72 G. F. BECKER FINITE STRAIN IN ROCKS. 



of no serious attempt to deal with the mechanics of the subject prior to 

 a paper already cited, in which I discussed those cases of rupture in 

 which the deformation could properly be regarded as infinitesimal. The 

 results there reached have been born out by further observation. In 

 this paper the investigation has been extended to cases of faulted fissures 

 in which deformation of the rock is finite, and the conclusions certainly 

 accord with very numerous observations which I have made in the Sierra 

 Nevada of California and elsewhere, nor are any facts known to me which 

 seem in conflict with the theory presented. 



On the other hand, jointing and slaty cleavage have been much dis- 

 cussed. Some authorities regard them as closely allied, while others 

 refer them to radically different causes. Many experiments have been 

 made on slaty cleavage and various theories have been propounded to 

 account for it. The theory here advanced is new, and I may say that it 

 is a surprise to myself. I have long felt that the theory which refers 

 slaty structure to a pressure perpendicular to a fixed plane of resistance 

 and parallel to two lateral constraining planes was unsatisfactory. The 

 combination seems too artificial. The chances against its occurrence 

 seem too large when the frequency of slaty structure is considered. I 

 did not anticipate, however, that analysis would show so large a range 

 of conditions under which slaty structure might result, and I entertained 

 the idea that if a slanting force produced slaty cleavage the force would 

 slant in the direction of the grain of the slate. I have been led by purely 

 algebraical reasoning to believe that the force may be inclined to the 

 fixed plane within very wide limits, covering at least 60°, and that in 

 all cases the plane of the force is at right angles to the grain of the 

 slate. 



Under these circumstances it is absolutely necessary to pass in review 

 the principal theories hitherto advanced and to compare the new theory 

 with the results of experiment and observation. 



Origin of Jointing. — Joints are commonly nearly plane surfaces dividing- 

 rock masses and arranged in groups, the members of which are parallel 

 to one another. Fault fissures are also frequently arranged in similar 

 groups, and show similarly flat surfaces, the term joint being employed 

 when the amount of relative motion on the divisional surfaces is imper- 

 ceptible or is regarded as negligible. Jointing has been referred to ten- 

 sile stresses by distinguished authorities, including W. Hopkins, but the 

 correctness of this reference lias been questioned. Thus, Professor W. 

 King* after special investigation, stated his opinion that, in their original 

 condition, the walls of joints were in close contact, and protested against 

 the classification of columnar structure with jointing. Mr G. K. Gil- 



*Trans. K. Irish Acad., vol. 25, 1875, p. 605. 



