ORIGIN AND DEVELOPMENT OF ORE DEPOSITS. 127 



square cm (say 600 lbs. per square inch), the escaping mass was 

 found to possess a schistosity perpendicular to the pressure, and 

 longrain parallel to the direction of its escape. When the clay 

 was free to escape in all directions, cleavage perpendicular to 

 the pressure was produced, but not longrain* 



The origin of slaty cleavage may be better understood by 

 reference to figs. 3 to 6, Plate IX, where the upheaved stratified 

 rocks are supposed to be unsupported or insufficiently supported 

 from below, and therefore in a state of unstable equilibrium. 



Under such conditions, gravity acting in the direction of 

 the arrow g., fig. 3, there will be a tendency for the stratified rocks 

 to slide down the junction a., and also down the beds b., c, d. 

 There will also be a tendency to "shear" the beds vertically 

 in directions parallel to the arrowy., all these tendencies being 

 resisted by the lateral pressures acting in the direction of the 

 arrow I. 



In fig. 4 the pressures are supposed to be relaxed sufficiently 

 to allow of movements along all the junctions, until finally 

 equilibrium is restored, while in fig. 5 the first stratified bed is 

 supposed to be of such a nature as to yield in its particles to 

 shear instead of sliding over the junction a. In such a case 

 the rock would become cleavable in directions between those of 

 the junction a., and the bed b., as indicated by the fine lines 

 in the fig. In fig. 6 the case of rocks whose particles do not 

 yield to " shear," and some of whose beds do not admit of a 

 sliding motion over each other, is illustrated. In such there will 

 be a tendency for master-joints to open at right angles to the 

 bedding, taking the shortest course across the beds as shewn at 

 x x. In his diagram fig. 4,f Mr. Fisher shows how the cleavage 

 angles may be calculated under various specified conditions. 



The distortions produced by the slaty cleavage have been 

 already dealt with. These are especially noticeable in the 

 specimens of spirifera disjuncta found so abundantly in the slates 



*Ed. Jannettaz, Clivages des Eoches, Bull. Soc. Geol. France, 1884. It is 

 worthy of remark that M. Jannettaz was able to trace the existence of longrain by 

 means of varying thermal conductivity even when it was not mechanically evident. 



*loc. cit. 



