OROGENY AND EARTH’S ROTATION 
67 
strains of deformation the action of the earth as a whole is as a 
body having the rigidity of steel finds qualification in the circum¬ 
stance that the translocation of matter within is mainly analogous 
to that taking place in a highly viscous liquid. With the excep¬ 
tion of the thin, outer, so-called crust all internal change of mass- 
position is necessarily by normal flow-movement. Only in the 
exterior film where the load pressure is less than that of the 
crushing strength of rock, and where rocks of different strengths 
alternate, do folding without rupture, and faulting without flexure, 
take place. It is the finely straticulate character of this surficial 
zone that enables deformative effects to be quantitatively meas¬ 
ured. To this layered cuticle are, of course, the chief investiga¬ 
tions of earth-students severely limited. 
In their larger relations the differences in tectonic behavior of 
rock-terranes under orographic stress in the zones of rock-fracture 
and rock-flow remain as yet to be carefully analyzed. Near the 
earth’s surface, in all situations where the superincumbent load is 
less than the crushing strength of the rock composing a given 
prism, plication appears to be not always accomplished by true 
bending of the rock-plates, but to be effected mainly by their cross¬ 
rupture and a readjustment of the separate fragments along 
angles new and slightly different from one another. This phase 
of compressive phenomena seems to find best exemplification in 
such mountain chains as the Appalachians. In others it seems 
to be wholly absent. 
To this feature in a large way, as displayed by the so-called 
Basin Range, or Block-mountain, structure, and the corrugated 
type of the Appalachians I have recently called attention as a 
possible explanation of certain fundamental differences in Amer¬ 
ican geotectonics. On the one hand, the general presence of hard- 
brittle formations throughout the great Paleozoic section seems 
satisfactorily to account for the crustal adjustment by means of 
faulting. On the other hand, the reverse lithologic conditions 
appear to offer a clue to the real reason for compressive expres¬ 
sion through means of notable folding of strata. 
Experimentally, this difference is clearly foreshadowed by A. 
Fabre,^® when, in attempting to reproduce the compressive condi¬ 
tions of the Alps, he places layers of clay on a stretched band 
30 Nature, Vol. XIV, p. 104, 1878. 
