DEFORMATION OF ROCKS I 79 



hand the strongest, brittlest rocks in the deepest zone observable 

 may be partly deformed by complex fracturing along intersect- 

 ing shearing planes, but, however, without spaces between the 

 particles. In the deepest seated zone the fracturing of the min- 

 eral particles may be so uniformly distributed as to give slio-ht 

 undulatory extinction only, the ultimate particles between which 

 differential movements have occurred or differential stresses are 

 at work not being discriminated as such even with the most pow- 

 erful objective. However, in some of the deformed strong rocks 

 even such stress effects as undulatory extinction are not marked, 

 and in this case, the material must have been largely released 

 from strain, just as in the case of viscous liquids which for a time 

 after deformation show stress effects, but which later free them- 

 selves from them. Such profound changes are believed to involve 

 recrystallization, water being the agent through which alteration 

 took place. 



It is also conceivable that where the deformation is very 

 slow, even strong, brittle rocks may be deformed by plastic 

 flow comparatively near the surface. But as shown by deep 

 tunnels, some of which have in places a superincumbent load of 

 rock a mile thick, if flow does occur, it is very slow indeed. 

 This, too, is in spite of the fact that a tunnel is substantially a 

 cylinder, very long in comparison to its width, and therefore 

 that if the stress amounts to one half of the elastic limit of the 

 rock, flowage would result.^ But in estimating the stress in the 

 case of tunnels, it is to be considered that the mountain mass 

 does not have vertical but sloping sides, and hence is really a 

 flat cone. While from the above it is clear that the superincum- 

 bent weight of thousands of feet of rock is not sufificient to 

 cause flowage in very strong rocks, it is equally certain that in 

 softer rocks, such as shale and coal, flowage occurs under much 

 less weight than this, as shown by the creeping and closing of 

 some galleries in mines, which at a depth of one thousand feet or 

 less, have from time to time to be cut out, so as to compensate 

 for the creeping flow which has tended to close them. 

 ' Loc. cit., (A), pp. 592 ; (B), p. 199. 



