But the growth would take place almost exclusively around the
circumference of the reef, this being the only region in which the coral
polypes would find the conditions favourable for their existence. The
bottom of the lagoon would be raised, in the main, only by the
coral debris and coral mud, formed in the manner already described;
consequently, the margins of the reef would rise faster than the bottom,
or, in other words, the lagoon would constantly become deeper. And, at
the same time, it would gradually increase in breadth; as the rising
sea, covering more of the land, would occupy a wider space between the
edge of the reef and what remained of the land. Thus the rising sea
would eventually convert a large island with a fringing reef into a
small island surrounded by an encircling reef. And it will be obvious
that when the rising of the sea has gone so far as completely to cover
the highest points of the island, the reef will have passed into the
condition of an atoll.
But how is it possible that the relative level of the land and sea
should be altered to this extent? Clearly, only in one of two ways:
either the sea must have risen over those areas which are now covered by
atolls and encircling reefs; or, the land upon which the sea rests must
have been depressed to a corresponding extent.
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