494 W. J. McGee—Dynamical Geology. 
crust. Taylor and others have pointed out that any corrugations 
resulting from secular cooling of the terrestrial shell in combination 
with the stresses resulting from precession, nutation, retardation of 
axial rotation, etc., would tend to assume certain definite directions, 
and that these directions do not coincide with those of the mountain 
ranges actually existing nearly enough to give countenance to the 
hypothesis. Reade and others have recently found that tangential 
contraction due to secular cooling must have been confined to a 
limited shell, even thinner than the strata known to be corrugated ; 
and it might be demonstrated that the concentration of montanic 
corrugation along certain lines, leaving vast intervening areas quite 
undisturbed, does not agree with the hypothesis, and could not occur 
in accordance with it under any conditions of rigidity and internal 
friction of the rocks which it is reasonable to assume—the arches 
are too long and rest too heavily upon the terrestrial nucleus to 
convey crushing strains to their extremities without greater com- 
pression about their keystones. While therefore the “ contraction 
theory” is so conditioned as to explain antecedent deformation, it 
cannot be accepted as a quantitatively adequate cause of such 
movement. 
There is another hypothesis of earth-movement frequently re- 
garded as alternative with the last. Geologists and physicists have 
long ageed that lines of mountain growth usually coincide with 
zones of rapid deposition during former times, and that in these 
zones the deposition was accompanied by depression (thus fore- 
shadowing the later conception of consequent diastatic movement) ; 
they have agreed further that in consequence of the combined 
sinking and thickening of the crust, the couches of equal temperature 
within the earth—the isogeotherms—must rise in the sediments 
until strata formed at the temperature of the sea-bottom are heated 
to hypogeal temperatures; and they have inferred that the con- 
sequent expansion of sediments developed stresses whereby further 
heat was generated, and that the rocks were thus flexed, corrugated, 
and sometimes metamorphosed. ‘This hypothesis, in one form or 
another, has had currency for a generation. It has indeed been 
questioned whether the assumed cause is commensurate with the 
effect—whether the expansion of sedimentary beds by local rise of 
isogeotherms from time to time and from place to place is sufficient 
to explain the extensive and profound corrugation observed in the 
mountains of the earth, the enormous shortening of the Alpine arc as 
observed by Heim and others, and the shortening of the Appalachian 
arc by 60 miles as computed by Claypole; but Reade has quite 
recently pointed out what the early advocates of the hypothesis had 
overlooked, viz. that since the strata are confined laterally, any ex- 
pansion due to rise of temperature must take place vertically, so 
that a given rise of temperature would produce thrice the elevation 
and perhaps thrice the corrugation inferred by the older geologists ; 
and the hypothesis has thus been rendered more acceptable.’ But 
1 Singularly, Reade and all his predecessors neglected an important (in the 
judgment of the writer the most important) factor in the movements contemplated 
