Barrell — Growth of Knowledge of Earth Structure. 155 



in 1858 as in 1842. There is no acceptance of the views 

 of Lyell concerning the uniformitarian principles 

 expounded by this British leader in 1830, or of the nature 

 of orogenic forces as published by Elie de Beaumont in 

 1833. Eogers rejects the view that cleavage is due to 

 compression and suggests "that both cleavage and folia- 

 tion are due to the parallel transmission of planes or 

 waves of heat, awakening the molecular forces, and 

 determining their direction. 9 Thus a mere maze of 

 words takes the place of inductive demonstrations 

 already published. 



In following the play of these opposing currents of 

 geologic thought we reach now the point where a period 

 of brilliant progress in the knowledge of mountains and 

 of continental structures begins in the work of J. D. 

 Dana. In 1842 Dana returned from the Wilkes Explor- 

 ing Expedition and the following year began the publica- 

 tion of the series of papers which for the next half 

 century marked him as the leader in geologic theory in 

 America. His work is of course to be judged against 

 the background of his times. His papers mark distinct 

 advances in many lines and are characterized throughout 

 by breadth of conception and especially by clear and log- 

 ical thinking. His work was published very largely in 

 the Journal, of which after a few years he became chief 

 editor. His first contribution on the subject of moun- 

 tain structures, entitled " Geological results of the earth's 

 contraction in consequence of cooling," was published in 

 1847 (3, 176). The evidence of horizontal pressure was 

 first perceived in France as shown by the features of the 

 Alps. Elie de Beaumont connected it, by means of the 

 theory of a cooling and contracting globe, with the other 

 large fact of the increase of temperature with descent in 

 the crust. Dana credits the Rogers brothers with first 

 making known the folded structures of the Appalachians, 

 but objects to their interpretation of origin. He showed 

 by means of diagrams that the folds are to be explained 

 by lateral pressure, the direction of overturning indicat- 

 ing the direction from which the driving force proceeded. 



The Eogers brothers and especially James Hall, in 

 working out the Appalachian stratigraphy, had noted 

 that the formations, although accumulating to a maxi- 

 mum thickness of between 30,000 and 40,000 feet, showed 



8 H. D. Rogers, Geology of Pennsylvania, vol. 2, p. 916, 185S. 



