JAMES DWIGHT DANA 253 



of continents and oceans. It is of course a fact familiar to all 

 students of geology that nearly if not quite the whole surface of 

 our existing continents has been covered at some time by the waters 

 of the sea. This naturally suggested the belief which was held by 

 Lyell, the great master of geology in the middle of the nineteenth 

 century, as it had been held in general by his predecessors, that 

 continent and ocean have repeatedly changed places. Moreover, 

 the strictly uniformitarian doctrine of Lyell was adverse to any 

 notion of progressive change in any definite direction. Lyell, 

 accordingly, conceived of a perfectly indefinite, kaleidoscopic 

 interchange of continent and ocean in the course of geological 

 time. The Lyellian doctrine finds beautiful expression in the 

 familiar lines of In Memoriam: 



"There rolls the deep where grew the tree. 

 O earth, what changes thou hast seen! 

 There, where the long street roars, hath been 

 The stillness of the central sea." 



Dana, on the contrary, believed that our present continents 

 and oceans were outlined as areas of relative elevation and depres- 

 sion, respectively, in the crust of the globe, in the very beginning 

 of geological time. The progress of geographical evolution has 

 been, in the broadest view, a subsidence of the ocean bottoms, a 

 withdrawal of the waters more and more into the deepening 

 basins, and consequently a progressive emergence of continental 

 lands. The substantial truth of this view, enunciated by Dana 

 in I846, 1 hardly admits of doubt; though there has been an amount 

 of oscillation, in connection with the progressive deepening of the 

 oceans and emergence of the lands, which Dana seems not to 

 have adequately appreciated. The greater density of the sub- 

 oceanic masses in comparison with the subcontinental masses, as 

 shown by pendulum observations, indicates that the distinction 

 between continent and ocean depends on the heterogeneity of the 

 material in the interior of the earth ; and the determining conditions 

 must therefore have had their origin in the initial aggregation of 



i American Journal of Science, series 2, vol. 2, p. 353. 



