MECHANICAL THEORY OF CLEAVAGE. 
593 
dicular to them; and the same Fig. m, 
bed exhibits cleavage planes 
in the direction of the greatest 
movement, although they are 
much fewer than in the slaty 
strata above and below. 
Above the sandy bed d /*, 
the stratum c is somewhat dis¬ 
turbed, while the next bed, 5, is 
much less so, and a not at all; 
yet all these beds, c, and a, 
must have undergone an equal 
amount of pressure with c?, the 
points a and g having approx¬ 
imated as much towards each 
other as have d and f. The 
same phenomena are also re¬ 
peated in the beds below d^ 
and might have been shown, 
had the section been extended 
downward. Hence it apjDears 
that the finer beds have been 
squeezed into a fourth of the 
space they previously occu¬ 
pied, partly by condensation, 
or the closer packing of their 
ultimate particles (which has 
given rise to the great specific 
gravity of such slates), and 
partly by elongation in the 
line of the dip of the cleavage, 
of which the general direction 
is perpendicular to that of the 
pressure. “These and numer¬ 
ous other cases in North Dev¬ 
on are analogous,” says Mr. Sorby, “ to what would occur if 
a strip of paper were included in a mass of some soft plastic 
material which would readily change its dimensions. If the 
whole were then compressed in the direction of the length 
of the strip of paper, it would be bent and puckered up into 
contortions, while the plastic material would readily change 
its dimensions without undergoing such contortions; and the 
difference in distance of the ends of the paper, as measured 
in a direct line or along it, would indicate the change in the 
dimensions of the plastic material.” 
By microscopic examination of minute crystals, and by 
Vertical section of slate rock in the cliffs 
near Ilfracombe, North Devon. Scale 
one inch to one foot. (Drawn by H. C. 
Sorby.) 
n, c, e. Fine-grained slates, the stratifi¬ 
cation being shown partly by lighter 
or darker colors, and partly by differ¬ 
ent degrees of fineness in the grain, 
d,/. A coarser grained light - colored 
sandy slate with less perfect cleavage. 
