252 HISTORY OF GEOLOGY AND PALEONTOLOGY. 



the Bermuda reefs. He found only evidences of elevation, 

 and came to the conclusion that coral reefs could be formed 

 wherever the fundamental conditions for the existence of the 

 polyps were satisfied, and a firm basis of support was present; 

 and it was quite indifferent whether the basis was a submerged 

 coast, a submarine plateau of elevation, or a submarine volcano. 

 Sir John Murray arrived at similar conclusions (Proc. Roy. Soc. 

 Edin.y 1880). He does not accept the hypothesis that the 

 atolls and barrier reefs of the Pacific Ocean are built upon 

 a submerged continent, but believes the coral polyps settle 

 upon isolated volcanoes which still are partly above the water, 

 but have been in some parts abraded to the limit of the 

 mechanical activity of the waves ; and he correlates the different 

 forms of the reefs with conditions of nourishment and processes 

 of erosion and corrosion. Murray's explanation of lagoon 

 reefs is that on the windward side the existence of the coral 

 colonies is more prosperous, and the reef grows more quickly 

 than on the leeward side, whose position is less advantageous 

 for the constant renewal of food supplies. The polyps on that 

 side die, and the reef passes through processes of decay; the 

 excavation of the saucer-shaped lagoon is due to the corrosion 

 of the reef limestone by sea-water strongly impregnated with 

 carbonic acid, and also to the erosive activity of the high tides. 



Another important point in which Murray differs from the 

 results attained by Darwin and Dana is the thickness of coral 

 reefs. He shows from numerous soundings taken along the 

 outer edge of atolls and barriers, that the reef-wall is precipitous 

 only to a depth of about 200 feet; below that there is a talus 

 slope occupied by broken blocks of coral limestone to depths 

 of about 1000 feet; and fragments of volcanic material begin 

 to occur at still greater depths. 



In the Salomon Isles Guppy found older coral reefs that 

 had been elevated to heights of more than 900 feet, but the 

 reefs were not more than 130 feet thick. 



In general, it may be said that most scientific authorities on 

 coral reefs at the present day no longer accept Darwin's theory 

 of widespread subsidence as applicable to the American and 

 Australian reefs, or to those of the Red Sea. On the other 

 hand, subsidence seems to be the most satisfactory explanation 

 of many atolls in the Pacific Ocean. Clearly the critical test 

 for subsidence is the thickness of a reef. The borings under- 

 taken at the Ellice Isles, under the guidance of Professor Sollas 



