ON CLEAVAGE AND FOLIATION IN ROCKS. 



391 



By inspection of this figure, the great tendency of a 

 mass so penetrated by secret fissures to split in planes ap- 

 proximately parallel is evident. This tendency may be 

 exhibited numerically for any particular angle of inclina- 

 tion to the plane of principal cleavage. 



" If we suppose (says Mr. Sorby) that in a mass of 

 rock thei'e were 600 particles having their longer axes 

 lying in the space included within 5° on each side of 

 positions inclined at 0°, 10°, 20°, &c. to the line of pressure, 

 so that they were uniformly distributed, as is nearly the 

 case in thick-bedded uncleaved rocks, then, after compres- 

 sion in the ratio 1 : 6, their distribution would be changed, 

 as shown in the following Table* : — 



Fig. 38. 



Inclination to the direction 

 of the pressure. 



Original 

 distribution. 



0° 



10 



20 



30 



40 



p„ J>-600 m each case.< 



60 



70 



80 



90 J I. 



Subsequent 

 distribution. 



. . . 100 



. . . 103 



... 113 



... 13* 



. . . 168 



. . . 236 



. . . 376 



. . . 733 



, . . 1825 



. . . 3324." 



These numbers exhibit the relative tendency to cleavage 

 in each arc of 10° in the compressed mass. If instead of 

 5° on each side of a given position we had assumed a very 

 small angle only, the tendency to fissility along the prin- 

 cipal cleavage plane, as compared to that perpendicular to 

 it in the line of strike, would have been as 36 : 1. 



The structure here assigned by calculation does actu- 

 ally occur in slaty rocks, but not in others. " The water 

 of Ayr stone, which has no cleavage, consists of mica and a very few grains 

 of quartz sand, imbedded in a large proportion of decomposed felspar ; the 

 peroxide of iron being collected to certain centres, and having the character 

 of peroxidized pyrites. The flakes of mica do not lie in the plane of the 

 bedding, but are inclined at all angles ; so that there is no definite plane of 

 structural weakness independent of that due to bedding." But in a rock of 

 similar composition having cleavage, a section cut perpendicular to cleavage 

 in the line of its dip, shows by far the greater part of the flakes of mica in- 

 clined at low angles, so that the majority lie within 20° on each side of it, 

 being most numerous in and nearly in the plane of cleavage, — twenty times 

 as many nearly in it as nearly in the plane of 45° to it, and very few at 90°. 

 In a section perpendicular to cleavage, and in the line of strike, there is still 

 a preponderance of flakes of mica in and near the plane of cleavage, but in 

 a less marked degree. On the plane of cleavage itself, a slight tendency to 

 arrangement of the flakes parallel to the line of dip is observable. 



One of the latest and most instructive of Mr. Sorby 's observations relates 

 to the cleavage of Devonian limestones. In a specimen from Kings Kerswell 

 near Torquay, the cleavage pressure has affected the whole mass of the rock, 



* Phil. Mag., January 1856. 



