188 THE FORMS OF WATER IN 



quarries of Bangor or Cumberland, and observe the 

 quarrymen in their sheds splitting the rocks. With a 

 sharp point struck skilfully into the edge of the slate, 

 they cause it to divide into thin plates, fit for roofing 

 or ciphering, as the case may be. The surfaces along 

 which the rock cleaves are called its planes of cleavage. 



477. All through the quarry you notice the direction 

 of these planes to be perfectly constant. How is this 

 laminated structure to be accounted for ? 



478. You might be disposed to consider that cleavage 

 is a case of stratification or bedding ; for it is true that 

 in various parts of England there are rocks which can 

 be cloven into thin flags along the planes of bedding. 

 But when we examine these slate rocks we verify the 

 observation, first I believe made by the eminent and 

 venerable Professor Sedgwick, that the planes of bed- 

 ding usually run across the planes of cleavage. 



479. We have here, as you observe, a case exactly 

 similar to that of glacier lamination, which we were 

 at first disposed to regard as due to stratification. We 

 afterwards, however, found planes of lamination crossing 

 the layers of the neve, exactly as the planes of cleavage 

 cross the beds of slate rocks. 



480. But the analogy extends further. Slate cleavage 

 continued to be a puzzle to geologists till the late Mr. 

 Daniel Sharpe made the discovery that shells and other 

 fossils and bodies found in slate rocks are invariably 

 Battened out in the planes of cleavage. 



