1053 
Fic, 4.—Diagrams to illustrate the effect of rain erosion on an island when all the eroded 
material is deposited about the shore. The black portions represent deposition. The 
dotted lines represent the original surface. The several diagrams represent successive 
stages in the process. (After Chamberlin and Salisbury.) 
It is seen from these figures that while there is a shifting of material 
from the highlands to the sea-border, there is no diminution in the bulk 
of the mass, but only a change in its form. It can also be seen that the 
portion between the levels of high and low water approach more nearly a 
true plain, for along this belt the waves, tidal or otherwise, are contin- 
uously at work levelling down any irregularities. Beginning from here, 
in either direction, toward the shore or toward the sea, the profile departs 
greatly from the horizontal. If the island were gradually to subside, the 
water would encroach on the land and the waves would tend to plane off 
the latter, carrying the horizontality of the profile curve farther and 
farther inland. If, on the other hand, the islands were rising, the plain 
would grow seaward and there would be little change in the upper portion 
of the profile. 
Now, if we suppose that reefs are forming around the island, which is 
rising at the same time, the growth of this platform seaward would be 
greatly quickened. Coral reefs make ideal foundations upon which to 
build a plain, because they grow up to a limiting level, namely, the surface 
of the water. When the coast becomes sufficiently elevated to bring these 
reefs above the water, we already have a flat surface which usually has 
a very slight slope toward deep water. At the edge of this coral platform 
the slope is very sudden and very great, though just at the edge the reef 
is slightly higher, the result of wave action, no doubt. For the most part 
the coastal plain of Cebu is of this origin. This is well seen by an 
examination of the plain north of the city of Cebu, just west of Mabolo, 
and in the neighborhood of Mandaue. 
Rivers build many parts of typical coastal plains in the form of delta 
deposits. The best example of this class of formation which Cebu affords 
is probably the one at Argao. A delta deposit may be distinguished some- 
times by its triangular shape and always by its peculiar cross section in 
which two series of beds can be seen. In the lower portion of a section of 
sufficient depth the layers are seen to dip at rather high angles, whereas 
