A CHAPTER IN THE PHYSICAL GEOGRAPHY OF THE PAST. 49 
the sediments forming the various portions of this huge pile of 
Carboniferous rocks were originally deposited. But before we 
attempt to do this let us turn for a moment to what is going on at 
the present day around our coasts, and see if we cannot deduce 
from our observations some guiding principles with regard to the 
phenomena of sedimentation, which may help us in our enquiry. 
We find that the material which is constantly being brought 
down by streams and rivers, and which has of course been 
derived from the degradation and waste of the land, is deposited 
on the bottom of the sea when the velocity of the currents 
bringing it down has been sufficiently checked. The particles 
thus carried down to the ocean vary in size from large rounded 
pebbles to the finest possible mud ; and, since 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 sedi- 
ment is deposited near the coast the finer material, under the 
combined influence of the outflow 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 may extend for a very considerable distance. 
From the somewhat intermittent 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 outer- 
most fringe of the land must come to an end somewhere, for, 
given a sufficient time, the very finest sediment will fall to the 
bottom. 
’ Asatule, except in shallow seas and opposite the mouths of 
great rivers, the very finest mud deposits do not extend more than 
