186 Geological Results of the Earth’s Contraction. 
VI. The folding of strata by subsidence of the plicated region 
can be only of small extent.—This subsidence may or may not 
be attended by a general contraction of the earth’s crust below 
ing, and the result could be accomplished only by fractures and 
openings. ‘The material moreover would be drawn off from the 
summits of the convexities, or very much thinned out in those 
parts; a supposition not warranted by facts to the extent required. 
in the explanation. The hypothesis moreover would not account 
for the greater steepness of the northwest slope. 
But if the material beneath may be supposed to have contracted 
correspondingly with the amount of plication, then folds might 
have been produced by the process. The hypothesis however 
has many weighty objections. It is at variance with the fact 
that this same region remained unplicated, at least in the parts oc- 
cupied by the coal formation, till after the coal epoch, although 
the contraction must have been more rapid during the preceding 
epochs of the earth’s cooling.* The non-plication of the Siluri- 
an rocks of the centre of our country, adds force to this objection. 
Why this long delay in the action of those violent forces suppo- 
sed to be imprisorfed beneath the earth’s crust ? 
Farther, a stiffened crust cannot be much folded by mere 
shrinkage, where the material is like that of the earth’s crust. 
The fact that the Silurian rocks of the interior are not plicated 
by contraction below them, is evidence of this. Instead of be- 
coming plicated, they have probably aided by lateral action in 
producing the elevations on the east or west, or the Ozark Moun- 
tains or other heights intermediate. " 
Moreover, the very close compacted folding illustrated in figs. 
4 and 6, a result which only lateral pressure could effect. 
VII. Position of voleanoes.—The occurrence of volcanoes 
mostly in the neighborhood of the sea, is a necessary result of 
these principles. For we have already stated that fractures of the 
earth would be likely to take place near the limits between the 
contracting and non-contracting areas:+ here they would hav 
that depth and extent which is necessary in order that they should 
remain open as the seat of perpetual eruptions; for there is ne- 
cessarily a wide difference as regards extent between those fissures 
* The writer has offered as an explanation of this non-plication till after the 
coal epoch, the suggestion that the crust over the oceanic (or igneous) portions, 
had so far cooled by that time, that the pressure or strain arisin contraction 
was no longer relieved to the same extent as before by rents and upliftings over 
the ign ion. This lateral action was exerted long previously, but its 
est effects on the earth’s features date subsequently to the carboniferous epoc 
t This volume, page 96. 
