American Geology, by T. S. Hunt. 411 
molten matter, such as he imagined might have once underlaid 
the Appalachians, suggested that the incumbent flexible strata, 
collapsing in obedience to gravity would be forced, if this con- 
traction took place along narrow and parallel zones of country, 
to fold into a smaller space as they conformed to the circumfer- 
ence of a smaller are, “ 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 
oldings 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 satisfactory 
explanation of the difference of slope on the two sides of the 
ppalachian anticlinals, where the dips on one side are uniformly 
steeper than on the other. p. 71 
n important question here arises, which is this;—while ad- 
mitting 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 ade- 
the Alleghanies. Mr. Billings in a recent paper in the Canadian 
Naturalist (Jan. 1860), has endeavored to show that the folding 
which we have already alluded (Am. Jour. Sci. [2], xxx, 188). 
It is the condensation which must take place when porous sedi- 
ments are converted into crystalline rocks like gneiss and mica 
slate, and still more when the elements of these sedimentstare 
changed into minerals of high specific gravity, such as 2 ames 
garnet, epidote, staurotide, chaistolite and chloritoid. ‘his con- 
traction can only take place when the sediments have become 
deeply buried and are — metamorphism, and is, as 
many attendant phenomena indicate, conn with a soften 
and yielding condition of the lower strata. — 
’ We have now in this connection to consider the hypothesis 
which ascribes the corrugation of portions of the earth’s crust to 
* Travels in N. America, Ist visit, vol. i, p. 78. 
