18 WALTER H. BUCHER 
must, therefore, have been developed essentially through irrotational 
strain. | 
The differential movement between adjoining beds required by 
the structural relations indicated in the text, was obviously largely 
limited to slippage parallel to the bedding within the thin slaty 
layers. It is important to note, however, that the direction of 
movement here, as in several cases referred to in the first part of 
this paper, has influenced the number and nature of the two 
opposed diagonal joint systems in the brittle beds. Those inclined 
in the direction of drag produced by the differential movement are 
more numerous, closed, and slickensided, while the opposite set is 
represented by few and gaping joints. 
The closely spaced joints in the slaty beds, on the other hand, 
may partly be due to true rotational strain. 
Essentially the same considerations apply to the case discussed 
in detail by Steidtmann' and by Leith in his Structural Geology. 
Here the joint system opposed to the ‘‘drag”’ action of the differ- 
ential movement between the beds is represented by but a few 
“‘open gashes or tension joints.” But their presence is sufficient 
to indicate that in the center of the quartzite beds the effect of 
rotational strain has been entirely subordinate to that of the 
irrotational strain produced by the component normal to the 
bedding planes. 
The writer has the suspicion that this will be found to be true 
most generally in bedded rocks, in which differential movement 
between the beds is largely accomplished by slippage along 
bedding planes chiefly within layers of soft rock acting as lubricants. 
V. PLANES OF SHEARING IN SHALES 
The absolute and relative values of the crushing and tensile 
strengths of a rock play a fundamental roll in the deformation of 
rocks along the face of natural and artificial excavations.? 
When the depth of an excavation has reached the point at which 
the vertical component of the stress set up by the removal of 
t Jour. Geol., Vol. XVIII (1910), p. 261, Fig. r. 
2 See the excellent analysis of the factors involved in D. F. MacDonald, ‘‘Excava- 
tion Deformations,” Congrés géologique international, Compte rendu de la XIIe session 
(Canada, 1913), pp. 779-92. 
