S Bary CLEAVAGE. - 521 

Pembrokeshire, where De la Beche has shown that the roofs and 
pavements of coal-seams are brought together, the coal itself, as 
having least resistance, being thrust into the loops (Fig. 246). 
Deformation.—During the intense compression to which rocks 
_ have been subjected their individual particles have been compressed, 
_ elongated and fractured, as is instructively shown by the deforma- 
tion of pebbles and of fossils. These effects have already (p. 311) 
been referred to. 
Part V.—CLEAVAGE. 
Cleavage-structure having been described at p. 310, we have to 
notice here the manner in which it presents itself on the large scale 
among rock-masses. The direction of cleavage usually remains per- 
sistent over considerable regions, and, as was shown by Sedgwick,’ 
corresponds, on the whole, with the strike of the rocks, It is, however, 
independent of bedding. Among curved rocks the cleavage planes 
may be seen traversing the plications without sensible deflection 
from their normal direction, parallelism, and high angle. But their 









MIN AAA 
WAY RS WQS 
NO \ 
anes 
\ WEST SSS 
: a b c 
| - Fig. 247,—CurveD AND ConTORTED DEVONIAN Rocks, NEAR ILFRACOMBE (B.). 
Bedding and cleavage planes are coincident at a and c, but nearly at right angles at b. 

_ general coincidence with the axes of plications serves to indicate a 
| community of origin for cleavage and folding, as results of the 
lateral compression of rocks. Among curved strata the planes of 
cleavage sometimes coincide with and are sometimes at right angles 
to the planes of bedding, according to the angles of the folding (Fig, 
247). ‘The persistence of cleavage planes across even the most diverse 
kinds of rock, both sedimentary and igneous, was first described by 
Sedgwick. Jukes also pointed out that over the whole of the south 
of Ireland the trend of the cleavage seldom departs 10° from the 
normal direction E. 25° N., no matter what may be the differences in 
character and age of the rocks which it crosses. But though cleavage 
is so persistent, it is not equally well developed in every kind of 
rock. As already explained (p. 311), it is most perfect in fine- 
grained argillaceous rocks, which have been altered by it into slates, 
and may be observed at once to change its character as it passes 
from such rocks into others of a more granular or gritty texture. 
Occasional traces of distortion or deviation of the cleavage planes 
may be observed at the contact of two dissimilar kinds of rock 
(Fig. 248). 
1 “On the Structure of Large Mineral Masses,” Trans. Geol. Soc. 2nd Ser. III.—an 
admirable memoir, in which the structure of a great cleavage region is clearly and 
graphically described. tie 
