‘278 
THE VOYAGE OF H.M.S. CHALLENGER. 
lu tropical and temperate regions there is likewise a much greater accumulation of 
carbonate of lime remains on the bottom than at like depths towards the polar areas, 
where the surface waters have a low temperature throughout the year. At the present 
time, then, it is evident that there is a decided tendency for carbonate of lime deposits to 
accumulate towards the equatorial regions of the ocean. In the central parts of the 
equatorial regions of the ocean basins this carbonate of lime is almost exclusively derived 
from the shells and skeletons of pelagic organisms whose habitat is in the warm surface 
and subsurface waters. That these pelagic shells should be abundant on the bottom in 
tro})ical regions at nearly all moderate depths, and wholly or almost wholly absent from 
the deposits in all the greater depths, has been regarded as one of the most remarkable 
facts brought to light by the Challenger investigations. This fact, however, admits of a 
ready explanation, if it be remembered that all these shells are subject to solution imme- 
diately on the death of the organisms, that only a small number of them — the more 
delicate ones — are wholly removed in falling through a moderate depth of water, while a 
very large proportion are wholly dissolved in falling through a depth of four or five miles. 
Mr. Murray made a large number of experiments during the expedition with the view 
of ascertaining the rate of fall of pelagic organisms in sea-water. The experiments were 
conducted in a long glass cylinder, and the rate was found to vary greatly according to 
the shape of the shell and the albuminoid matter associated with it. According to the 
results of these experiments it would take from three to six days for the shells to reach a 
de})th of 2500 fathoms. In the deeper layers the rate of fall would probably be much 
slower than in the surface layers, owing to the shells being less compressible than water. ^ 
It has also been shown that solution of carbonate of lime shells takes place more rapidly 
under pressure.'* In this dissolution of the carbonate of lime shells the reaction referred 
to on pages 255 and 25G appears to play an important role. Besides it must be remembered 
that in the greater depths of the ocean, those shells which may reach the bottom are not 
covered up so rapidly l>y other shells falling from the surface, as they undoubtedly are 
in the shallower de})tlis, where large numbers reach the bottom, and there accumulate. 
'I’he practically motionless water in contact with the large quantity of carbonate of lime 
in moderate de 2 )ths would in addition soon become saturated, and consequently be 
unable to take up more carbonate of lime, for sea-water can only take up a relatively 
small quantity of carbonate of lime in addition to what it normally contains. The water 
in contact with the deeper deposits, in which there is but little carbonate of lime, would 
not become thus .saturated. These considerations also ex})lain why the whole of the 
carbonate of lime shells are removed from the deposits at lesser depths in extra-tropical 
regions, where there are fewer living calcareous organisms at the surface, than in the tropics 
beneath the warm oceanic currents, where the surface shells are much more abundant.® 
• Murriiy and Irvine, I’ror. Roy Hoc. lulin., vol. xvii. p. 98. 
• Iteid, I'roc. Roy. Hoc. Kdin., vol, xv. pp. 151-157, 1888. ’ Murray and Irvine, Proc. Roy. Soc. Edin., vol. xvii. p. 97. 
