286 
GEOGRAPHY: W. M. DAVIS 
development once sets in, it is likely to continue indefinitely, as long as 
the island stands still. The grinding of the detritus upon itself, as it is 
shifted by the waves, would soon reduce it to so fine a texture that it 
would all be swept off the platform into deep water, leaving the platform 
bare; but the loss thus occasioned is continually made good by a new sup- 
ply from the valleys and cliffs of the island; hence the detrital sheet is 
always maintained. The rasping of the detritus on the platform, espe- 
cially at the time of storms, slowly wears the rock surface down to greater 
and greater depth, but the degradation thus caused is probably 20 or 40 
or more times slower than the retrogradation of the cliffs. The 'edge' at 
the outer margin of the abraded platform may be somewhat accentuated 
and stript as it is worn down, because the slow abrasion of its rock sur- 
face may be exceeded by the more rapid abrasion of the first deposited 
detritus next outside of it: here corals and other reef building organisms 
might grow, but for the constant outward passage of detritus which 
would smother them. The important matters to note thus far are 
first, that when the series of normal changes begins around the simple 
shoreline of the completed island, sector K, the shore waves will at once 
have to deal with so abundant a supply of detritus washed down by tor- 
rents, that a continuous shoreHne beach and off-shore detrital sheet will 
be formed at an earlier stage of shoreline development than is usually 
the case on the shores of strongly sloping coasts; second, that the detrital 
sheet, once established, will be thereafter well supplied with waste from 
the deepening valleys and the retreating cliffs, and will thus be main- 
tained indefinitely, as long as the island stands still; and third, that as 
long as the detrital sheet exists, the growth of corals is greatly discour- 
aged if not entirely prevented. 
Now let it be supposed that when a certain stage of cliff recession is 
reached, as in sector N, the island subsides by a succession of starts and 
stops, so that the shoreline crosses the cHffs at mid-height and enters the 
larger valleys, as in sector O. Several significant changes will follow. 
The detritus from the larger valleys of the dissected cone will now be 
pocketed in the valley-mouth embayments; the waves, beating the face 
of the cliffs without the aid of the boulders and cobbles, with which they 
had previous attacked the cliff base so successfully, will do Kttle work in 
the way of cutting the upper half of the cliffs farther back. Under these 
conditions corals and their associates may attach themselves to the cliff 
face and form a fringing reef; as soon as such a reef broadens a little, it 
will hold nearly all the detritus that is washed from the Httle hanging 
valleys and weathered from the non-submerged part of the cHff face : thus 
the detrital sheet on the abraded platform, no longer receiving a renewal 
