ON CLBAVAGfi AND FOLIATION IN ROCKS> 393 



§ 8. Secondary Cleavage of Slate. 



It is difficult to break slates of the usual thickness (about ^th of an inch) 

 so as to produce surfaces even rudely rectangled to the plane of cleavage ; a 

 circumstance which need occasion no surprise. But in this respect two 

 lines may be chosen in the slate, along one of which the rudely perpendi- 

 cular fracture may occasionally be looked for; this is the line of dip, — on 

 the other it can hardly be produced even with the utmost care ; this is 

 parallel to the strike. In experiments for this purpose, it should be observed 

 whether the surfaces produced by fracture on lines parallel to the strike 

 tend to parallelism. If a sheet of slate be laid on two supports parallel to 

 and equidistant from the strike edges, it may be found that at one of these 

 edges fracture will be more easy than at the other. Then turn over the 

 slate to see if the facts will be reversed, and the other edge give the easiest 

 fracture. [An observation in the affirmative is in my note book, for 1836, 

 at Llanberis. I shall be glad to know if it has been noticed by others in 

 this or other localities.] 



Hence it appears probable, that besides the principal cleavage, some slates 

 contain a secret lamination, or ' secondary cleavage,' which occasions a par- 

 tial fissility ; but in general this kind of structure produces no such distinct 

 appearances in the blocks and masses as to be often recognized on a great 

 scale. Some cases in which I had supposed such a structure to be real and 

 important, turned out on further research to be merely examples of symme- 

 trical jointing. Prof. Sedgwick, however, refers me for satisfactory instances 

 to the old black slates of Buttermere, and to the vicinity of Yspytty Evan, in 

 North Wales. 



One of the cases in which a second set of cleavage planes was supposed to 

 cross the principal cleavage frequently and regularly, is the " pencil bed " 

 of Skiddaw slate dug in Westmoreland, near Shap. Mr. Sharpe has exa- 

 mined this curious rock, and finds in one case (Thornthwaite Gill) the prin- 

 cipal cleavage parallel to the original beds and dipping N.W. 60° ; the 

 secondary cleavage crosses it nearly at right angles and dips S.E. between 

 20° and 30°. In another case (Rosgill Moor) the beds dip N.E. 30° ; the 

 principal cleavage N. by W. 60°; the secondary cleavage S. by E. 15°. By 

 natural decomposition, small square prisms are produced, whose sides mea- 

 sure one-quarter to half an inch across, and these may be sometimes split 

 again parallel to the faces. Mr. Sorby has found proof that this so-called 

 'secondary cleavage' is due to many small parallel joints. 



The following case occurred to me in North Wales, in 1836: — A sheet of 

 slate was excavated into a notch on one dip-edge, and the other struck by a 

 heavy tool on the opposite point (the plane of cleavage being held vertical) ; 

 it yielded along a zigzag line so as to show two sets of planes on the fracture 

 meeting each other at 90° + on the plane of cleavage, but with a common 

 edge oblique to the plane (70° and 110°). This I regard as a case of secret 

 jointing, and wish to know if any thing of the kind has been observed by others. 



§ 9. Relation of Cleavage to Joints, 

 The joints which traverse cleavage, in well-cleaved and massive slate rocks, 

 show much regularity for short spaces, and often present the same or nearly 

 equal angles of intersection. After examining and measuring innumerable 

 instances, I believe that this apparent symmetry is not delusive, and that by 

 a careful classification of joints with reference to the plane of stratification 

 and the plane of cleavage, some data of importance in the theory of their 

 origin may be obtained. An example of joints seen on a plane of stratifica- 



