424 TERENCE T. QUIRKE 



All these researches seem to indicate that the zone of fracture, 

 folding, and of deformation in general which may be called plastico- 

 frangible is probably confined to depths which properly may be 

 called shallow. 



T. Mellard Reade^ used the terms sheet or strata-plate to 

 describe the deformable parts of the earth's crust, and Chamberlin 

 and Miller^ suggest that the deformation of some folded mountain 

 chains is perhaps analogous to the deformation of a thin prism or 

 wall. 



Probably few geologists would maintain that the frangible 

 part of the earth's crust is more than fifteen miles deep. 



The probability of cruslal members as such. — Thus, the crustal 

 members are conceived to be sheets conforming to the curvature 

 of the earth, from 200 to 300 times as wide and long as they are 

 thick. Their proportions may be compared to those of a pave- 

 ment one foot thick covering a city block about three hundred 

 feet square. This conception of strata-plate members of great 

 length and width might appear unreasonable were it not for the 

 fact that each member is not an unsupported arch. Each member 

 is very appreciably stiffened by the support of the plastico-rigid or 

 elastico-rigid mass beneath. 



A consideration of the probable effects of the yielding of such 

 members is interesting. Supposing that there should be moun- 

 tain folding near the border of a continent, the yielding decreases 

 the stresses at the place of failure and throughout a certain dis- 

 tance on either side. Were the crust rigid instead of plastico- 

 frangible, the stresses would be relieved throughout the whole of 

 the neighboring sections, and the stress relief would be distributed 

 promptly throughout all the members of the earth's crust. Thus, 

 distribution of relief would tend to become world-wide, but the 

 plastic-like behavior of the members retards such a distribution of 

 stress and in some cases absorbs it within a relatively short dis- 

 tance. In such cases conditions for the continental mass may 

 include a relieved, small, residual stress on one side of the conti- 

 nent and a large, almost unrelieved stress on the other side of the 

 continent. This may result in a later break on the unrelieved 



' T. Mellard Reade, The Evolution of Earth Structure (1903), p. 134. 

 ^ Op. cit., p. 21. 



