430 WILLIAM BARTON ROGERS. 



By far, however, the most memorable contribution of Professor 

 Rogers to geology was that made iu connection with Henry D. Rogers, 

 in a paper entitled " The Laws of Structure of the more Disturbed 

 Zones of the Earth's Crust," presented by the two brothers at the 

 meeting of the Association of American Geologists and Naturalists, 

 held at Boston in 1842. This paper was the first presentation of what 

 may be called in brief the Wave Theory of Mountain Chains, This 

 theory was deduced by the brothers Rogers from an extended study 

 of the Appalachian chain in Pennsylvania and Virginia, and was sup- 

 ported by numerous geological sections and by a great mass of facts. 

 The hypothesis which they offered as an explanation of the origin of the 

 great mountain waves may not be generally received ; but the general 

 fact that the structure of mountain chains is alike in all the essential 

 features which the brothers Rogers first pointed out, has been con- 

 firmed by the observations of Murchison in the Ural, of Darwin in 

 the Andes, and of the Swiss geologists in the Alps. " In the Appa- 

 lachians the wave structure is very simple, and the same is true in all 

 corrugated districts where the crust movements have been simple, and 

 have acted in one direction only. But where the elevating forces have 

 acted in different directions at different times, causing interference of 

 waves like a chopped sea, as in the Swiss Alps and the mountains of 

 Wales or Cumberland, the undulations are disguised, and are with 

 extreme difficulty made out." The wave theory of mountain chains 

 was the first important contribution to dynamical and structural geology 

 vrhich had been brought forward in this counti-y. It excited at the 

 time great interest, as well from the noveltv of the views as from the 

 eloquence with which they were set forth ; and to-day it is still re- 

 garded as one of the most important advances in orographic geology. 



A marked feature of mountain regions is that rupturing of the strata 

 called faults ; and another of the striking geological generalizations of 

 the brothers Rogers is what may be called the law of the distribution 

 of faults. They showed that fiiults do not occur on gentle waves, 

 but in the most compressed flexures of the mountain chains, which 

 in the act of moving have snapped or given way at the summit where 

 the bend is sharpest, the less inclined side being shoved up on the plane 

 of the fault, this plane being generally parallel to, if it does not coincide 

 with, the axis plane ; and, further, that " the direction of these faults 

 generally follows the run of the line of elevation of the mountains, the 

 length and vertical displacement depending on the strength of the dis- 

 turbing force." 



The last of the general geological results to which we referred 



