EEPORT ON THE DEEP-SEA DEPOSITS. 
185 
Notwithstanding some exceptions, due to special conditions, as, for instance, on deep 
ridges between oceanic islands, where gravelly deposits are found, or in bays, fjords, and 
enclosed seas, where mud is met with in shallow water, it may be said that, along all 
coasts situated in or fronting the great oceans, 100 fathoms is the average depth at 
which fine mud or ooze commences to form. At about this depth the deposits on the 
whole assume a greater uniformity of composition and grain, and the signs of mechanical 
action tend to diminish or completely disappear. The greater the extent and depth 
of the ocean, the greater the depth to which wave-movement extends, and consequently 
the greater is the depth at which the mud-line is formed around the coasts,^ but the 
average depth of this mud-line may be taken as approximately about 100 fathoms. 
Not only is the 100-fathom line important as a dividing line between Deep-Sea and 
Shallow-Water Deposits, but in the physical relief of the globe it appears to mark the 
outer and upper limits of the continental masses, all within that line (the continental 
shelf) belonging to the continental plateaus, while beyond the 100-fathom line there 
is a relatively rapid descent of the sea-floor to the level of the depressed regions of the 
oceanic basins. This is shown by the fact that, while the area of tfie ocean between the 
shore-line and a depth of 100 fathoms is estimated at over ten millions of square miles, 
the area between the 100-fathom line and the 500-fathom line — in other words, the area 
of the ocean’s bed taken in by a descent of the next 400 fathoms — is estimated at only 
about seven millions of square miles. 
Marine Deposits as a whole may be arranged, from the point of view of their relative 
geographical and bathymetrical position, into three groups, viz., (l) Deep-Sea Deposits, 
formed beyond the 100-fathom line; (2) Shallow-Water Deposits, formed between the 
100-fathom line and low-w^ater mark ; and (3) Littoral Deposits, formed in the space 
between high and low water marks. From the point of view of their composition, as 
well as of their geographical and bathymetrical position. Marine Deposits may be separ- 
ated into two great divisions, viz., (I.) Pelagic Deposits — those formed towards the 
centres of the great oceans, and made up chiefly of the remains of pelagic organisms along 
with the ultimate products arising from the decomposition of rocks and minerals ; and 
(II.) Terrigenous Deposits — those formed close to continental and other lands, and 
largely made up of transported materials immediately derived from the disintegration of 
the land masses. 
The relations of these large groups to each other, and their subdivisions, are exhibited 
in the following scheme, which is the first attempt at a systematic classification of Marine 
Deposits as a whole.^ 
1 Stevenson, “ On the Destructive Effects of the Waves of the Sea on the North-East Shores of Scotland,” Proc. Eoy. 
Soc. Edin., vol. iv. pp. 200, 201, 1859. 
2 A very large number of names have been given to deposits in the littoral and shallow-water zones by geologists, 
physical geographers, and marine surveyors, viz., muds, oozes, sands, boulders, gravels, with various qualifying 
words indicating their colour, physical aspect, or composition, such as blue, red, yellow, black, soft, coarse, angular, 
(deep-sea deposits chall. exp. — 1890.) 24 
