THE OCEAN. 677 



file a cliff on the coast of New South Wales, near Port Jackson), the 

 horizontal strata of the foot of the cliff extend out in a platform, a 

 hundred yards beyond the cliff. The tide rises on the platform ; and 



Fig. 1096. Fig. 1007. 



Cliff, New South Wales. » The Old Hat," New Zealand. 



the waves, unable to reach its rocks to tear them up, drive on to bat- 

 ter the lower part of the cliff. At the Bay of Islands, New Zealand, 

 the rocks have no horizontal stratification ; and still there is the same 

 seashore platform; and an island in the bay (Fig. 1097) is called 

 M The Old Hat." The seashore platform of coral islands has the same 

 origin. The stability of sand-flats in the face of the sea is owing to 

 this cause. In seas of high tides and frequent storms, the platform is 

 narrow or wanting, owing to the tearing action of the heavier waves. 



2. Transportation and Deposition. 



1. Characteristics of Sea-water. — Owing to the greater density of 

 sea-water, stones immersed in it lose l-40th more of their weight than 

 when in fresh water, and thus in the transportation of the same load 

 the former would have less weight to carry. But, on the other hand, 

 sediment falls in salt water in a fifteenth of the time it does in fresh, 

 as first shown by Mr. W. M. Sidell, salt water having less cohesion 

 than fresh ; so that the transporting power is actually less. 



Mr. Sidell, while studying the rate of deposition of the Mississippi at its mouth, ex- 

 perimented with solutions of common salt, epsom salt, and alum, as well as sea-water, 

 and found that in river water the sediment took 10 to 14 days to settle, while the same, 

 in saline solutions, settled in 14 to 18 hours. (Appendix A, Humphreys' & Abbot's 

 Mississippi Report.) E. W. Hilgard observes that clay immediately settles if any con- 

 siderable amount of lime or other neutral salt is present in solution; yet, should the 

 water be alkaline, it will remain suspended indefinitel\ r . He also states that the "set- 

 tling" effect of alum appears to be mainly due to the precipitation of alumina by the 

 calcium and magnesium carbonates which are present. 



This is an important means of preventing the detritus of rivers from 

 being carried into and wasted in the deep waters. Mr. Sidell thus ex- 

 plains the fact that the bars are formed where the saltwater is first met. 

 Moreover, since the tidal wave is a translation wave, and appreciably 

 so in soundings, its action and that of wind-waves is to beat back the 

 detritus contributed to the ocean by rivers and hasten its deposition. 

 From these facts it is plain that no continent can contribute to the 

 detrital accumulations of another continent, except through the aid of 



