OF THE MORE DISTURBED ZONES OF THE EARTH'S CRUST. 469 



d. In sucli groups of alternating cleavable and non-cleavable beds, the cleavage 

 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. 



e. 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. 



b. 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. 



c. 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. 



e. The hypothesis of the origin of flexures from a sinking of the ground by re- 



