1888.] natural sciences of philadelphia. 419 



December 4. 

 Mr. Charles Morris in the chair. 

 Twenty-five persons pressnt. 



Theories of the Formation oj Coral Islands. — Mr. Charles Morris 

 remarked that there exist, as is ,well known, two theories of the 

 formation of coral islands, the subsidence theor}' of Charles Darwin, 

 and the recent theory propounded by John jNIurray and others, 

 which claims that the phenomena can be explained without calling 

 in the aid of subsidence. It was not his purpose to offer an}- argument 

 on this controverted question, and he would simply say that the 

 Darwin theory seemed to him much the most probable, the objec- 

 tions to it being, in his view of the case, far less cogent than those 

 to the Murray theory. 



If the subsidence theory were accei^ted, however, there was one 

 consequence necessarily deducible from it which, so far as he was 

 aware, had not yet been definitely considered, and which was not 

 without scientific importance. 



The area occupied by coral islands in the Pacific is, as stated by 

 Dana, 6000 miles in length and from 2000 to 2500 miles in width, 

 thus covering from 12,0t)0,000 to 15,000,000 square miles. This 

 includes a blank central area of 1,000,000 square miles in which the 

 subsidence is supposed to have been too rapid to permit coral growth, 

 beyond which is a region of small atolls, and outside this the region 

 of ordinary atolls. Outside this again is a region in which bari'ier 

 and fringing reefs replace atolls, and if this region be included the 

 total area of subsidence must have been, according to Le Conte, 

 about 20,000,000 square miles. 



The depth of subsidence is variously stated. Dana considers that 

 the extreme subsidence was at least 9000 or 10,000 feet. Later 

 authorities o-ive it at about three miles. As resrards the averasre sub- 

 sidence of the whole area it may perhaps be safely assumed as not less 

 than 5000 feet, possibly considerably more. If the Darwin subsidence 

 theory be accepted, then, an area of sea bottom equal to that of the 

 largest continent must have sunk bodily to a depth of at least a 

 mile. 



This subsidence may have been correlative with a considerable 

 elevation of the land surface, but there is no reason to believe that 

 there was any equal elevation of other portions of the ocean bed. 

 There are many evidences of local elevation, but all of them taken 

 together are unimportant as compared with the great subsidence 

 over the coral islana area, and may have been balanced bv local 

 subsidence elsewhere. Yet such an immense subsidence, with no 

 corresponding elevation of the ocean bottom, could not take place 

 without adding greatly to the capacity of the ocean basin. It formed 

 what we may speak of as a huge valley in the ocean bed, of 20,000,000 



