ESTIMATE BY PROF. H. S. WILLIAMS 



tention ; and yet it is by these alone that continents and 

 ocean basins have been gradually formed. 



Now it was just these slowly acting causes and these 

 grander effects that took strongest hold on Dana's mind. 

 Igneous agencies became for him the interior vital forces 

 of the earth, which, reacting on the exterior crust, pro- 

 duced the greater features, and by their eternal conflict 

 with external, sun-derived, sculpturing forces determine 

 the evolution of the earth as a whole. 



The mention of this line of his thought introduces 

 us naturally to the next head, and that the one which 

 most deeply interests this Society,* namely, Dana as a 

 geologist. 



Prof. H. S. Williams has already given an admir- 

 able account of this in the Journal of Geology for Sep- 

 tember, 1895. I am indebted to him for much that 

 follows. For other details I would refer the reader to 

 that article. 



Development of the Earth as a Unit 



As already said, the idea underlying all Dana's geo- 

 logical work is that of development of the earth as a 

 unit. Before Dana, geology was doubtless in some sense 

 a history that is, a chronicle of interesting events; but 

 with Dana it became much more, it became a philosophic 

 history, a life history, a history of the evolution of the 

 earth, and of the organic kingdom in connection with 

 one another. For the first time there was recognized a 

 time-cosmos governed by law as the true field of geology, 

 as the space-cosmos governed by law is the field of astron- 

 omy. Before Dana, geology was the study of a succes- 

 sion of formations; with Dana it was the study of a 

 succession of eras, periods, epochs, during which geo- 

 graphic forms and organic forms were both developing 

 toward a definite goal. The underlying idea of his 



* The American Society of Geologists. 

 253 



