Dr. TyndaW on Slaty Cieavaffe. 133 



Journal, and decide for himself the extent to which I have erred 

 in my statement of Mr. Sorby^s opinions. My experiments on 

 this subject were all finished on the 27th of October, 1855, and 

 at that time the paper from which the above citations are drawn 

 was the only one which, as far as 1 am aware of, Mr. Sorby had 

 ever published on the subject of cleavage. From November 1855 

 to June 1856 1 was far too busy with other matters to be able 

 to devote any attention to the subject of cleavage, so that the 

 lecture which has been the cause of this discussion was merely 

 the utterance of views to which experiment had led me upwards 

 of seven months before. 



My special object in the lecture was to account for the cleavage 

 of ordinary roofing- slates, like those of Bangor and Borrodale — 

 of such slates, in fact, as Mr. Sorby had in view when he pro- 

 pounded his theory. In the above critique, however, Mr. Sorby 

 refers exclusively to a paper on the cleavage of Devonian lime- 

 stone, drawn up, 1 believe, at my suggestion, and published 

 in the Philosophical Magazine for January 1856. ^'In no 

 case," he observes, "do I attribute it [the cleavage] to the 

 presence of mica ; and do not even use that word in the whole 

 paper, except .... at page 23." Mr. Sorby surely cannot 

 imagine that 1 attach any particular value to mica as such. It 

 is shape, and not composition, that affects the question—" elon- 

 gated particles," as 1 have somewhere expressed it in my lecture. 

 And if mica occupies a prominent place there, it is because Mr. 

 Sorby himself has given it that prominence, as the foregoing ex- 

 tracts prove, while accounting for the cleavage of the particular 

 kind of slate which formed the subject of my discourse. But even in 

 this paper on the Devonian limestone, though mica is absent, it 

 has its equivalent in elongated particles of sand and organic 

 fragments, which perform the same functions in the limestone 

 as the mica does in the Welsh and Cumbrian slate. So evident, 

 indeed, does this equivalence appear to Mr. Sorby himself, that 

 he makes use of the same experiment with the oxide of iron scales 

 to illustrate both the action of the plates of mica in common 

 slate, and of the "unequiaxed particles" in Devonian lime- 

 stone. In proof of all this 1 refer the reader to the papers of 

 Mr. Sorby ; and 1 would here once for all express the hope that 

 he will accept none of my statements without this verification ; 

 otherwise I may, although quite unwittingly, lead him into error. 



Figs. 5 and 6, to which Mr. Sorby calls particular attention, 

 and which occur towards the conclusion of his observations on 

 the cleavage of Devonian limestone, refer to a cleaved organic 

 clay composed almost entirely of minute crystals of calcareous 

 spar, which " appears as though the compression indicated by the 

 joints of the encrinites and larger ci^stals, had afiected the 



