irably explains a great
variety of complicated phenomena, we ought to remember that the upheaval
to the height of 4000 feet of atolls in which the coralline limestone
would be 4000 feet thick, implies, first, a slow subsidence of 4000
feet, and, secondly, an elevation of the same amount. Even if the
reverse or ascending movement began the instant the downward one ceased,
we must allow a great lapse of ages for the accomplishment of the whole
operation. We must also assume that at the commencement of the period in
question, the equatorial regions were as fitted as now for the support
of reef-building zoophytes. This postulate would demand the continuance
of a complicated variety of conditions throughout a much longer period
than they are usually persistent in one place.

To show the difficulty of speculating on the permanence of the
geographical and climatal circumstances requisite for the growth of
reef-building corals, we have only to state the fact that there are no
reefs in the Atlantic, off the west coast of Africa, nor among the
islands of the Gulf of Guinea, nor in St. Helena, Ascension, the Cape
Verdes, or St. Paul's. With the exception of Bermuda, there is not a
single coral reef in the central expanse of the Atlantic, although in
some parts the waves, as at Ascension, are charged to excess with
calcareous matter. This capricious distribution of coral reefs is
probably owing to the absence of fit stations for the reef-building
polypifers, other organic beings in those regions obtaining in the great
struggle for existence a mastery over them. Their absence, in whatever
manner it be accounted for, should put us on our guard against expecting
upraised reefs at all former geological epochs, similar to those now in
progress.

_Lime, whence derived._--Dr. Maculloch, in his system of Geology, vol.
i. p. 219, expressed himself in favor of the theory of some of the
earlier geologists, that all limestones have originated in organized
substances. If we examine, he says, the quantity of limestone in the
primary strata, it will be found to bear a much smaller proportion to
the siliceous and argillaceous rocks than in the secondary; and this may
have some connexion with the rarity of testaceous animals in the ancient
ocean. He farther infers, that in consequence of the operations of
animals, "the quantity of calcareous earth deposited in the form of mud
or stone is always increasing; and that as the secondary series far
exc