238 CORALS AND: CORAL ISLANDS. 
AK 
to the height varying with the tides, we observe that in the | 
Paumotus, where the water rises but two or three feet, the | 
platform is seldom over four to six inches above low tide, >~ 
which is proportionally less than at Australia and New Zea- 
land, where the tide is six and eight feet. From these ob- 
servations it appears that the height of least wave-action. 
as regards the degradation of a coast under ordinary seas, 
is situated near one-fifth tide in the Paumotus, and above 
half-tide at New Zealand, showing a great difference between 
the effect of the comparatively quiet work of the middle Pa- 
cific, and the more violent of New Zealand. Within the Bay 
of Islands, where the sea has not its full force, the platform, 
as around the ‘‘ Old Hat,” is but little above low-water level. 
The exact relation of the height of the platform to the height 
and force of the tides, and the force of wave-action, remains 
to be determined more accurately by observation. While, 
therefore, the height of the shore platform depends on the 
tides, and the degree of exposure to the waves, the breadth 
of it will be determined by the same causes in connection with 
the nature of the rock material. 
On basaltic shores it is not usual to find a shore platform, 
as the rock scarcely undergoes any degradation, except from 
the most violent seas; such coasts are consequently often cov- 
ered and protected by large fragments of the basaltic rocks. 
But on sandstone shores, if the rock is not too firm to yield 
sensibly under the stroke of the breakers, this gradual action 
keeps the platform of nearly uniform breadth. Moreover, 
any masses torn from the edge of the platform and thrown 
upon it by storm waves, or the heavier earthquake waves, 
may be soon destroyed by the same action, and carried off ; 
and thus the platform may be kept nearly clean of débris, 
even to the base of the cliff. 
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