JAMES DWIGHT DANA 259 



which he had not yet solved to his own satisfaction. Darwin 

 had been at work at the problem three years earlier, and at Sydney 

 Dana learned of Darwin's theory. It seemed to him then to 

 explain the phenomena he had studied in the regions of barrier 

 reefs and atolls which he had already visited; and the larger 

 acquaintance with coral formations which he gained in the course 

 of the next two years seemed to him only to bring ampler evidence 

 of its truth. Although the original conception was Darwin's, 

 Dana had the opportunity to study a vastly greater number and 

 variety of coral formations than Darwin had ever seen, so that he 

 was able to support the theory with a greater wealth of evidence 

 than Darwin himself. Darwin welcomed most cordially so power- 

 ful an ally. Writing to Lyell, after receiving a copy of the Report 

 on the Geology of the Exploring Expedition, he refers to the 

 substantial agreement of Dana's views with his own, and adds, 

 " Considering how infinitely more he saw of coral reefs than I did, 

 this is wonderfully satisfactory to me. He treats me most cour- 

 teously." 



The theory of Darwin and Dana may be summed up in a single 

 word subsidence. If there occurs, along a coast of continent or 

 island bordered by a fringing reef, a subsidence not more rapid 

 than the upward growth of the reef, the coral growth and conse- 

 quent reef formation will be most rapid on the outer margin of 

 the reef, where the water is purest, and the supply of oxygen and 

 of floating life available for food is greatest; and the channel 

 between the reef and the shore will consequently become wider 

 and deeper. Thus the fringing reef becomes a barrier reef. If 

 an island is girt with a coral reef, the ultimate effect of a progres- 

 sive subsidence will be to carry the original island entirely under 

 water, leaving an atoll as a monument to mark its place of burial. 

 The most important difference between Darwin's own conception 

 of the theory and that of Dana was that Darwin, in the spirit of 

 the Lyellian geology, thought of the Pacific area of coral islands 

 as very likely marking the site of a drowned continent; while 

 Dana, in accordance with his own doctrine of the essential perma- 

 nence of continent and ocean, conceived the drowned lands to be 



