



302 STRUCTURAL AND FIELD GEOLOGY 
Occasionally, necks are occupied wholly by crystalline 
rock, the junction between which and the adjacent rocks may 
similarly be concealed, so that the observer may be in doubt 
at first as to whether the igneous rock may not be an isolated 
patch or cake resting unconformably on the strata that crop 
out in its immediate neighbourhood. Were such its origin, 
its jointing should be vertical. On the other hand, if it be of 
the nature of a plug occupying a pipe of eruption, the joint- 
planes will be arranged horizontally. A geologist having 
satisfied himself on this point would, of course, seek to 
strengthen the evidence by such additional observations as 
are referred to above in connection with necks of agglomerate. 
Slaty Cleavage.—This structure, we have seen, occurs among rocks 
which have been more or less folded and compressed. In fine-grained 
slates the original planes of lamination and bedding are usually obscured, 
and may even be entirely obliterated, and when such is the case the 
superinduced cleavage-structure might readily be mistaken for planes of 
sedimentation. The geologist, therefore, must be on his guard, and 
when any thick belt of finely divided argillaceous rock is encountered in 
a region of steeply inclined and much-folded strata, he should at once 

FIG, 119,—CLEAVAGE AND BEDDING, 
suspect that the division-planes may be those of cleavage. If the rock 
be really a slate, careful examination will probably result in the detection 
of the original lines of bedding. These may be indicated by alternating 
bands of differently tinted slate or by variations in the texture of the 
slates—such differences of colour and texture being visible in section, as 
it were, on the cleavage-planes. By splitting the slate open we can see 
that the varying tint and texture are not merely superficial but, penetrating 
the rock, are as conspicuous on one face of the slate as on the other. 
Usually, however, bands and beds of greywacké, quartzite, or other less 
cleavable rock occur interbedded with slates—and the presence of these 
at once discloses the true bedding. It is not uncommon, moreover, to 
find the cleavage-structure restricted to the argillaceous rocks of a series 
of folded strata, and as the structure in question frequently traverses the 
original bedding-planes at a high angle, the junction between cleaved 
and non-cleaved rocks often resembles an unconformity (Fig. 119). In 
