REVIEWS. 
413 
last the land has entirely disappeared, and we have a circular line of coral 
reef, representing what was originally the coast line, and enclosing within it 
a quantity of water, partly brackish, and having an opening at one part of the 
reef by which boats can pass into the circle from the outside. Eventually 
cocoa-nuts and seeds of different kinds get among the coral reefs and grow up, 
and eventually a perfect wall of coral is found enclosing a vast pond of calm 
water, many miles in length and breadth. Now there remains one question 
to be asked, and that is, How came there to be any channel as an entrance 
from the surrounding sea P This seems' difficult to understand, but Mr. 
Darwin furnishes us at once with a lucid and satisfactory answer. It is 
this. While the island which formerly existed was in progress of 
descent, of course during many years, it attracted rain, and thus there was 
fresh water, which flowed down and most probably formed a river at some 
point. Well, as this river flowed out to the sea, it utterly prevented the 
growth of coral at the point of exit, and so it left at last a point of entrance 
to the atoll. 
The above is but a very imperfect sketch of the several operations em- 
ployed, still it may help the reader to understand the general principle of 
the process; and we trust he will be led by it to get Mr. Darwin’s book, and 
study the operations more fully for himself. We have given only the descrip- 
tion of an atoll-growth, because we doubt not that those who understand 
it will readily be led to perceive that there is no material difference between 
the growth of an atoll and that of a barrier- and a fringing-reef. We may 
therefore pass on to the consideration of some of the points at issue between 
Mr. Dana and Mr. Darwin. And firstly of the question as to the supposed 
recent elevation of some of the lagoon islands, Keeling atoll, for example. 
Of this one Mr. Darwin says : — “ Mr. Liesk informed me that he had seen 
an old chart in which the present long island on the north-east side was 
divided by several channels into as many islets ; and he assures me that 
the channels can still be distinguished by the smaller size of the trees on 
them. On several islets also I observed that only young cocoa-nut trees 
were growing on the extremities, and that older and taller trees rose in 
succession behind thfem, which shows that these islets have very lately 
increased in length. In the upper and south-eastern part of the lagoon 
I was much surprised at finding an irregular field, of at least a mile square, 
of branching corals still upright, but entirely dead. . . . They were of a 
brown colour, and so rotten that in trying to stand on them I sank half 
way up the leg, as if through decayed brushwood. The tops of the branches 
were barely covered by water at the time of lowest tide. Several facts 
having led me to disbelieve in any elevation of the whole atoll, I was at 
first unable to imagine what cause could have killed so large a field of 
coral. Upon reflection, however, it appeared to me that the closing up of 
the above-mentioned channels would be a sufficient cause ; for before this a 
strong breeze, by forcing water into them from the head of the lagoon, would 
tend to raise its level. But now this cannot happen, and the inhabitants 
observe that the tide rises to a less height during a high south-east wind at 
the head than at the mouth of the lagoon.” 
Thus, in this instance, Mr. Dana is answered ; but we must confess that 
Mr. Darwin does not appear so satisfied with the view that he completely 
