188 A CENTUKY OF SCIENCE 



through a wholly new branch of knowledge, demonstrated 

 to be true. 



Nevertheless, all students of orogeny are agreed that 

 profound compressive forces have been the chief agents 

 in developing mountain structures. Chamberlin was 

 the first to arrive at the idea that the shrinkage may 

 originate in the deeper portions of the earth under the 

 urgency of the enormous pressures, apparently by giving 

 rise to slow recombinations of matter into denser 

 forms. 17 



The New Era in the Interpretation of Mountain Structures. 



In the meantime, between 1874 and 1904, another 

 advance in the knowledge of mountain structures was 

 taking place in Europe. Suess studied the distribution 

 of mountain arcs over the earth and dwelt upon the 

 prevalence of overthrust structures ; the backland being 

 thrust toward and over the foreland, the rise of the 

 mountain arc or geanticline depressing the foredeep or 

 geosyncline. Bertrand and Lugeon from 1884 to 1900 

 were reinterpreting the Alpine structures on this basis. 

 They showed that the whole mountain system had been 

 overturned and overthrust from the south to an almost 

 incredible degree. Enormous denudation had later dis- 

 severed the northern outlying portions and given rise to 

 "mountains without roots,"- isolated outliers, consist- 

 ing of overturned masses of strata which had accumu- 

 lated as sediments far to the southward in another por- 

 tion of the ancient geosyncline. 



On a smaller scale similar phenomena are exhibited in 

 the Appalachians. Willis showed that the deep subsi- 

 dence of the center of the geosyncline gave an initial dip 

 which determined the position of yielding under compres- 

 sion. Laboratory experiments brought out the weakness 

 of the stratigraphic structure to resist horizontal com- 

 pression. The nature of the stratigraphic series was 

 shown to determine whether the yielding would be by 

 mashing, competent folding, or breakage and overthrust. 

 The problem of mountain structures was thus brought 

 into the realm of mechanics. These results were pub- 

 lished in three sources in 1893, the Transactions of the 



