198 Sir J. F. W. Herschel on Slaty Cleavage. 



But as the thickness of the whole increases, the weight will 

 overcome the friction, and the still soft strata will yield inwards 

 from all sides towards the central portion of the bed, increasing 

 the thickness of the strata there, and at the same time crumpling 

 up the strata into contortions, &c. The whole affair may go on 

 with any degree of slowness*. 



Now as this process goes on, the middle thickness will increase^ 

 and that by a general lateral compression and vertical dilatation 

 of the whole central mass. And here we have the condition 

 which your theory requires for the slaty cleavage, wfiich (be it 

 observed) does not absolutely require violent force (for mere force 

 cannot produce any rearrangement of molecules), but only inter^ 

 molecular movement in a given fixed direction. Thus we see the 

 blebs in slow-moving lava currents drawn out in length ; and 

 thus we see liquids, in which excessively fine spicular crystals 

 are floating, assume a silky appearance when stirred or agitated 

 (I have a most beautiful case of the kind under my eye at this 

 moment) by the tendency of the particles to arrange themselves 

 when in motion all in one direction, according to the laws, not 

 of pressure but of friction, a distinction which is quite necessary 

 to be borne in mind. Infinitesimal films of mica disseminated 

 through amorphous mud would afford a precise parallel to this 

 case. 



Your account of the fibrous structure in tough iron is quite 

 beautiful, and perfectly accounts for the exceeding toughness 

 and tenacity of bar iron formed by welding together smaller bars, 

 &c. In fact, such a bar is a rope of iron wire held together as 

 the fibres of a hempen rope are, by lateral friction, and each 

 portion being per se of trivial length compared with the whole. 



I am sure you will excuse me for writing to you in this desul- 

 tory manner, which perhaps I am not entitled to do; but the 

 fact is, that since reading, a great many years ago, Wells's little 

 tract on dew, I have not met with anything which seems to bring 

 so many loose- lying phsenomena under a general and very simple 

 principle. 



I remain, dear Sir, 



Yours truly, 

 J, Tyndall, Esq., F.R.S. ^c. J. F. W. Herschel. 



♦ In forcing down tlie strata, while still in some degree yielding, down 

 ike general slope of the ocean bed, they must often be driven over its larger 

 inequaUties in a way very likely to contort and corrugate them ; and this, 

 too, equally whether the softness be that remaining in them previous to 

 their consolidation, or be produced anew by invasion of heat from below 

 producing semifusion and weakening their support. 



