310 
this fact is therefore very good evidence of the slate having 
had originally such a structure as would be changed into its 
present form, if its dimensions had been altered in the manner 
and to the extent indicated by the breaking up of other 
rounded grains of mica seen in the same thin section. 
What I therefore contend is, that there is abundance of 
proof that slate rocks have suffered such a change of dimen- 
sions as would necessarily alter the arrangement of their 
ultimate particles from what is found in rocks not having 
cleavage, to that in those which have, and hence develop a 
line of structural weakness in the direction in which it really 
does occur. 
Some slates have a very poor cleavage, although their 
mineral composition is similar to that of such as often have 
a most perfect one. In these the green spots indicate a 
comparatively small change of dimensions ; whence it would 
appear that the perfection of cleavage depends both upon 
the ultimate mineral composition, and the amount of change 
of dimensions of the rock. 
If the direction of the cleavage be examined in the various 
parts of the section given at page 302, I cannot conceive 
how they could possibly be explained except by such a 
theory as I am now advocating. In the coarser-grained 
sandy bed it coincides with the axes of all the contortions, 
and is in the line of greatest elongation of the thickness of 
the bed, and perpendicular to the line of pressure. It is 
arranged in fan-shape in all the contortions, as though they 
had been squeezed together after the sandy bed had suffered 
as much compression as it admitted of. The cleavage in 
the fine-grained beds at some distance from the contorted 
one, is perpendicular to the line of squeezing, as indicated 
by its puckering up, and the increase and diminution of its 
thickness in passing round the contortions ; but when ap- 
proaching their rounded ends, though the cleavage passes 
