414 The Rev. S. Haughton on Slaty Cleavage, 



about by a force acting very nearly in the line perpendicular to 

 cleavage, because there is scarcely any difference in the amount 

 of distortion in different directions in the plane of cleavage itself, 

 as is shown by the close agreement of the axes a and b. The 

 ellipsoid of distortion in this case is nearly an oblate ellipsoid of 

 revolution, whose equatorial diameter is nearly two and a half 

 times its polar diameter. 



The slate of Ardoginna is merely a compressed and hardened 

 mud ; it is a bluish shale, with occasional black partings. 



Example 2. — Carboniferous Slate of South Petherwin, 

 Cornwall. 



The next example of cleavage which I shall select is the car- 

 boniferous slate of South Petherwin, near Launceston in North 

 Cornwall. The slate of this district and its fossil remains have 

 a striking resemblance to the carboniferous slate beds of many 

 parts of the south of Ireland, which I believe to lie on the same 

 geological horizon. The fossils I have used to determine the 

 amount of cleavage are Spirifer disjunctus and Athyris conceU" 

 trica. The former of these fossils is closely allied to Spirifer 

 clathratus, of which it is probably only a variety. In calculating 

 the distortion, I have assumed for the normal proportions of this 

 fossil — 



Length =189 



Breadth of hinge-line =474 



Length of rib making 60° with hinge =308 



For Athyris concentrica I have used the normal proportions 

 already given. The data obtained as the mean of many mea- 

 surements are as follow : — 



<^==17°, i«l'480 



<^'=12°, i^= 1-263. 

 From these data we obtain, by means of equations (4) and (5),— 



a 

 i=0'990 !> (B) 



^=8-889 J 



This result is very similar to that already found for the car- 

 boniferous slate of the Co. Waterford, and proves that the 

 ellipsoid of distortion is nearly an oblate eUipsoid, whose polar 



