OF THE MORE DISTURBED ZONES OF THE EARTH’S CRUST. 469 
d. In such groups of alternating cleavable and non-cleavable beds, the cleavage 
i 
i} 
é. 
planes curve from the normal dip they possess to approach to a parallelism 
with the planes of separation of the strata as they near their surfaces. 
The cleavage susceptibility is alternately greater and less in closely adjacent 
parallel planes. 
The ribbon structure of glaciers is probably analogous to the cleavage struc- 
ture of argillaceous rocks. 
8. Foliation. 
In districts of crystalline, metamorphic, or gneissic strata, not much disturbed 
or corrugated, the foliation generally coincides with the stratification. In 
regions much corrugated the foliation, on the contrary, is often at a steep 
angle to the stratification, and shows a tendency to dip, as cleavage does, paral- 
lel to the axis planes of the flexures. Generally the direction of the foliation 
appears to conform to that which the waves of heat metamorphosing the rocks 
would take in slowly flowing through them. 
9. Theories of elevation. 
a. A common hypothesis of the cause of the elevation of strata is that of a wedge- 
like intrusion of melted matter. But this implies a function in semifluid or fluid 
matter incompatible with the dynamic properties of liquids. Some force must 
have first cracked the strata before the molten rock could insert itself. Veins 
and dykes tapering upward do not belong to lines of anticlinal elevation, where 
geologists so frequently indicate them, but to synclinals or concave curves. 
The kindred idea of the intrusion of igneous rocks in solid wedges separating 
and lifting the crust is also at variance with sound mechanical laws. To exert 
this lifting and thrusting force, the assumed wedges must have moved freely 
through the fissures they fill; but we see no proofs of discontinuity between 
the igneous and stratified rocks, but evidences of the closest cohesion. 
. A modified view of the wedging up of the flexible strata, conceives them to have 
been simply carried up by the lifting of the igneous nucleus. Such movements 
have no doubt occurred, and have served to steepen the strata leaning against 
the igneous rocks, but they cannot have corrugated the strata, which would be 
rather stretched than compressed by the elevation. 
d. The hypothesis of a simple upward pressure at points or lines in the crust, 
which does not include an explanation of the wave structure of disturbed dis- 
tricts, cannot be a true theory ; it must show how the pressures have shifted to 
new and parallel lines, and lines constantly receding, and also show why, if 
the linear pressures were simultaneous, they should not have produced a wide 
general arching, rather than a series of contiguous sharp waves. 
. The hypothesis of the origin of flexures from a sinking of the ground by re- 
