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SCIENCE 



[N. S. Vol. XXV. No. 631 



parison with the subcontinental masses, as 

 shown by pendulum observations, indicates 

 that the distinction between continent and 

 ocean has its basis in the heterogeneity of 

 the material in the interior of the earth; 

 and the determining conditions must, there- 

 fore, have had their origin in the initial 

 aggregation of that part of the primitive 

 nebula which formed the earth; or, per- 

 haps, as suggested by Chamberlin and 

 Salisbury, in changes attendant upon the 

 beginning of the formation of the ocean. 

 The study of the sedimentary rocks which 

 cover our existing continents shows that 

 almost all of them were deposited in shal- 

 low waters; many of the strata, indeed, in 

 waters so shallow that the layers of mud 

 and sand were from time to time exposed 

 by the receding tide or the subsiding fresh- 

 et, to dry and crack in the sun or to be 

 pitted by raindrops. None of the sedi- 

 mentary deposits seem to have been 

 formed in waters of truly oceanic depth. 

 Certain it is, however, that Dana made 

 the evolution of the continents too simple 

 an affair. He recognized, indeed, that the 

 progressive emergence of the continental 

 lands was attended by continual oscilla- 

 tion; yet, even in the last edition of his 

 'Manual,' it appears that he did not duly 

 appreciate the magnitude of those oscilla- 

 tions. We now know that in early Cam- 

 brian time the Mississippian sea was only 

 a sound or strait, most of the area in which 

 the Trenton limestone was subsequently 

 deposited being then dry land. Only grad- 

 ually did the Appalachian strait widen out 

 into the Mississippian sea. A true concep- 

 tion of continental evolution must recognize 

 two complementary truths : a wide range 

 of oscillatory movement, and yet on the 

 whole a progressive deepening of ocean 

 basins and a progressive emergence of con- 

 tinental lands. Chamberlin has formu- 

 lated the doctrine of an alternation of 

 marine and continental periods due to an 



intermittently progressive deepening of the 

 ocean basins, and has connected therewith 

 an ingenious and beautiful theory of cli- 

 matic changes in geological time. 



The doctrine of the progressive evolution 

 of continents, as taught by Dana, gave new 

 clearness and emphasis to the general con- 

 ception of geology as a history of the globe. 

 Le Conte, in his cordial and generous 

 eulogy of Dana, declared that 'geology 

 became one of the great departments of 

 abstract science, with its own characteristic 

 idea and its own distinctive method, under 

 Dana.' There is certainly somewhat of 

 exaggeration in this commendation, yet the 

 statement contains an important truth. 

 More or less clearly, all geological investi- 

 gators must have felt that the distinctive 

 idea of geology is that the structures of the 

 rocks of the earth's crust have their su- 

 preme significance as monumental inscrip- 

 tions, the deciphering of which may reveal 

 to us the history of the earth. Yet this 

 conception was never before so clearly 

 formulated, and the whole treatment of the 

 subject so consistently adjusted thereto, as 

 in the writings of Dana. The portion of 

 previous manuals dealing with the local 

 distribution of the series of strata had gen- 

 erally borne some such title as 'Strati- 

 graphical Geology'; and very commonly, 

 as in the well-known works of Lyell and 

 De la Beche, the series had been traced 

 backward, beginning with the most recent 

 strata. In Edward Hitchcock's 'Elemen- 

 tary Geology,' with which, in my boyhood, 

 I commenced the study of the science, the 

 stratigraphic chapter bears the title, 

 ' Lithological Characters of the Stratified 

 Rocks.' It occupies only twenty pages in 

 a book of more than four hundred pages. 

 It traces the formations backward in Lyell- 

 ian fashion. Separate from the strati- 

 graphic chapter, is another and longer 

 chapter on paleontology, which is arranged 

 botanically and zoologically, and not chro- 



