GEOLOGICAL EVIDENCE. 
331 
would prevail in a lagoon, when its waters had become 
unsuitable for coral life, would be those which would be 
exceptionally favourable to the formation of dolomite. It 
seems, then, from the above considerations that we cannot 
regard the corrosive effect of sea-water as an agent of more 
than very secondary importance in modifying the structure 
of an atoll. 
4. In regard to the negative geological evidence. Here 
we must not overlook two considerations — one that the 
structure of a coral reef is very commonly more or less 
composite ; broken coral, shells, &c., forming a part, and 
sometimes predominating when from one cause or another 
the growth of the polyps is temporarily checked (p. 155) ; 
hence in some cases, what is really a continuous reef may 
be supposed, if only an occasional section be visible, to be 
a series of thin reefs — the other (the more important and 
general) that the characteristic structure of dead coral 
becomes rapidly inconspicuous and may be only discover- 
able in thin sections under the microscope. Where dolo- 
mitisation has occurred it may be actually obliterated, for 
the molecular changes involved in the process are often 
sufficient to destroy every trace of an organism. We may 
thus be prevented from recognising many ancient coral 
reefs. Moreover, the aporosa and madreporaria, which 
are now the chief reef- builders, have only become common 
since the conclusion of Palaeozoic ages, so that the largest 
volume of the geological history of the earth is excluded 
from consideration, because in the times which if covers 
the habits of the reef-builders may have been different. 
Reefs also, it must be remembered, are restricted at the 
present day to almost tropical regions, so that, notwithstand- 
ing any variation of climate, they must always have been 
less frequent and less luxuriant in northern latitudes — that 
is to say, in those regions with which geologists are best 
acquainted. Still, instances of thick reefs of comparatively 
