311 
straight forward in the line of their axes, it spreads out on 
each side, and curves down into the sharp-ended spaces 
between them, in just such a manner as would necessarily 
occur if the coarser-grained bed had been less compressed 
than the other. It would also follow that the above-men- 
tioned fan-shaped arrangement would be of greatest amount 
in such beds as offered much resistance to change of dimen- 
sions, whereas in fine-cleaved slates it woulaYbe very small, 
or even not occur at all ; and such is the fact observed in 
the rocks themselves. It would also necessarily follow from 
this theory, that the strike of the cleavage would usually 
coincide with the general strike of the beds, and be parallel 
to the main axis of elevation of the district, as has been 
found to be so commonly the case. The dip of the cleavage 
planes over any extensive district would likewise be as has 
been observed ; and it is easy to explain why it should differ 
in contiguous interstratified beds of different nature, and 
why. their junctions should often be irregularly wrinkled. 
There are also many other facts connected with the subject 
which may thus be readily explained, but it would occupy 
too much time to describe them. 
Perhaps it may be said, How is it possible that hard rocks 
could have had their dimensions changed to the extent de- 
scribed ? To this I would reply, If the rocks be examined, 
it will be seen that it really has occurred ; and I would 
suggest that solidity is but a comparative property, and that 
the intensity of the forces in action during the elevation of 
a range of mountains could gradually change the dimensions 
of rocks ; for it is well known that many hard and brittle 
substances will admit of such movements, as, for instance, 
the ice of glaciers, and hard and brittle pitch. 
I would now ask, How is it possible to reconcile all the 
mechanical facts I have described, which are so clearly 
related to the cleavage, with the supposition of its being 
