418 The Rev. S. Haughton on Slaty Cleavage y 



a proportionably less-developed cleavage, is a solid argument in 

 support of the mechanical theory of cleavage. 



In addition to the examples already given, I have measured 

 several others in which I could only obtain a single position of 

 the plane of bedding in relation to the cleavage plane ; this is 

 not sufficient to determine the three axes of the ellipsoid of dis- 

 tortion ; but if we assume, as is probable, that this ellipsoid is 

 one of revolution, we can readily obtain an approximate value of 

 the compression due to cleavage as follows : — Assuming b equal 

 to unity in one of the equations (3), we find, after some simple 

 reductions, — 



c sin^ • • • • ^ ^ 



The following examples of the application of equation (6) are 

 taken from my note-book : — 



Example 6. — Silurian Black Slates of Moel Benddu, Dinas 

 Mowddy, South of Bala, North Wales. 



The fossil whose distortion was measured in this case was the 

 Asaphus Powisiij and its normal shape was assumed as follows : — 



Asaphus Powisii . Length of head =338 

 Length of body =386 ' 

 Length of tail =448 ^ 

 Breadth of body =838 



From these measurements, compared with the distorted spe- 

 cimens from Moel Benddu, I found — 



1 Asaphus Powisii. 



<^=65°, -r=:3-020 whole fossil 



3-230 tan 



2-562 body 



3-265 head 



2-584 body 



8-033 tail 



Mewi . . 2*949 



From these data, I find by equation (6), — 



a 



i;= 8-697 



(F) 



