2O2 GEOLOGY [CHAP. IX 



Letter 537 greatest difficulty on any mechanical doctrine ; though I did 

 not expect ever to find actual displacement, as seems to be 

 proved by your shell evidence. I am extremely glad you 

 have taken up this most interesting subject in such a philo- 

 sophical spirit ; I have no doubt you will do much in it ; 

 Sedgwick let a fine opportunity slip away c I hope you will 

 get out another section like that in your letter ; these are 

 the real things wanted. 



Letter 538 To. D. Sharpe. 



Down, [Jan. 1847]. 



I am very much obliged for the MS., which I return. 

 1 do not quite understand from your note whether you have 

 struck out all on this point in your paper : I much hope not ; 

 if you have, allow me to urge on you to append a note, 

 briefly stating the facts, and that you omitted them in your 

 paper from the observations not being finished. 



I am strongly tempted to suspect that the cleavage planes 

 will be proved by you to have slided a little over each other, 

 and to have been planes of incipient tearing, to use Forbes' 

 expression in ice ; it will in that case be beautifully analogical 

 with my laminated lavas, and these in composition are inti- 

 mately connected with the metamorphic schists. 



The beds without cleavage between those with cleavage 

 do not weigh quite so heavily on me as on you. You 

 remember, of course, Sedgwick's facts of limestone, and mine 

 of sandstone, breaking in the line of cleavage, transversely 

 to the planes of deposition. If you look at cleavage as I do, 

 as the result of chemical action or crystalline forces, super- 

 induced in certain places by their mechanical state of tension, 

 then it is not surprising that some rocks should yield more or 

 less readily to the crystalline forces. 



I think I shall write to Prof. Forbes 1 of Edinburgh, 

 with whom I corresponded on my laminated volcanic rocks, 

 to call his early attention to your paper. 



Letter 539 To D - Sharpe. 



Down, Oct. i6th [1851]. 



I am very much obliged to you for telling me the results 

 of your foliaceous tour, and I am glad you are drawing up an 



1 Prof. D. Forbes. 



