ON A PIECE OF LIMESTONE. 175 



like a stone. This is especially the case with the Meandnna, or brain- 

 stone coral, so named from the resemblance which its furrowed sur- 

 face bears to the convoluted surface of the brain ; hemispherical mass- 

 es of this coral are not unfrequently to be seen in museums having a 

 diameter of from two to three feet ; and in the upraised coral-cliffs 

 of Bermuda they are reported to be five or six feet in diameter. The 

 polyps lie in rows along the furrowed surface, and the active life of 

 the composite mass does not extend far down ; its stony interior being 

 the product of its earlier life, as the heart-wood of a tree is the prod- 

 uct of previous successions of leaf-buds. But it is no more correct to 

 say that the polyps have built up the stony mass, than it would be to 

 say that the leaves of a tree build up its woody stem, or that our own 

 soft parts build up our bony skeleton. The hard parts are formed in 

 each case by a process of groicth ; soft tissue being first produced as 

 a "part of the animal body, and this being subsequently solidified by 

 mineral deposit, the material for vrhich is absorbed by the animal from 

 the sea-water in which it lives. 



The admirable researches of Mr. Darwin have shown us that, 

 although the reef-building corals seem unable to live and grow at 

 depths greater than twenty fathoms (one hundred and twenty feet), 

 yet that if their base gradually subsides, at a rate not greater than 

 that of coral-growth, the reef or island will be kept up to the surface 

 by such growth ; so that,*if we could bore down into it, we might 

 find the coral-structui'e to have a depth of many hundreds or even 

 thousands of feet. The recent soundings of the Challenger around 

 the Bermuda islands, which are entirely composed of coral, indicate 

 that they form the summit of a pillar rising from a depth of twelve 

 thousand feet ; and as we have no instance of a mountain having such 

 a shape, it seems probable that the upper part of this pillar, at any 

 rate, must have been formed of coral, which kept growing upward, in 

 the manner indicated by Mr. Darwin, while the bottom was slowly 

 subsiding. It is commonly supposed by geologists that the lime- 

 stone beds of which I have been speaking are the result of the meta- 

 morjjhosis of ancient coral formations, which attained their great 

 thickness by continuous growth at their living surface, as their base 

 gradually subsided. But it appears to me that all we know of exist- 

 ing coral formations renders it unlikely that there should have been 

 such a continuity of area in ancient coral formations, as would be re- 

 quired to account for the continuity in the area of our great beds of 

 carboniferous limestone ; and that this continuity is far better account- 

 ed for by supposing them to have been formed in the manner I pre- 

 viously indicated by the foraminiferal life which recent researches 

 have shown to be even now forming a calcareous deposit over vast 

 areas of the ocean-bottom. 



Thus, then, we should regard the beds which show distinct coral- 

 structure as representing reefs or islands of limited extent in the 



