SUMMARY. 89 



and the existence of undistributed faults are shown to arise in particular 

 cases of the formula. This formula is applicable only when the rupture 

 is not brought about by a very rapid strain. When the strain is impul- 

 sive it is shown that the interference of vibrations attending rupture 

 may cause further parallel ruptures. The suggestion is made that thick 

 slates and flags may possibly be due to plastic deformation attended by 

 vibrations. 



As jointing has been referred to tensile stress, rupture through tension 

 is discussed. It is shown that curved or broken lines, and not plane 

 partings, must result; and the columnar structure of lavas receives a 

 seemingly sufficient explanation. 



The last portion of the paper is occupied by a review of the theories 

 and observations on jointing and slaty cleavage. It is maintained that 

 joints are always attended by macroscopic or microscopic faults, and that 

 they are closely allied to slaty cleavage. The ascription of slaty struc- 

 ture to the presence of deflected mica scales and flattened particles is 

 pronounced unsatisfactory. Glass, wax and other substances in which 

 slaty cleavage has been artificially produced can hardly owe their cleavage 

 to such a distribution of flat particles, while sedimentary rocks in which 

 the fiat particles are mostly parallel to the bedding do not show slaty 

 cleavage. 



Analysis of certain well-known experiments and of some made for 

 this paper shows that artificial slaty cleavage is always attended by rota- 

 tional strains, such as those to which slaty cleavage is ascribed above. 

 The theory of this paper (that slate is due to pressures inclined at small 

 angles to the cleavage plane and standing at right angles to the grain of 

 the slate) is shown to account for grain, " side " and " end," for elongation 

 of fossils in the direction of the grain, contraction in the cleavage plane 

 at right angles to the grain, and for master joints which intersect 

 the cleavage plane along the grain and make a large angle with this 

 plane. 



The most important result of the investigation is that jointing, schis- 

 tosity and slaty cleavage all imply relative movement, and are thus as 

 truly orogenic as faults of notable throw. They may all be regarded ns 

 orogenically equivalent to distributed faults. The great number of joints 

 and planes of slaty cleavage compensates for the minute movement on 

 each, and the sum of their effects is probably at least as important as 

 that of the less numerous faults of sensible throw. 



In the light of this conclusion it appears that if one could reproduce 

 the orogenic history of the Sierra in a moderate interval of time on a 

 model made to a scale of one mile to the inch, it would seem to yield 



