
: ‘ 5 ee phe pe ro, 
7 3 r a - = - Ars . x * ef 
a 7 oe 3 = , oh ee ae 
\ \ : ; _ 
438 ‘DYNAMICAL GEOLOGY. [Boo TIL. _ 
sand, the sediment, though liable to irregular distribution, yet 
tending to arrange itself according to coarseness and specific gravity, 
the rougher and heavier detritus lying at the upper, and the finer 
and lighter towards the lower edge of the shore. The nature of 
the littoral accumulations on any given part of a coast-line must 
depend either upon the character of the shore-rocks which at that 
locality are broken up by the waves, or upon the set of the 
shore-currents, and the kind of detritus they bear with them. Coasts 
exposed to heavy surf, especially where of a rocky character, are apt 
to present beaches of coarse shingle between their projecting pro- 
montories. Sheltered bays, on the other hand, where wave-action is 
comparatively feeble, afford a gathering ground for finer sediment 
such as sand and mud. Estuaries and inlets into which rivers enter 
frequently show wide muddy flats at low water. Deposits of com- 
minuted shells, coral-sand, or other calcareous organic remains 
thrown up on shore, may be cemented into compact rock by the 
solution and redeposit of carbonate of lime (p. 324). Where tidal 
currents sweep along a coast yielding much detritus, long bars or 
shoals may form parallel with the shore. On these the shingle and 
sand are driven coastwise in the direction of the prevalent current.’ 
8. Infra-Littoral and Deeper-Water Deposits.—These extend 
from below low-water mark to a depth of sometimes as much as 2000 
fathoms, and reach a distance from land varying up to 200 miles or 
even more. Near land, and in comparatively shallow water, they 
consist of banks or sheets of sand, more rarely mixed with gravel. 
The bottom of the North Sea, for example, which between Britain 
and the continent of Europe les at a depth never reaching 100 
fathoms, is irregularly marked by long ridges of sand enclosing here 
and there hollows where mud has been deposited. In the English 
Channel large banks of gravel extend through the Straits of Dover 
as far as the entrance to the North Sea. ‘These features seem to 
indicate the line of the chief mud-bearing streams from the land, — 
and the general disposition of currents and eddies in the sea which 
covers that region, the gravel ridges marking the tracts or junctions 
of the more rapidly moving currents, while the muddy hollows point 
to the eddies where the fine sediment is permitted to settle on the 
bottom. It is possible, however, that the inequalities on the floor of 
the North Sea, and their peculiarities of sediment, are not wholly 
modern, but may be partly due to irregular deposition of glacial drift 
and partly to the contour of the ground before it was submerged and 
the land connection between Britain and Europe was destroyed. 
During the course of the voyage of the Challenger, the 
approach to land could always be foretold from the character of the 
bottom, even at distances of 150 and 200 miles. ‘The deposits 
were found to consist of blue and green muds derived from the 
degradation of older crystalline rocks. At depths of 100 to 700 
1 See Bristow and Whitaker on Chesil Bank, Dorset, Geol. Mag. (4869), vi. p. £33; 
Kinahan, Geol. Mag. (Decade 2.) (1874), i. 

