V.] THE ORIGIN OF MOUNTAINS. 55 



the variations of local pressure acting upon the yielding crust 

 of the earth, agreeably to the view of Sir John Herschel. 

 This subsidence of the ocean's bottom would, according to 

 Mr. Hall, cause plications in the soft and yielding strata. 

 Lyell, in speculating upon the results of a cooling and con- 

 tracting sea of molten matter, such as he imagined might have 

 once underlaid the Appalachians, had already suggested that 

 the incumbent flexible strata, collapsing in obedience to grav- 

 ity, would be forced, if this contraction took place along 

 narrow and parallel zones of country, to fold into a smaller 

 space as they conformed to the circumference of a smaller arc, 

 " thus enabling the force of gravity, though originally exerted 

 vertically, to bend and squeeze the rocks as if they had been 

 subjected to lateral pressure." * 



Admitting thus HerscheFs theory of subsidence and Lyell's 

 theory of plication, Mr. Hall proceeds to inquire into the great 

 system of foldings presented by the Appalachians. The sink- 

 ing along the line of greatest accumulation produces a vast 

 synclinal, which is that of the mountain ranges, and the result 

 of such a sinking of flexible beds will be the production within 

 the greater synclinal of numerous smaller synclinal and anti- 

 clinal axes, which must gradually decline toward the margin 

 of the great synclinal axis. This process, the author observes, 

 appears to furnish a satisfactory explanation of the difference 

 of slope observed on the two sides of the Appalachian anticli- 

 nals, where the dips on one side are uniformly steeper than 

 on the other, (p. 71.) 



An important question here arises, which is this : while 

 admitting with Lyell and Hall that parallel foldings may be 

 the result of the subsidence which accompanied the deposition 

 of the Appalachian sediments, we inquire whether the cause 

 is adequate to produce the vast and repeated flexures presented 

 by the Alleghanies. Mr. Billings, in a recent paper in the 

 Canadian Naturalist (Jan., 1860), has endeavored to show 

 that the foldings thus produced must be insignificant when 

 compared with the great undulations of strata ; whose origin 

 * Travels in North America, First Visit, Vol. I. p. 78. 



