254 LEADING AMERICAN MEN OF SCIENCE 



the part of the primitive meteoric swarm which formed the earth ; 

 or, perhaps, as suggested by Chamberlin and Salisbury, in the 

 changes attendant upon the beginning of the formation of the 

 ocean. The study of the sedimentary formations which cover our 

 existing continents shows that almost all of them were deposited 

 in shallow 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 subsiding freshet, to dry and crack in the 

 sun or to be pitted by raindrops. Scarcely any of the strata bear 

 evidences of deposition in water of very considerable depth. Even 

 the chalk of England and of Texas was probably not deposited 

 in waters of oceanic depth. 



Another general topic in dynamical geology for whose elucida- 

 tion we are greatly indebted to the writings of Dana is the process 

 of mountain-making. 1 That the main cause of mountain eleva- 

 tion is the tangential pressure in the earth's crust resulting from 

 internal contraction, is now somewhat generally acknowledged; 

 though there may be doubt whether the main cause of contrac- 

 tion is the cooling of the earth from an incandescent condition, 

 as assumed in the commonly accepted form of the nebular theory, 

 or the gravitational adjustment of an incoherent aggregation of 

 planetesimals, as assumed in the more recent hypothesis of 

 Chamberlin and Moulton. Whatever the cause or causes of 

 internal contraction, its effect in causing crustal wrinkles would 

 be the same. As, in Dana's homely illustration, the smooth skin 

 of the plump, fresh apple becomes wrinkled when the apple dries 

 and shrivels, so the earth's skin must wrinkle if the interior 

 decreases in volume. The idea of the contractional origin of 

 mountains was not original with Dana. A glimmer of the idea 

 appears in the writings of Leibnitz. Constant Prevost appears 



1 The views of Le Conte on this subject are in most -points similar to those 

 of Dana; but, while Le Conte's discussions have been of great value, the 

 priority in the general development of the theory belongs to Dana. See the 

 noble and generous tribute of Le Conte, in his obituary of Dana, read before 

 the Geological Society of America, and published in vol. 7 of the Bulletin of 

 the Society. 



