RISE OF THE THEORY OF GEOSYXCLINES 155 



developed "a theory of mountains with the origin of mountains left out." 

 To this Le Conte added that Hall left "the sediments just after the whole 

 preparation had been made, but before the actual mountain formation 

 has taken place." ^ 



^ Hall's ideas of mountain origin, as we see his theory now, refer rather 

 to mountains of sculpture, since he held that their internal structure was 

 impressed upon them during the time of their sedimentary accumulation. 

 ^^It is certainly one of the great glories of American geology to have 

 clearly shown by the study of the Appalachian chain the immensity of 

 the work of erosion, and that the present sculpture owes its origin to this 

 cause alone." ^ 



HALL'S THEORY OF SYNCLINES AND CRUST AL FOLDING 



Putting together all of Hall's various statements, his theory as to the 

 making of the Appalachian Mountains is briefly as follows : 



Mountains occur only in areas of greatest sedimentary accumulation, 

 and never where formations are thin. A^Tiere the strata are thickest, 

 there they accumulated in shallow seas, and the whole subsiding area, 

 whether the sinking was gradual or periodic, always remained shallow. 

 A northeastern ocean, spreading southwestward into Canada and the 

 United States, gathered the detritals of Laurentia, and more especially 

 of Ap23alachia, and laid them down along the northwestern side of the 

 latter land, where the currents were strongest. This interior ocean, as 

 he called it, extended westward to the Rocky Mountains. In the Appa- 

 lachian seaway, he said, the Paleozoic formations are j)ossibly ten times, 

 and certainly six times, thicker than the equivalent deposits of the same 

 seas in the Mississippi Valley. In consequence the area of greatest sedi- 

 mentary accumulation formed a very long and comparatively narrow 

 mass of stratified rock having eventually the general structure of a vast 

 and very deep syncline that lay closely adjacent to an eastern land which 

 rose periodically in compensation for the sinking syncline. 



During the accumulation of the sediments in the syncline, according 

 to Hall, the bottom strata would gradually become stretched and rent 

 with fractures, while from time to time in the upper or younger deposits 

 a folding movement would take place, making land areas which sooner or 

 later were eroded to sealevel and may or may not have become buried 

 beneath subsequent seas. 



Hall distinctly stated that the folding of strata seemed to him to be 



^ Joseph Le Conte : On the formation of the features of the earth surface. Amer. 

 Jour. Sci. (3), vol. 5, 1873, p. 450. 

 * Le Conte : Op. cit., p. 451. 



