vi.] THE SLATES ON THE ROOF. 145 



suffered enough to change them into something very- 

 different from mud, and, therefore, probably, into 

 what they are now namely, slate. 



And now, at last, we have got to the slates on the 

 roof, and may disport ourselves over them like the 

 cats. 



Look at any piece of slate. All know that slate 

 splits or cleaves freely, in one direction only, into flat 

 layers. Now any one would suppose at first sight, and 

 fairly enough, that the flat surface the " plane of 

 cleavage " was also the plane of bedding. In simpler 

 English we should say The mud which has hardened 

 into the slate was laid down horizontally ; and therefore 

 each slate is one of the little horizontal beds of it, 

 perhaps just what was laid down in a single tide. We 

 should have a right to do so, because that would be 

 true of most sedimentary rocks. But it would not be 

 true of slate. The plane of bedding in slate has 

 nothing to do with the plane of cleavage. Or, more 

 plainly, the mud of which the slate is made may have 

 been deposited at the sea-bottom at any angle to the 

 plane of cleavage. We may sometimes see the lines 

 of the true bedding the lines which were actually 

 horizontal when the mud was laid down in bits of 

 slate, and find them sometimes perpendicular to, some- 

 times inclined to, and sometimes again coinciding 

 with the plane of cleavage, which they have evidently 

 acquired long after. 



Nay, more. These parallel planes of cleavage, at 

 each of which the slate splits freely, will run through 

 a whole mountain at the same angle, though the beds 

 through which they run may be tilted at different 

 angles, and twisted into curves. 



SO. L 



