
earns 2 Ree 
ies 
a 
Parr Il. Secr. ii. § 6] MARINE TRANSPORT. 435 
mud of different degrees of fineness, were discharged into a sea 
having a uniform depth of 1000 feet over a great extent, four 
varieties of silt falling respectively through 10, 8, 5, and 4 feet of 
water per hour would be distributed as in the following table. 




| | 
: ; . Greatest distance 
Velocity of fall | Nearest distance of , : 
No. per hour. deposit to river. Length of deposit. | of bee Se: from 
feet miles. miles mniles 
1 10 180 20 200 
2 8 225 25 250 
3 5 360 40 400 
4 4 450 50 500 

It must be borne in mind, however, that mechanical sediment 
sinks faster in salt than in fresh water. The fine mud in the iayer 
of river water which floats for a time on the salter and heavier sea- 
water begins to sink more rapidly as soon as the two waters com- 
mingle. 
Near the land, where the movements of the water are active, much 
coarse detritus is transported along shore or swept farther out to sea. 
A prevalent wind, by creating a current in a given direction, or a 
strong tidal current setting along a coast-line, will cause the shingle 
to travel coastwise, the stones getting more and more rounded and 
reduced in size as they recede from the sources. The Chesil Bank, 
which runs as a natural breakwater 16 miles long connecting the 
Isle of Portland with the mainland of Dorsetshire, consists of rounded 
shingle which is constantly being driven westwards. On the Moray 
_ Firth the reefs of quartz-roek about Cullen furnish abundance of 
shingle, which, urged by successive easterly gales, moves westwards 
along the coast for more than fifteen miles. The coarser sediment 
probably seldom goes much beyond the littoral zone. From a depth 
- of even 600 fathoms in the North Atlantic between the Faroe 
Islands and Scotland small pebbles of volcanic and other rocks are 
dredged up which may have been carried by an Arctic under-current 
from the north. But recently Mr. Murray and Captain Tizzard have 
brought up large blocks of rounded shingle from the bank (3800 
fathoms) between Scotland and Faroe. This coarse detritus can 
hardly be due to any present action of the sea, for at such depths 
the force of currents at the bottom must be too feeble to push along 
coarse shingle. It may be glacial detritus dating back to the Glacial 
Period. Much fine sediment is carried in suspension by the sea for 
long distances from land. The Amazon pours so much silt into 
the sea as to discolour it for several hundred miles. After wet 
weather the sea around the shores of the British Islands is 
sometimes made turbid by the quantity of mud washed by rain 
1 Q. J. Geol. Soe. xii. 368. 
2 For a suggested explanation of this fact sce Ramsay, Q. J. Geol. Soc. xxxu. p. 129. 
AF 2 
