174 ^^^ POPULAR SCIENCE MONTHLY. 



remains abundant in specimens of this limestone which do not show 

 any indications of organic structure that are obvious to the naked eye. 

 If the Globigerina-mud were to be subjected to the pressure of an 

 enormous weight of rock deposited above it, and then to the heat and 

 pressure which we know must have accompanied the great crumpling 

 of the earth's crust that made the marked separation between the 

 Paleozoic and the Secondary epochs, we may well believe that it 

 would have been metamorphosed into a limestone closely resembling 

 the least fossiliferous of the Avonside rocks ; and we have no difficulty 

 in accounting for the vast thickness of these beds, if we regard them 

 as having been progressively formed on the bottom of a very deep 

 ocean, through a long succession of ages. 



That certain beds of the Avonside rocks are ancient Coral-Reefs, 

 cannot be a matter of question ; for we find them to be entirely made 

 up of fossil corals, together with the fossilized shells and crinoids 

 which such reefs would have supported. This was especially the 

 case with what used to be called the " black rock " under the sea- 

 wall, which has been nearly all quarried away since, when a boy, I 

 brought home a piece of it as large as I could carry, wondering at 

 such an accumulation of fossils, but without any such understanding 

 of their import as that which I am endeavoring to give you. Every 

 one has heard of the coral reefs and islands, which are popularly 

 said to be " built up " in tropical seas by the agency of " insects," as 

 bees build their waxen combs. ' And I suppose that every one of you 

 is familiar with specimens of some kind of coral brought home by a 

 seafaring friend, or has seen such in your musgum. Now, the fact is, 

 that all these corals are the production of animals resembling in es- 

 sential points the common sea-anemone, but differing from it in de- 

 positing a stony skeleton in the fleshy substance which forms its base, 

 and also in the radiating partitions which surround its stomach. We 

 have on our own shores a small type of the coral-forming polyps, in 

 the little Garyophyllla, which, when the animal is expanded, you 

 would take to be a small sea-anemone, but which, when contracted, 

 shrinks down into its stony cup. The Fangia of tropical seas is a 

 much larger solitary polyp of the same kind ; and you will often 

 meet with its stony disk, four or five inches in diameter, with beauti- 

 ful thin vertical plates radiating from the centre to the circumference, 

 very much like the " gills " of the under-side of a mushroom (fungus), 

 whence its name is derived. But all the more massive corals are the 

 skeletons of composite animals ; that is, of polyps which bud like 

 plants, and thus grow to large dimensions. In some cases they form 

 tree-like structures, in which you will find a multitude of polyp-cells, 

 sometimes very small, each having its characteristic arrangement of 

 radiating plates. But in the reef-building corals, the polyp-cells are 

 packed closely together ; and the older portion becomes so complete- 

 ly solidified by calcareous deposit that, when broken across, it looks 



