W. J. MceGee—Dynamical Geology. 493 
upon gradational transfer of matter—that the earth is in a condition 
of isostacy, and that as the rains bear detritus from the mountains 
into the seas, the unloaded mountains rise and the loaded sea- 
bottoms sink, and thus that a part of the deformation of the outer 
shell of the earth and certain volcanic movements are consequent 
upon the processes of gradation. Although at first blush it might 
appear that the great problem of earth-movements is solved by this 
discovery, consideration shows that these consequent diastatic and 
volcanic movements are but the indirect result of antecedent 
movements for which no adequate cause has been assigned (unless 
the “contraction theory ” be accepted) ; for it is evident that 
if deformation were dependent solely upon transfer of sediments, it 
would diminish with the lapse of time, that the mechanism of 
mountain elevation would soon become clogged by increasing friction, 
and that the terrestrial surface would quickly be graded so completely 
that further movement would cease; yet the rocks record diastatic 
activity throughout geologic time, now increasing, now diminishing, 
but on the average probably increasing rather than diminishing, and 
perhaps as potent to-day as during any past time. So the pro- 
cesses by which heteromorphism are produced are separable into two 
classes—that depending upon transfer of sediments, which may be 
designated consequent, and that for which adequate cause has not 
yet been assigned, which may be called antecedent. Discriminated 
upon a different basis they fall into classes approximately but not 
exactly coinciding with these, viz. orogenic, or mountain-making 
movements, and epeirogenic,’ or continent-making movements. 
Now it is only within a decade that diastatic and volcanic move- 
ments of the consequent class have been separated from the 
primary class; even yet there are geologists who do not recognize 
the distinction ; and so most of the hypotheses thus far framed to 
explain the deformation of the terrestrial shell rest upon the explicit 
or implicit assumption that all the movements involved belong to 
the class here called antecedent. The appositeness of the definitions 
therefore needs illustration. 
A primary hypothesis ascribed the corrugation of the terrestrial 
crust to contraction of the interior of the earth accompanying 
secular cooling more rapid than that of the exterior shell. The 
common conception of the mechanism of this process was familiarly 
illustrated by likening the corrugated globe to a withered apple, and 
the inequalities of the terrestrial surface to the wrinkles on the 
apple’s skin; and to the surprise of most American geologists, at 
least, this hypothesis has been prominently advocated within a year 
or two. Fisher and others have shown, however, that the postu- 
lated cause is far from commensurate with the observed effect—that 
even upon the most liberal estimates of radial contraction due to 
secular cooling, the concomitant tangential contraction would scarcely 
produce a tithe of the observed corrugation of the terrestrial crust. 
Dutton maintains that equitable contraction of a spheroid would not 
tend to produce corrugations such as those characterizing the earth’s 
1 A term proposed by Gilbert. 
