392 REPORT — 1856. 



including the encrinites, which are found with their substance compressed 

 and crushed so as to occupy, in the direction of the perpendicular to cleavage, 

 only a quarter of the space they fill in the direction of cleavage dip. Thus 

 the originally nearly equiaxed cells of the encrinital stem are altered by 

 cleavage to elongated fusiform shapes, whose longer axes are parallel, and 

 four times as great as the shorter axes. Even crystals of calcareous spar and 

 dolomite are found crushed, bent and broken up, so as to be with difficulty 

 recognizable. 



The instances thus collected of the movements of the parts of the rocks 

 subject to slaty cleavage, in directions normal to the planes of cleavage, have 

 been, if possible, made more convincing by imitative experiments, which 

 show that some of the phaenomena of cleavage are attainable by means of 

 pressure in materials composed of particles capable of change of figure, or 

 change of position. Mr. Sorby, observing by the microscope that iu certain 

 uncleaved stones (e.g. water of Ayr stone) mica occurred in plates inclined 

 evenly in all directions, — while in slates in which cleavage was manifest the 

 mica was found more collected on the cleavage planes and inclined at low 

 angles to it, — a circumstance directly deducible from the phaenomena of com- 

 pression already proved, — made a cleavable mass in the following manner*: — 

 He mixed scales of oxide of iron with soft pipe-clay, so that the scales lay 

 evenly in all direction as in water of Ayr stone, and then pressed it so as 

 as to alter the dimensions of th.e mass in the same proportion as the slate of 

 Llanberis already referred to. Having then dried and baked it, he examined 

 the interior state of the substance by rubbing smooth faces, one face perpen- 

 dicular to pressure and in the line of elongation or dip; another in what 

 represented the line of strike, and a third face in the plane of the pressure 

 corresponding to the cleavage plane. The particles of oxide of iron were 

 found distributed just as mica is in well-cleaved slate; the mass was capable 

 of easily splitting parallel to the pressure planes, but not across them. 



Professor Tyndall has more recently taken up this part of the subject, 

 and has produced a variety of results, confirming and extending the inge- 

 nious reasonings and experiments of Mr. Sorby f- Perhaps his most re- 

 markable experiment is tliat made with pure white wax, which in the ordi- 

 nary state admits of fracture in all dii^ections equally, and contains no 

 unequiaxed particles like mica and scales of oxide of iron. This substance, 

 being subject to pressure:!:, is found to have acquired true slaty structure, 

 even in a higher degree than any known slate, for it splits to much finer and 

 more equal laminae. " The finer the slate the more perfect will be the resem- 

 blance of its cleavage to that of the wax," is the conclusion of the author of 

 this instructive experiment. 



The experiments and reasonings of Professor Tyndall, Mr. Sorby, and 

 Mr. Sharpe, will again come under review in a future Report, when the theory 

 of slaty cleavage may be examined, and the 'mechanical pressure' which 

 these authors advocate may be placed in comparison with the crystalline 

 polarity, formerly advanced by Prof. Sedgwick. The veined structure of 

 glaciers, which reminded Professor J. Forbes of the analogous lamination in 

 slates, — an idea since expressed by Rogers and Tyndall, — and Mr. Fox's in- 

 genious imitation of slaty cleavage by electrical currents passing through 

 clay, will then receive the attention which they merit. 



* Edinb. New Phil. Journal, July 1853. 



+ Lecture to the Roy.il Institution, June 6, 1856. 



X The wax is kneaded with the fingers, and pressed between thick plates of glass pre- 

 viously wetted. In cold weather, or when cooled by a freezing mixture, it splits beau- 

 tifully". 



