Crystals and Slate Rocks. 4& 



In reference to Mr. Sorby^s contorted bed, I have said that 

 by supposing it to be stretched out and its length measured, it 

 would give us an idea of the amount of yielding of the mass 

 above and below the bed. Such a measurement, however, would 

 not quite give the amount of yielding ; and here I would beg 

 your attention to a point, the significance of which has, so far as 

 I am aware of, hitherto escaped attention. I hold in my hand 

 a specimen of slate with its bedding marked upon it ; the lower 

 portions of each bed are composed of a comparatively coarse 

 gritty material something like what you may suppose this con- 

 torted bed to be composed of. Well, I find that the cleavage 

 takes a bend in crossing these gritty portions, and that the 

 tendency of these portions is to cleave more at right angles to 

 the bedding. Look to this diagram : when the forces com- 

 menced to act, this intermediate bed, which though compara- 

 ratively unyielding is not entirely so, suffered longitudinal pres- 

 sure ; as it bent, the pressure became gradually more lateral, 

 and the direction of its cleavage is exactly such as you would 

 infer from a force of this kind — it is neither quite across the 

 bed, nor yet in the same direction as the cleavage of the slate 

 above and below it, but intermediate between both. Supposing 

 the cleavage to be at right angles to the pressure, this is the di- 

 rection which it ought to take across these more unyielding strata. 



Thus we have established the concurrence of the phsenomena 

 of cleavage and pressure — that they accompany each other ; but 

 the question still remains. Is this pressure of itself sufficient to 

 account for the cleavage ? A single geologist, as far as I am 

 aware, answers boldly in the affirmative. This geologist is 

 Sorby, who has attacked the question in the true spirit of a 

 physical investigator. You remember the cleavage of the flags 

 of Halifax and Over Darwen, which is caused by the interpo- 

 sition of plates of mica between the layers. Mr. Sorby exa- 

 mines the structure of slate-rock, and finds plates of mica to be 

 a constituent. He asks himself, what will be the effect of 

 pressure upon a mass containing such plates confusedly mixed 

 up in it ? It will be, he argues, and he argues rightly, to place 

 the plates with their flat surfaces more or less perpendicular to 

 the direction in which the pressure is exerted. He takes scales 

 of the oxide of iron, mixes them with a fine powder, and on 

 squeezing the mass finds that the tendency of the scales is to set 

 themselves at right angles to the line of pressure. Now the 

 planes in which these plates arrange themselves will, he con- 

 tends, be those along which the mass cleaves. 



pressure has been exerted ; and provided this remain firm, the fate of any 

 minor portion of the evidence by which it is here estabUshed is of compa- 

 rativelv httle moment. 



