144 Dr. J. D. Dana on the Origin of 



which give the best shadows are those made with single loop, 

 but it has also been observed on Swan lamps with double 

 twist. By making lamps with clamps of various metals, it 

 might be possible to obtain metallic films of various kinds. 

 Interesting magneto-optic phenomena might perhaps present 

 themselves in the case of transparent iron films, if they could 

 be obtained. 



XVII. Origin of Coral Reefs and Islands. 

 By James D. Dana, LL.D* 



[Plate L] 



ri^HE Presidential Address of Dr. Archibald Geikie, Di- 

 JL rector- General of the Geological Survey of Great 

 Britain, before the Royal Physical Society of Edinburgh in 

 1883 t? reviews the subject of the origin of coral-reefs and 

 islands. In the course of the discussion, the author sustains 

 and enforces the objections which have been presented by 

 others, and concludes that " the existence of such reefs is no 

 more necessarily dependent on subsidence than on elevation." 

 The existing state of doubt on the question has led the writer 

 to reconsider the earlier and later facts, and in the following 

 pages he gives his results J. That both sides may be fairly 

 before the reader, the views of Darwin and the evidences in 

 favour of his theory are first considered, and afterwards the 

 arguments that have been urged against it. Part of the 

 objections are based on misunderstandings of the facts, and 

 hence a general presentation of the subject has been thought 

 necessary. 



Paet I. — The Darwinian Theory and its Evidences. 



1. According to the Darwinian theory, islands with fringing 

 reefs have been often changed through a slow subsidence of 

 the region into islands with barrier reefs; and, as the last 

 summit of the sinking land disappeared, the latter have be- 

 come atolls, that is, barrier reefs enclosing simply a piece of 

 the ocean (or a lagoon). Darwin added to this conclusion, a 

 second, in view of the wide distribution of atolls and their 



* From an advance proof from Silliman's i American Journal/ com- 

 municated by the Author, to whom we are also indebted for copies of the 

 plate and cliches of the woodcuts. 



t Proceedings Edin. Roy. Phil. Soc. viii. p. 1 (1883). 



J The writer's account of his original observations is contained in his 

 Wilkes Expedition, Geological Report, 1840 (756 pp. 4to), pp. 29-154; 

 and, less completely, along with a review of facts from other regions, in 

 his < Corals and Coral Islands; 398 pp. 8vo (1872, 1875). 



