248 
THE PHYSICAL GEOGRAPHY OF THE PAST. 
the carrying power of water is dependent upon its velocity, 
it is not surprising that we find a sorting action going on ; 
that whilst the coarser sediment is deposited near the coast 
the finer material, under the combined influence of the out¬ 
flow of rivers, and wave and tidal action of the sea, is carried 
out to a far greater distance from land before it is deposited 
on the more or less shelving bottom. 
If we could make a horizontal section at right angles to 
the shore line of any large body of water, fed by running 
streams, we should find, as a general rule, a belt of coarse, 
roughly stratified shingle, giving place gradually to a less 
coarse and more sandy sediment, and this again graduating 
further from shore into beds of fine mud, which mav extend 
for a very considerable distance. From the somewhat inter¬ 
mittent character of the streams and currents we should not 
expect anything like a sharp or invariable line dividing off 
these various sediments horizontally, but we should observe 
them dove-tailing, as it were, into each other laterally. 
It is evident that the beds of fine mud forming the very 
outermost fringe of the land must come to an end somewhere, 
for, given a sufficient time, the very finest sediment will fall 
to the bottom. 
As a rule, except in shallow seas and opposite the mouths 
of great rivers, the very finest mud deposits do not extend 
more than 100 miles from land. Farther out than this in 
deep water soundings have shown the bottom to consist of a 
widely spread deposit of a white sticky ooze , which, when 
dried resembles chalk in appearance and also in composition.* 
This is a veritable limestone now in course of deposition, and 
is the product of minute specks of living jelly, which abstract 
the carbonate of lime from sea water wherewith to form their 
shells, which, after the organisms are dead, are showered 
down upon the sea bottom. These microscopically minute 
animals are known as Foraminifera, and many of the lime¬ 
stones known to geologists have been built up almost entirely 
by their agency. They are, however, by no means the only 
limestone builders. Coral Polyps play a most important 
part in the production of modern limestones, and that they 
have played as important a part as far back as Devonian 
and Carboniferous times is equally certain. Then again we 
find some limestones made up almost entirely of the remains 
of Encrinites or sea-lilies, and of the shells of molluscs. 
Muddy water is absolutely inimical to the life and growth of 
these limestone builders, and we may be quite sure when we 
* In the abyssal depths of the ocean this calcareous ooze is replaced 
by a red clay, the origin of which is at present unknown. 
