

i dee eS eee” Se ee 
224 STRUCTURAL AND FIELD GEOLOGY 
folded rocks, therefore, foliation sometimes coincides with original bedding- 
planes, or it may cross these at any angle. Owing to metamorphism, 
however, the original rock-structures are often wholly obliterated, 
and it is then impossible to say what influence these may have had 
in determining the direction of foliation. Along great thrust-planes the 
immediately adjacent rocks are often rendered crystalline and schistose, — 
and in such cases the foliation coincides in direction with the plane of 
rock displacement. 
Slaty Cleavage.—In a preceding chapter (p. 141) the 
phenomena of rock-folding were discussed, and it was there 
pointed out that the constituent ingredients of a folded rock 
were often more or less deformed or distorted. Deformation 
of the kind referred to is often conspicuously developed in 
areas of dynamo-metamorphism, more especially along their 
outer margin. In this peripheral zone the rocks may be 
arranged in more or less steeply inclined positions, but they 
are neither crystalline nor foliated. Nevertheless they usually 
afford evidence of having been compressed. ‘This is shown 
by the superinduced structure known as S/aty Cleavage, a 
structure which renders a rock capable of being cleaved or 
split into slabs, plates, or laminze in a direction independent 
of the planes of bedding. When such a rock is examined 
under the microscope, the particles of which it is composed 
are seen to be flattened out in one and the same direction 
—an arrangement which obviously accounts for the fissile 
character of clay-slate. A rock of this kind, therefore, cleaves 
or divides most readily along planes of compression, and not, 
as in the case of shale, along planes of deposition. That 
slaty cleavage is one of the concomitant results of crustal 
deformation is shown by the fact that the planes of cleavage 
are always parallel to the axes of anticlinal and synclinal 
folds. When the structure is well developed, not only does 
the original lamination disappear, but even the planes of 
bedding may be rendered obscure or altogether obliterated. 
Cleavage may intersect the bedding-planes at any angle, or 
may now and again coincide with these where the limb of a 
fold is inclined in the same direction and at the same angle as 
the planes of compression (see Plate XLIX.). Slaty cleavage 
is best developed in fine-grained, homogeneous clay-rocks, 
which are sometimes so fissile that they divide with ease into 
very thin smooth plates. It is not confined, however, to 

