292 NATURAL SCIENCE. April. 



These mechanical products of denudation are sifted out and 

 arranged by the ocean waves, tides and currents, in the order of their 

 sizes and specific gravities, so that the boulder-beds are mostly beach 

 deposits, while the pebbles, gravel, grains of sand, and finely-triturated 

 material are carried out further and further from the coast and into 

 deeper water in the inverse order of the size of the particles. 



It is well known that the thickest deposits accumulate nearest to 

 the land-masses ; I do not indeed think that any fixed line or margin can 

 be dravv^n, within which limit deposits may be considered terrigenous 

 and outside oceanic, but it is a general principle to be kept in mind. 

 When coast waters are shallow and what is called the continental 

 sub-aqueous plateau extends far out from land, then these mechanical 

 sediments will under ordinary circumstances extend the furthest.^ 

 But in the extremely interesting " Reports on the Dredging operations 

 off the west coast of Central America to the Galapagos, to the west 

 coast of Mexico and the Gulf of California," ^ by the U.S. Fish 

 Commission steamer " Albatross," Alex. Agassiz says, " I was struck 

 while trawling on our second line between the Galapagos and 

 Acapulco to observe the great distance from shore to which true 

 terrigenous deposits were carried. There was not a station there 

 occupied of which the bottom could be characterised as strictly 

 oceanic." .... "A very fine mud was the characteristic bottom 

 we brought up, often very sticky, and enough of it usually remained 

 on the trawl even when coming up from depths of over 2,000 fathoms 

 materially to interfere with the assorting of the specimens contained 

 in our trawls"; along with the mud, logs, branches of trees, twigs, and 

 decayed vegetable matter usually came up. The distance of the 

 Galapagos Islands from the nearest land in South America is between 

 500 and 600 geographical miles, while the line between Acapulco and 

 the Galapagos is about 1,100 miles. 



The 2,000-fathom line often comes within ion miles of the coast. 

 Doubtless opposite to the mouths of great rivers, such as the Amazons 

 and the Congo, terrigenous deposits will have a wide extension over 

 the ocean floor. The Indus and Ganges spread their deposits over 

 700,000 and 900,000 square miles respectively. (" Conditions of 

 Sedimentary Deposition," p. 500.) 



There is, however, no doubt that the matters in solution have a 

 much wider extension, only limited, in fact, by the area of the ocean 

 itself. These matters are removed from the water mostly by organic 

 agencies and consist, in the largest proportion, of carbonates and sul- 

 phates of lime. Whether any direct precipitation of the matters in 

 solution takes place in the ocean bottom is a subject of surmise, but 



1 See " Conditions of Sedimentary Deposition," by Bailey Willis, Journal of 

 Geology, vol. i., no. 5, p. 498. 



2 Bulletin of the Museum of Comparative Zoology at Harvard College, 

 vol. xxiii., 1S92. 



