Mr J. D. Dana on the Origin of Continents. 235 



sequent to it, the ocean's bottom has actually subsided several 

 thousand feet within a late period, as shewn by the coral 

 islands scattered over the wide Pacific* 



By reference, therefore, to the principle of unequal con- 

 traction, and to those subordinate causes of change of level 

 usually appealed to by Geologists (though treated of com- 

 monly as primary in importance), we may obtain a general 

 view of the origin of the earth's features. I propose at this 

 time to offer a few remarks in illustration of this subject, 

 derived from the features of our own continent, reserving a 

 fuller discussion for another occasion. t 



The effects of contraction as a geological cause, though 

 long admitted, have been first brought out in their various 

 bearings by M. Constant Prevost, before the Geological So- 

 ciety of France. J The facts adduced substantiate his views, 



* If we consider that two hundred islands have subsided in the Pacific, which 

 had there been no corals, would have disappeared without a record, we perceive 

 that the comparative absence of islands from the Atlantic, whose waters are, to 

 a large extent, too cold for corals, proves nothing against the hypothesis. On 

 the contrary, so large a bare surface of waters is probable evidence of the dis- 

 appearance of some points of land by submergence. All existing Atlantic 

 islands are of igneous origin except the Falklands, to the east of Tierra del 

 Fuego. 



t We may here mention one or two facts in corroboration of the general 

 theory, that the more igneous portions of the globe have contracted most and 

 thereby became submerged. For example we find the continent of America 

 reduced to a narrow strip of land, just where the great American tract is 

 crossed from east to west by a region of igneous action, not yet entirely ex- 

 tinct ; that is, about the West Indies and the adjoining isthmus. This region 

 became thus depressed and submerged, in consequence of greater contraction 

 below ; and hence North and South America are nearly disjoined by a broad 

 arm of the ocean. This single instance is the only one, through the continent 

 of America, of volcanic eruptions east of the great western chain of mountains. 



Again, the East Indies, another region of perpetual fires, in the earth's his- 

 tory, constitute a cluster of islands separating from Asia the large non-volcanic 

 New Holland, properly a part of a soutli-eastern extension of the continent. 

 Moreover, we may account for the fact that this Archipelago has not farther 

 subsided, so as to become a deep ocean with few islands, on the ground that 

 extensive areas of land, without fires, exist in the midst of the group, Borneo 

 being one example, equalling in extent half the United States, east of the 

 Mississippi. The Indian Ocean, at the same time, bears evidence, in its coral 

 islands of a much more extensive subsidence. 



* See American Journal of Science, vol. ii., scr. ii., p. 355. While thus men- 



