DIASTROPHISM AND THE FORMATIVE PROCESSES 417 
and much crustal shorting indicate the deformation of a thin shell; 
otherwise the resulting upward bulging would be enormous. (6) 
Open, gentle folding may signify either a thin shell or a thick shell, 
according as there has been little or much upbowing. (c) For a 
given crustal shortening, the greater the vertical uplift the thicker 
the shell which has been actively deformed. 
The detection of the wedgelike shape of the deformed mass 
led to a consideration of the nature of the plunging planes which out- 
line the block. If the wedge was defined by fracturing, the borders, 
as shown by the Daubrée experiment,’ by the familiar crushing- 
strength tests upon building stones, as also by an analysis of the 
stress-strain relations, should be fault planes dipping beneath the de- 
formed block at angles in the general vicinity of 45° though in most 
cases somewhat less. ‘This is the result to be expected in a case of 
non-rotational strain, in which the axes of strain do not change posi- 
tion with respect to the axes of stress. If the developing strain be 
rotational in character, the angles of the shearing planes will be 
lowered from 45°, in proportion to the extent of the rotational 
element.? If, on the other hand, definite shearing planes do not 
develop, and the deformation is largely by folding, it is possible 
that the folding dies out below by affecting successively narrower 
and narrower belts. Though such a process would make the 
deformed block taper downward, just as in the preceding case, 
the borders would be much less sharply defined. With increas- 
ing resistance to deformation with increasing depth, in accordance 
with the results of the experimental work of Adams and his 
colleagues,? it seems mechanically logical that folds should die out 
in this fashion. But whatever the nature of the bordering zones 
of accommodation may be, the results of the computations in the 
case worked out show that the folded tract becomes narrower 
*G. A. Daubrée, Etudes synthéliques de géologie expérimentale, T.1., p. 316, Plate II. 
2R. T..Chamberlin and W. Z. Miller, ‘‘Low Angle Faulting,” Jour. Geol., Vol. 
XXVI (1918), pp. 1-44. 
3 Frank D. Adams, ‘‘An Experimental Contribution to the Depth of the Zone of 
Flow in the Earth’s Crust,” Jour. Geol., Vol. XX (1912), pp. 97-118; F. D. Adams 
and J. A. Bancroft, ‘‘On the Amount of Internal Friction Developed in Rocks during 
Deformation, and on the Relative Plasticity of Different Types of Rocks,” Jour. Geol., 
Vol. XXV (1917), pp. 597-637. 
