68 G. F. BECK Kit — FINITE STRAIN IN ROCKS. 



volved in the formation of slate should ever be unattended by vibrations 

 of sensible amplitude.* 



Though this hypothesis of thick slates seems probable enough, I am 

 not able to offer a detailed explanation .of the process or to show under 

 what conditions thick slates would form rather than thin ones. 



Secondary Action on ruptured Rock. — It often happens that the pressure 

 which causes such systems of fissures as have been treated above is not 

 fully relieved by these ruptures and the small relative movements in- 

 separable from rupture. If pressure continues on the divided mass the 

 results will vary with circumstances. When the pressure is oblique and 

 the mass is divided only into sheets, faults of considerable throw may 

 take place. I have previously discussed the distribution of motion in 

 such a system.t It appears from the present investigation that a simple 

 fault arises from pure scissive stress, while distributed faults are due to 

 pressure combined with scissive stress, or, in other words, to oblique 

 pressure. Thus a distributed fault is much the more general case and, 

 in my observation, much the more common. A solitary fault is an ex- 

 treme and very special case of a distributed fault. 



At the instant of rupture there is very little normal pressure between 

 the sheets or blocks of rock, but as rotation progresses the pressure tends 

 to increase. Extensive movements are therefore accompanied by a forci- 

 ble grinding of the fragments against one another. At first the stresses 

 called into play are but little inclined to the surfaces, and the result is 

 often to produce slaty structure close to the surfaces of the fragments. 

 I have observed very many cases in which large blocks of granite showed 

 slaty cleavage close to the bounding surfaces which had evidently been 

 produced in this way. The cleavage was certainly in part due to close 

 jointing, but in mosl cases the cleavage faded out at some little distance 

 from the edge of the mass, and the inner portion of the slaty selvedge 

 must therefore have arisen from strain without rupture (Heim's Um- 

 formung ohne Bruch). Such slaty selvedges thus furnish important 

 evidence as to the manner in which slate is formed in nature, evidence 

 entirely accordant with that afforded by experiment. 



When the secondary pressure becomes more nearly perpendicular to 

 the faces of the sheets of rock, these may themselves be divided into 

 secondary sheets, and a continuance of the process will reduce the rock 

 to a confused rubble. 



Effect of tensile Stresses. — Jointing has been referred to tensile stresses 

 by several authors. It is therefore desirable to examine what effects 

 tensile si ress can have upon homogeneous substances. To give abstract 



See Am. Journ. Sci., vol. xxxi, 1886, page 1 1 5. 

 f Geology of the < omstock Lode,U. - I. Surv., monograph iii, chapter 4. 



