lyS REPORTS ON INVESTIGATIONS AND PROJECTS. 



deduction from the present imperfect data which places the shearing 

 zone from 3 to 5 miles beneath the surface is in fair accord with the 

 deduction of Hoskins, which places the limit of resistance to crush- 

 ing at about 6 miles. 



It is inferred from the nature of the case that the shear horizon is 

 not a definite plane, but rather a zone of some depth which is affected 

 by more or less distributive shear, accompanied b}^ drag and foliation, 

 the phenomena being analogous to distributive faulting and attend- 

 ant drag and foliation. The shear zone is thus conceived to involve 

 the development of foliation and schistosity through a considerable 

 depth. Ideally each individual foliation plane is assumed to cross 

 the .shear zone by a double curve whose middle portion is nearly par- 

 allel to the earth's surface and coincident with the main plane of 

 shear. Generalized, this grows into the conception of a nearly or 

 quite universal zone of foliation or schistosit}- enveloping the body 

 of the earth beneath the superficial shell, for though the shearing at 

 any specific time would involve an appreciable motion in only a por- 

 tion of the shell, the shearings of different times in geological history- 

 taking place under different conditions and from different centers of 

 movement are assumed to have given the shear zone a nearly or quite 

 universal extent. 



This conception is enforced by a study of the distribution of the 

 folded tracts of any great period of deformation. The.se do not 

 seem to be distributed throughout the earth generally, or at frequent 

 inter\'als, which would involve many small foldings and limited 

 shear, but rather to be concentrated in a few great tracts which 

 involve a common shear movement for large sections of the shell. 

 This is confirmed by the great extent of the shear that is implied 

 by the amount of the folding. If the conception of concentration 

 from wide areas be not entertained, the necessary alternative seems 

 to be the assumption of a very large total shrinkage and of more 

 tracts of folding than observation warrants. Under this alternative 

 also the total amount of deformation becomes embarrassingly large 

 from the dynamic point of view. 



The conception of a shear zone acting as a zone of accommoda- 

 tion between the superficial shell and the great body of the earth 

 leads on to two suggestive geological applications : 



(i) In the nature of the case, volcanic extrusions in pa.ssing from 

 beneath the shear zone to the surface may reasonably be supposed 

 to follow preferentially the flexed foliation planes of the shear zone 

 as the lines of least resistance. As this is a zone of cool rocks, the 

 lavas must be presumed to suffer more or less stiffening and lodgment 

 within it by progressive cooling. This lodgment should normally 



