248 HISTORY OF GEOLOGY AND PALEONTOLOGY. 



Lesson and Garnot. The English navigator, Captain Beechey, 

 took a number of soundings round the edges of coral reefs, 

 and also arrived at the conviction that they were based upon 

 submarine mountains, whose summits were never covered by 

 more than 400-500 feet of water. 



The considerable size of many atolls made it seem some- 

 what improbable that they had been erected upon isolated 

 volcanoes, and this theory was opposed by Ainsworth in 1831. 

 He thought that, in addition to the coral polyps in shallow 

 waters, there might be certain species whose habitat was at 

 greater depths. In explanation of the higher edge on the 

 windward side of an atoll, he called oceanic currents to his 

 assistance, and thought they compelled the polyps to build 

 vertically, whereas on the leeward side nothing prevented them 

 from extending the reef in horizontal direction. Charles Lyell 

 was favourably inclined to the theory of a volcanic basis, but 

 also stated in the first edition of the Principles that the 

 inequality in the height of the atoll edges might be due to 

 local variation of level, more particularly to local subsidences 

 after earthquakes. 



The famous memoir by Ehrenberg, " On the Structure and 

 Form of the Coral Growths in the Red Sea," published in 1834 

 in the Abhandlungen of the Berlin Academy, represented the 

 result of eighteen months' study in the particular localities. 

 The treatise begins with an exhaustive historical account of 

 the previous literature on reef-building corals and reef-forms. 

 Ehrenberg then describes the form of the reefs in the Red Sea 

 as ribbon-like submarine banks extending parallel with the 

 coast-line, based upon gentle beach-slopes, and having their 

 water surfaces about \-2 fathoms below the water-level at high 

 tide. There are no exposed reef-surfaces in the Red Sea, and 

 the outer side of the reef has a steep cliff edge descending 

 rapidly into greater depths. The rock underlying the reefs is 

 either a porous limestone or volcanic nfaterial; the coral lime- 

 stone itself forms only a thin surface layer about i J fathoms thick 

 upon the basal rock. Hence Ehrenberg regards the corals not 

 as the builders of new islands, but only as the preservers of 

 islands already existing. 



The German zoologist agrees with Quoy and Gaimard on 

 one of the leading points of controversy, namely, the small 

 thickness of coral structures, and confirtns their conclusion 

 that the polyps can only exist in warm water not more than six 



