1848.] SHARPE ON SLATY CLEAVAGE. 115 



sition offer greater resistance than along the two directions just men- 

 tioned, but the greatest resistance will be along the plane a bfe and 

 all planes parallel to it, as they have to cut through a greater number 

 of the constituent almond-shaped particles of the rock than any others : 

 these therefore are the planes of greatest resistance. In the quarries 

 in Langdale these planes are parallel to the bedding, nevertheless the 

 rock will not split along them. 



In splitting up slate, the workmen have observed that the sheets 

 will bear a greater force without breaking, when the tool is applied 

 " at the end " of the sheet, than if it is applied " at the side." This 

 is an illustration of what has just been stated ; when the tool is ap- 

 plied at the end of the sheet, it opens the slate down the dip of the 

 cleavage-planes, and produces a strain on the sheet in the direction 

 of its greatest power of resistance ; but when the tool is applied " at 

 the side " of the sheet, the strain falls along the planes of secondary 

 cleavage, and the sheet easily breaks. 



Slate-pencil Rock. — Slate-pencils are cut from a slate which is 

 soft enough not to scratch, and which can be split down the planes 

 of secondary cleavage with nearly the same facility as along the true 

 cleavage. The latter character is not common. There are two 

 quarries near the village of Shap which have long been worked for 

 pencils, but the manufacture has declined in consequence of the 

 softer-grained pencils imported from Holland being preferred. 



The Southern Quarry is on Thornthwaite Gill in Ralphland, about 

 a mile and a half west of Shap. The rock is the Skiddaw slate, ap- 

 parently hardened from its proximity to the Shap granite ; the beds 

 strike N.E., and dip to the N.W. at about 60° : they contain many 

 layers of large ferruginous clay nodules ; the principal cleavage co- 

 incides with the bedding and dips N.W. 60°, which is about the 

 usual direction of the cleavage along that line of beds : the secondary 

 cleavage is nearly at right angles to the true cleavage, and dips S.E. 

 between 20° and 30°. 



The other quarry is on Rosgill Moor, between two and three miles 

 north of the former, and about three miles north-west of Shap ; the 

 rock is a soft black clay-slate, readily decomposing from exposure, 

 and containing beds of large ferruginous clay nodules : it belongs to 

 the Skiddaw slate, and agrees in all its mineral characters with the 

 upper part of that formation as seen near Keswick. The beds dip 

 N.E. 30° ; the principal cleavage dips N. by W. 60°, the secondary 

 cleavage dips S. by E. 15°. In both these quarries the beds are in- 

 tersected by a remarkable number of joints cutting through them in 

 various directions. At Rosgill Moor the weathered surfaces of rock 

 break up along both the planes of cleavage, and the sides of the 

 quarry present the ends of innumerable four-sided prisms of slate, 

 from a quarter to half an inch square, resembling those cut by the 

 workmen to make pencils from ; in this the pencil-rock contrasts with 

 the ordinary slate rocks which when weathered break up into thin 

 sheets. 



I was anxious to examine all the circumstances connected with 

 pencil-rock, in hopes of finding the cause of the peculiar development 



