MODE OF ORIGIN. ial 
Besides a line of the greatest wave-action, we may also distinguish 
a height where this action will entirely cease; and it is evident, from 
facts already stated, that the point will be found somewhat above 
low tide level. ‘The lower waters of the surge, instead of causing de- 
gradation, are accumulative in their ordinary action, when the mate- 
rial exposed to them is movable: they are constantly piling up, 
while the upper waters are rending and preparing material to be car- 
ried off. ‘The height at which these two operations balance one ano- 
ther will be the height, therefore, of the line of no degradation. As 
the sea at low tide is mostly quiet, and the lower of the surging waters 
swell on to receive the upper, and parry the blow, and, moreover, 
there is next a return current outward,—we should infer that the line 
would be situated more or less above low tide, according to the height 
of the tide, and the surges accompanying it. We are not left to 
conjecture on this point; for the examples presented by the shores of 
New Holland and New Zealand afford definite facts. Degradation has 
there taken place sufficient to carry off cliffs of rock, of great extent ; 
yet below a certain level, the sea has had little or no effect. ‘This 
height, at New Holland, is three feet above ordinary low tide, and at 
New Zealand, about five feet. With regard 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 New Holland and 
New Zealand, where the tide is six and eight feet. From these obser- 
vations, it appears that the height of no 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 surges of the middle Pacific, and the more violent of New Zea- 
land. 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 remains to be determined more accurately by 
observation. While the height of the shore platform depends, there- 
fore, on the tides, and the usual strength of the waves, the breadth of 
it will be determined by the same causes in connexion with the nature 
of the rock material.* 
* On basaltic shores it is not usual to find a shore platform, as the rock scarcely un- 
dergoes any degradation, except from the most violent seas ; such coasts are consequently 
often covered with large fragments of the basaltic rocks. But on sandstone shores, this 
