Mr. Sorby on the Theory of the Origin of Slaty Cleavage* 127 



It seems to be a mixture of hydrated antimonite of lead^ with 

 variable proportions of the oxides of lead and antimony. 



Since the examination of the Cornish ore by Mr. Dick, it has 

 been analysed by Dr. Heddle with the following results, which 

 agree with the supposition of its being a variable mixture of its 

 component parts : — 



I. II. 



Oxide of lead . . 47-045 46-68 



Antimonious acid . 42*216 42-44 



Water . * . . 11*497 11-98 



A brown variety consisted of — 



Oxide of lead 43-94 



Antimonious acid 



and Antimony ochre 



Water ....;... 6-625 



\. . . 46-70. 



XVIII. On the Theory of the Origin of Slaty Cleavage. 

 By H. C. Sorby, F.G.S."^ 



I SHOULD not now have trespassed on the pages of the 

 Philosophical Magazine, if it had not been that in Dr. 

 TyndalFs paper on the comparative view of the cleavage of cry- 

 stals and slate-rocks, published in its last Number, there is a most 

 serious misapprehension of my opinions on that subject. In 

 alluding to these, at page 44, he says, " I cannot accept his ex- 

 planation of slate-cleavage. I believe that even if these plates 

 of mica were wholly absent the cleavage of slate-rocks would be 

 much the same as it is at present. I will not dwell upon minor 

 facts, — I will not urge that the perfection of the cleavage bears 

 no relation to the quantity of mica present ; but I will come at 

 once to a case which to my mind completely upsets the notion 

 that such plates are a necessary element in the production of 

 cleavage.'^ Also somew^hat further on he speaks of " the plates 

 of mica assumed in Mr. Sorby^s theory.''' Now this statement 

 of my views respecting the origin of slaty cleavage is so entirely 

 different from what is really the case, that I think it would not 

 be proper for me to let it pass uncorrected. In proof of this I 

 need only refer to my paper on the cleavage of the Devonian 

 limestones of Devonshire, published in this Magazine (January 

 1856, 4th series, vol. xi. p. 20). 1 there describe that of a 

 considerable number of specimens, including all the principal 

 varieties I have seen in the district, and yet in no case do I 

 attribute it to the presence of mica ; and do not even use that 

 word in the whole paper, except in saying, at page 23, that 1 do 



. * Communicated by the Author. 



