184 Dr. Tyndall on Slaty Cleavage. 



smallest, constituting the ultimate structure of the rock/' This 

 case is so like that of iron, adduced in illustration of my views, 

 that had it occurred to me, I should undoubtedly have referred 

 to it in my lecture. To this class of facts Mr. Sorby ingeniously 

 endeavours to attach my experiment with the wax. When a 

 thin stratum of this substance, melted on a plate of glass and 

 permitted to solidify, was examined by the microscope, he found 



, It to be composed of unequiaxed prismatic ci7stals. Without 

 entering into the question as to the amount of cohesion between 

 these crystals, I would say let them be chopped up into microscopic 

 mincemeat, let the mass be rubbed away by emery, or pared into 

 the finest shavings by glass ; let it be pounded, crushed, — in 

 short let all mechanical means be applied to abolish this cry- 

 stalline structure; let the wax be kneaded, as in my experi- 

 ments, into dough, and submitted to pressure, — the same per- 

 fect cleavage will be produced. To one and the same cause I 

 have attributed the cleavage of such wax, and that of the slates 

 of Wales and Cumberland. Mr, Sorby deduces from his expe- 

 riments, that the volume of slate- rocks has been reduced one- 

 half by the pressure which produced the cleavage. Without 

 expressing any opinion as to the probability of this conclusion, 

 let the structure of such a mass of fine slate mud, before com- 

 pression, be considered. Its water has drained away from it, 

 oozing through it where it encountered least resistance, and leaving 

 what, to my mind, presents an innumerable assemblage of little 



' mud masses clinging together at some parts and separated at 

 others. The effect of pressure upon such, a mass must manifestly 

 be to develope cleavage. The presence of unequiaxed particles 

 may aid in producing this result, but the finer the mud, the more 

 in fact it approaches to the wax in structure, the more perfect, to 

 my mind, will the cleavage be. 



A day or two ago the presence of cream* cheese upon the 

 table of a hotel suggested to me the idea of trying whether the 

 squeezing of the mass had not developed in it a kind of cleavage. 

 I was much pleased to find this to be the case. The experiment 

 was performed by inserting the knife a quarter of an inch into 

 the cheese and tearing the latter asunder. It is still simpler 

 and better to pull the mass asunder with the fingers. The 

 cheese yielded freely along the planes perpendicular to the direc- 

 tion of pressure, but not across these planes. If this can be 

 referred to the compression of crystals, it is something won for 

 •science ; if not, it illustrates the action by which 1 suppose the 

 structure of ordinary roofing-slate to have been produced. 



* The term * cream * ought, properly speaking, to have the negative sign 

 prefixed ; the cheese was made with milk from which the cream had been 

 withdmvm. 



