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XXVI. Remarks on Slaty Cleavage, and the Contortions of Rocks. 

 By Sir John F. W. Herschel, Bart., F.R,S. ^c. 



Dear Sir, CoUingwood, July 27, 1856. 



LET me thank you for several instances of obliging attention 

 in forwarding me copies of your scientific papers on diamag- 

 netism, and quite recently of your lecture on the Slaty Cleavage^ 

 which appears to me a singularly happy explanation of that odd 

 phsenomenon. I may mention an illustration of the same nature 

 with some others you have introduced, which occurs in the 

 manufacture of the coin of the realm. One part of the process 

 consists in '^ ringing ^^ each piece of gold or silver coin (after 

 receiving the impression) . Every piece which will not ring on 

 dashing it against a stone is rejected and re-melted ; and such are 

 numerous. Now the cause of this can be no other than minute 

 air-blebs in the bars out of which the laminae of metal are rolled 

 by a flatting-mill, each bleb being flattened out into an infinite- 

 simally thin film of air parallel to the faces of the coin, and so 

 producing a solution of continuity, and of course destroying the 

 harmonic vibration. 



There are one or two points suggested by your views of the 

 slaty cleavage which harmonize so entirely with certain ideas I 

 have long held about geological stratification in general, that I 

 cannot forbear mentioning them. I refer to the contortions of 

 strata which are usually accounted for (as Poulett Scrope has 

 done) by the protrusive violence of igneous rocks crushing 

 together and doubling up the strata. To me it has always 

 seemed that this feature might be more tranquilly produced 

 thus : — 



Suppose A A the ocean surface, and B B B its bed, forming a 

 basin having irregularities C C in it. Let sediment be deposited 

 over the whole, uniformly, till the inequalities are filled in and 

 the bottom is reduced to a, generally speaking, basin-like depres- 

 sion. Then up to a certain point the friction of the bed will 

 retain the deposited mud on its slope, and in this way strata 

 nearly of uniform thickness (and parallel) will be formed. 



