two miles an hour, the first kind would be carried 180 miles before 



