6Q6 DYNAMICAL GEOLOGY. 



the waters first retreat to an unwonted distance, and then advance in 

 their might, striking deep, and tearing up strata that at other times are 

 under the protection of the waters. 



In the wave-movement on soundings, and not close in-shore, the pro- 

 pulsion of each wave is very small ; and its power of accomplishing 

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

 beat back the detritus thrown out by rivers, and cause them to be de- 

 posited 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. 661), the boats float- 

 ing 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 at times sends a deluging flood over the shores. (Am. Jour. Sci., 

 II. xx. 305.) 



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 cpntinent 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. 



Since oceanic currents can transport only the finest detritus, the depo- 

 sitions from them can be of no other kind ; 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 Atlantic, between Ireland and Newfound- 

 land, consists almost solely of the shells of microscopic organisms (p. 

 615) ; and in the deeper waters, 3,000 fathoms down, as examined by 

 Wyville Thomson, there is a red ooze, with little life and no sand. 



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 the ordinary rock-formations over the continent. 



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

 a size, shape, and density that.thej r would sink — the first kind 10 feet 

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



