1 02 Mr. T. Slerry Hunt 



sure 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 yield- 

 ing strata. Lyell had already in speculating upon the results of a 

 cooling and contracting sea of molten matter, such as he imagined 

 might have once underlaid the Appalachians, suggested that the in- 

 cumbent flexible strata, collapsing in obedience to gravity would 

 be forced, if this contraction took place along narrow and paral- 

 lel zones of country, to fold into a smaller space as they con- 

 formed 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 Herschel's theory of subsidence and Lyell's of 

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

 foldings presented by the Appalachians. The sinking 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 anticlinal axes, which 

 must gradually decline toward the margin of the great synclinal 

 axis. This process the author observes appears to furnish a satis- 

 factory explanation of the difi'erence of slope on the two sides of 

 the Appalachian anticlinals, where the dips on one side are uni- 

 formly steeper than on the other, p. 71. 



An important question here arises, which is this ; — while admit- 

 ting with Lyell and Hall that parallel foldings may be the result 

 of the subsidence which accompanied the deposition of the Appal- 

 achian sediments, we inquire whether the cause is adequate to 

 produce the vast and repeated flexures presented by the Alle- 

 ghanies. Mr. Billings in a recent paper in the Canadian Natu- 

 ralist (Jan. 1860), has endeavoured to show that the foldings thus 

 produced must be insignificant when compared with the great undu- 

 lations of strata, whose origin Prof. Rogers has endeavoured to ex- 

 plain by his theory of earthquake waves propagated through the 

 igneous fluid mass of the globe, and rolling up the flexible crust. 

 We shall not stop to discuss this theory, but call attention to another 

 agency hitherto overlooked, which must also cause contraction and 

 folding of the strata, and to which we have already alluded. (Am* 

 Jour. Sci.(2)xxx. 138.) It is the condensation which must take place 

 when porous sediments are converted into crystalline rocks like 



* Travels in North America, 1st visit, vol. i. p. *78. 



