303 
Referring to the accompanying figure, it will be seen that 
the bed of sandstone has been contorted in just such a 
manner as if it had been compressed laterally, parts which 
must have been about thirty-eight inches apart when the bed 
was level, being now only nine. Just above and below this 
bed is fine-grained, dark-coloured, shaly slate, in which the 
bedding is shewn by bands of coarser grain and lighter 
colour, which are not continuous, but, as usual, broken up 
into spots, as seen in a section perpendicular to cleavage in 
the line of dip, but as bands in its plane. Near the greatly 
contorted bed these bands are also somewhat disturbed, but 
at a short distance, both above and below it, they are quite 
regular and not at all contorted. This interstratification of 
much contorted beds with regular, seen in slate rocks, has 
long been an unexplained difficulty, but I think it is readily 
accounted for by supposing that the contorted sandy bed 
could not give way and change its dimensions so much as 
the finer-grained slate above and below it. It is in my 
opinion analogous to what would occur if a strip of paper, 
for instance, was included in a mass of some soft plastic 
material, which would readily change its dimensions. If 
the whole was then compressed in the direction of the length 
of the strip of paper, it would be bent and puckered up into 
contortions, whilst the plastic material would readily change 
its dimensions without such being the case; and the differ- 
ence in distance of the ends of the paper, as measured in a 
direct line, or along it, would indicate the change in dimen- 
sions of the plastic material. It will here be observed that 
the cleavage in the fine-grained slate is perpendicular to the 
line of pressure, as indicated by these facts. Moreover, if 
the contorted bed be examined, it will be seen to have had 
its dimensions altered in different parts by this pressure. Of 
course it would originally be of nearly uniform thickness, 
but now it is about one quarter as thick in those parts lying 
