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* Parr Il. Sror. ii. §6.] MARINE DEPOSITS. 437 
- animals (see p. 461, seg.). The inorganic deposits of the sea-floor are 
(1) chemical and (2) mechanical. 
i. Of Chemical deposits now furming on the sea-floor we know 
as yet very little. At the mouth of the Rhone a crystalline cal- 
eareous deposit accumulates in which the débris of the sea-floor 
is enveloped. As sea-water contains so minute a proportion of 
carbonate of lime and so much larger a proportion of carbon 
dioxide than is needed to keep this carbonate in solution, Bischof 
estimated that no precipitation of carbonate of lime could take place 
from sea-water until after +2 of the water had evaporated.’ It is 
thus evident that no deposit of lime in the open sea is possible from 
concentration of sea-water. But the calcareous formation on the 
sea-bottom opposite rivers like the Rhone may be explained by 
supposing that as the layer of river water floats and thins out over 
the surface of the sea in warm weather with rapid evaporation, its 
comparatively large proportion of carbonate of lime may be partially 
precipitated. It has been observed near Nice, as well as on the African 
coast and other parts of the Mediterranean shores, that the shore rocks 
within reach of the water have a hard varnish-like crust deposited 
upon them. This substance consists essentially of carbonate of lime. 
As it extends over rocks of the most various composition, it is probably 
due to a deposit of lime held in solution in the shore sea-water, and 
rapidly evaporated in pools or while bathing the surface of rocks 
exposed to strong sun-heat.? 
During the researches of the Challenger expedition, important 
facts in the history of marine chemistry have been obtained from 
the abysses of the Atlantic and Pacific oceans. Some of these are 
referred to on pp. 441, 469.. 
ii, The Mechanical deposits of the sea may be grouped into 
subdivisions according as they are directly connected with the waste 
of the land, or have originated at great depths and remote from land, 
when their source is not so obvious. : 
A. Land-derived ov Terrigenous.—These may be conveniently 
grouped according to. their relative places on the sea-bed. 
a. Shore Deposits.—The most conspicuous and familiar are the 
layers of gravel and sand which accumulate between tide-marks. 
As a rule, the coarse materials are thrown up about the upper limit 
of the beach. They seem to remain stationary there; but if watched 
and examined from time to time, they will be found to be continually 
shifted by high tides and storms, so that the bank or bar of shingle 
retains its place though its component pebbles are being constantly 
moved. During gales coincident with high tides, coarse gravel may 
be piled up considerably above the ordinary limit of the waves in 
the form of what.are termed storm-beaches.* Below the limit of coarse 
shingle upon the beach lies the zone of fine gravel, and then that of 

1 Chem. Geol. i. p. 178. 
2 Bull. Soc. Géol. France (3), ii. p. 219, iii. p. 46, vi. p. 84. 
* See Kinahau on Sea-beaches, Proc. Roy. Irish Acad. (2nd, ser:), iii. p. 101, 
