MODE OF ORIGIN. 109 
swept by the waves, across the patch of reef, into the lagoon or 
channel beyond, while other portions lodge on its surface. But be- 
sides the small fragments, larger masses will be thrown on the reefs, 
by the more violent waves, and commence to raise them above the sea. 
The clinker fields of coral, by this means produced, constitute the 
first step in the formation of dry land. Afterwards, by farther contri- 
butions of the coarse and fine coral material, the islets are completed, 
and raised as far out of water as the waves can reach—that is, from 
six to ten feet. The ocean is thus the architect, while the coral 
polyps afford the material for the structure: and when all is ready, it 
sows the land with seed brought from distant shores, covering it with 
verdure and flowers. 
The growth of the reefs and islands around high lands, is the same 
as here described for the atoll. 
Among the peculiarities of coral islands, the shore platform appears 
to be one of the most singular. It will be remembered that it lies but 
little above low tide level, and is often three hundred feet in width, 
with a nearly flat surface throughout. 
Though apparently so peculiar, the existence of this platform is due 
to the simple action of the sea, and is a necessary result of this action. 
Passing to New Holland, from the coral islands of the tropics, we 
there found the same structure exemplified along the sandstone shores 
of this semi-continent, where it is continued for scores of miles. At 
the base of the sandstone cliff, in most places one or more hundred 
feet in height, there is a layer of sandstone rock, lying, like the shore 
platform of the coral island, near low tide level, and from fifty to one 
hundred and fifty yards in width. It is continuous with the bottom 
layer of the cliff: the rocks which once covered it, have been removed 
by the sea. Its outer edge is the surf-line of the shore. At low tide it 
is mostly a naked flat of rock, while at high tide it is wholly under 
water, and the sea reaches the cliff. New Zealand, at the Bay of 
Islands, afforded us the same fact, again, in an argillaceous sandrock ; 
and there was no stratification in this case to favour the production of 
28 
