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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Nov. 1, 



direction. Its tendency to split with different degrees of resistance 

 in different directions is the consequence of this arrangement and 

 form of its constituent parts, and is therefore only a secondary cha- 

 racter. 



Two planes of cleavage in slate. — In preparing slates, the first 

 process is to split them along the planes of cleavage to the thickness 

 required, they are then to be cut to size ; most slates of good quahty 

 can be split tolerably straight down the sides of the sheet by apply- 

 ing the tool at the top, from which we see that besides the true or 

 principal planes of cleavage, there is also another direction in which 

 the rock has a tendency to split in a slighter degree ; this may be 

 called the plane of secondary cleavage ; the workmen call it the " side 

 of the sheet." The ends of the sheet must be chipped or sawed to 

 the size required, as the rock will not split at all in this direction, 

 which is across both the other planes of cleavage : in fact it will give 

 way in any other direction more easily than in this, which may be 

 called the plane of greatest resistance. . 



A diagram will make this more intelligible, and will also explain 

 Y\o- 3 . the relation which these va- 



rious planes have to the po- 

 ^^^^^-^^czzjj •=> .=ir~' cS'A .^^^^-^h ^ ®^t^^^ ^^ ^^ constituent par- 

 "'iXly \y i j/ \ ry^j^TlT/V)/ tides of the slate. Let us 



suppose fig. 3 to represent a 

 block of slate, of which ah cd 

 is one of the planes of clea- 

 vage, ad and he showing the 

 dip, a h and d c the strike of 

 the cleavage ; h cgf is the 

 plane of secondary cleavage, 

 a bfe is the plane of greatest 

 resistance. The component 

 parts of the rock are repre- 

 sented enlarged, of an almond 

 y- shape, with their largest and 

 flattest sides parallel to the 

 true cleavage-plane ah cd, and with their longest diameters in the 

 direction of the dip of the cleavage a d. 



It is obvious that wdth such a distribution of its particles, the rock 

 can offer less resistance to an attempt to split it down the plane of 

 cleavage ahcd, than in any other direction ; for that plane, and all 

 planes parallel to it, pass along the flat surfaces of many of the par- 

 ticles, and intersect a smaller number of them than any other plane 

 which can be drawn through the block. 



Next to the true cleavage-plane, the direction in which the rock 

 will offer least resistance is along the planes of secondary cleavage 

 hfg c, for the particles being all lengthened in that direction, it will 

 more readily separate down that plane than in any cross direction. 

 Nevertheless the cleavage along these planes is very imperfect, and 

 the sheets often fly off in a curve instead of splitting straight down. 

 In any other direction the particles must from their form and po- 



