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Show ~98 EXTENT OF CORAL FORMATIONS, [Ch. XVIII. are not wanting*, it is by-no means improbable that the era of the elevation of this island may not be very remote. The calcareous masses which we have now considered, constitute, together with the associated volcanic formations, the most extensive of the groups of rocks which can be demonstrated to be now in progress. The space in the sea which they occupy is so vast, that we may safely infer that they exceed in area any group of ancient rocks which can be proved to have been of contemporaneous origin. We grant that each of the great archipelagos of the Pacific are separated by unfathomable abysses, where no zoophytes may live and no lavas flow, where not even a particle of coral sand or volcanic scorire may be drifted : we confine our view to the extent of reef ascertained to exist, and assume that a certain space around each volcanic or coral isle has been covered with ejections or matter from the waste of cliffs, and it will then be seen that the space occupied by these formations may equal, and perhaps exceed in area that part of our continents which has been accurately explored by the geologist. That the increase of these calcareous masses should be principally, if not entirely, confined to the shallower parts of the ocean, or, in other words, to the summits of submarine ranges of mountains and elevated platforms, is a circumstance of the highest interest to the geologist ; for, if pai·ts of the bed of such an ocean should be upraised, so as to form large continents, mountain-chains might appear, capped and flanked by calcareous strata of great thickness, and replete with organic remains, while in the intervening lower regions no rocks of contemporary origin would ever have existed. A modern writer has attempted to revive 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 "' Se~ Captain Beechey'a Voyage to the Pacific, &c., pp. 159 and 191. Ch.XVIII.] ANIMAL ORIGIN OF LIMESTONE CONSIDERED, ~99 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 exceeds the ptimary in this respect, so a third series may hereafter arise from the depths of the sea, which may exceed the last in the proportion of its calcareous strata*." If these propositions went no farther than to suggest that every particle of lime that now enters irtto the crust of the globe, may possibly in its turn have been subservient to the purposes of life by entering into the composition of organized bodies, we should not deem the speculation improbable; but when it is hinted that lime may be an animal product combined by the powers of vitality from some simple elements, we can discover no suffici~nt grounds for such an hypothesis, and many facts which militate against it. If a large pond be made, in almost any soil, and filled with rain water, it may usually become tenanted by testacea, for carbonate of lim~ is · almost universally diffused in small quantities. But if no calcareous matter be supplied by waters flowing from the surrounding high grounds or by springs~ no tufa or shell-marl are formed. The thin shells of one generation of molluscs decompose, so that their elements afford nutriment to the succeeding races; and it is only where a stream enters a lake, which may introduce a ftesh supply of calcareous matter, or where the lake is fed by springs, that shells accumulate and form marl. All the lakes in Forfarshire which }jave produced deposits of shell-marl, have been the sites of springs which still evolve much carbohic acid, and a small quantity of carbonate of lime. But there is no marl in Loch Fithie, near Forfar, where there are no springs, although that lake is surrounded by these cal- . careous deposits; and although, in every other respect, the site is favourable to the accumulation of aquatic testacea. We find those charre which secrete the largest quantity of * Macct~Uoth's Syst, of Geol., vol. i. p. 219. |