SCIENCE-GOSSIP. 



[Oct. 1, 1865. 



mucli-disputed point. By some, especially by the 

 earlier autlioritics, it was thouglit to be a very slow 

 process, and it was supposed that, on the average, 

 little more than a foot could be added to a reef 

 during a whole century. But this cannot always be 

 the case, for Darwin speaks of a ship-bottom which 

 was covered to the thickness of two feet in twenty 

 months. Loose masses also have been known to 

 become firmly cemented in six months by the growth 

 of new coral. A further instance of the rapidity of 

 growth was found in the Keeling Reef, a chanuel in 

 which became entirely stopped up, through which a 

 schooner had floated only ten years before. 



Prom this fact Darwin draws the foUowiug con- 

 clusion : — " First, that considerable thickness of 

 rock has certainly been formed within the present 

 geological era by the growth of coral, and by the 

 accumulation of its detritus ; and, secondly, that 

 the increase of individual corals and reefs, both out- 

 wards or horizontally, and upwards or vertically, 

 under peculiar conditions favourable to such in- 

 crease, is not slow when referred either to the 

 standard of the average oscillations of level in the 

 earth's crust, or to the more precise but less im- 

 portant one of a cycle of years." 



Coral reefs, partaking as they do of the depression 

 or elevation of the sea-bottom, and also being sub- 

 ject to the waves and breakers, form a barrier of 

 limestone more or less compact ; and as the polype 

 ceases on reaching the surface of the water, the top 

 of the reef frequently becomes weathered and con- 

 verted into soil capable of sustaining vegetation. 

 When the sea-bottom to which the zoophytes are 

 attached partakes of a gradual elevation, they build 

 outwards and seawards, and should it be undergoing 

 depression, they strike upwards. 



These sea mountains abound in the Pacific, Indian, 

 and Southern Oceans, Masses of them abound in 

 the Pacific, on both sides of the equator, but not 

 beyond the 30th degree of latitude. They are also 

 very numerous in the southern part of the Indian 

 Ocean. For hundreds of miles, we find them 

 trending along the north-east coast of Australia, and 

 they occur more or less in the Persian, Arabian, 

 Bed, and Mediterranean Seas. 



Owing to the great amount of volcanic agency 

 which is constantly at work, upheaving and sub- 

 merging in the Pacific, there are found there many 

 peculiar phases of the coral reef; such, for instance, 

 as the atolls or lagoon islands, fringing or shore 

 reefs, coral ledges, and encircling reefs. 



Atolls consist of coral reefs forming low circular 

 slands, enclosing lagoons. Shore reefs are those 

 which surround islands of igneous and other origin. 

 Coral ledges are masses of coral thrown up by 

 volcanic agency on to the top of other reefs already 

 upheaved. The encircling reef is that which 

 stretches along shore in surf-beaten ridges, often 

 extending for many leagues, and varying in thick- 



ness from 20 to 200 feet. The great reef which 

 lies off the coast of New Holland is described by 

 Captain Plinders as being more than 1,000 miles 

 long, and varying in thickness from 20 to 100 feet. 



In this same reef there is one continuous portion 

 of more than 350 miles, without a single break or 

 passage through it. 



So much for coral reefs and islands, commonly 

 so called. But modern discoveries go to show that 

 the work of the polype is not confined to those 

 mountains which are still covered by the sea. On 

 the contrary, it is, I believe, the general belief 

 among the great geologists of our day, that the 

 Dolomite mountains, hitherto the cause of so much 

 doubtful controversy, were the v.ork of the coral 

 zoophyte. 



These unstratified rocks, deeply divided by ver- 

 tical fissures, and scattered about over porphyritic 

 platforms, with so little connection with the neigh- 

 bouring rocks, that they look like "icebergs 

 strandedj" have been shown by Dr. Bichthofen, 

 Mr. Churchill, &c., to bear a most remarkable re- 

 semblance to coral reefs. 



And here I cannot do better than quote some 

 passages from a review on Gilbert and Churchill's 

 book on " The Dolomite Mountains : " — 



" Mr. Churchill suggests that the formation of 

 Dolomite may be going on in coral reefs at the present 

 day ; for the specimens of coral rock brought home 

 by Dana from the raised island of Mantea, or 

 Aurora, were found to contain in one case 5 per 

 cent, and in another as much as 38 per cent, of 

 carbonate of magnesia. And if we suppose the 

 Dolomite mountains of Carinthia to have been 

 formed on a gradually subsiding basis, they may 

 have grown up like the low islands of the Pacific, 

 till the sea attained the depth of a thousand fathoms, 

 preserving their original contour from first to last, 

 the group of corals, like a forest of tree trunks 

 without tops, rising upwards together, and becoming 

 partially solid by lateral growth, or by fillmg up 

 with sediment. 



" We have no fossil coral in England wherewith 

 to compare the Dolomite mountains. Our mague- 

 siau limestone affords only bryozoa, for it has not 

 been suspected that the remarkably concentric and 

 radiated concretions are metamorphosed corals. In 

 one Silurian coral reef of the Wenlock Edge and 

 Dudley there may be masses of branching coral a 

 yard across, and convex Stromatoporce (which are 

 not corals) of nearly equal size. But the coral beds 

 are separated by clay partings, and never attain a 

 great thickness. The Devonshire marbles have 

 much the appearance of coral reefs, so far as respects 

 the scattering of small masses over a region of 

 argillaceous schists. In the carboniferous limestone 

 layer above layer of branching corals may be seen 

 in the lofty cliffs of Cheddar and the weather-beaten 

 shores of Lough Erne. There the corals are slightly 



