1856.] the Cleavage of Crystals and Slate llocks, 303 



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 contorted 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 commenced to act, this intermediate bed, which though com- 

 paratively 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 direction which it ought to 

 take across these more unyielding strata. 



Thus we have established the concurrence of the phaenomena 

 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 investi- 

 gator. You remember the cleavage of the flags of Halifax and 

 Over Darwen, which is caused by the interposition of plates of 

 mica between the layers. Mr. Sorby examines 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 contends, 

 be those along which the mass cleaves. 



I could show you by tests of a totally different character from 

 those apj)lied by Mr. Sorby, how true his conclusion is, that tiie 

 effect of pressure on elongated particles, or plates, will be such as 

 he describes it. Nevertheless, while knowing this fact, and admi- 

 ring the ability with which Mr. Sorby has treated this question, I 

 cannot accept his explanation of slate- cleavage. I believe that 



Vol. IL x 



