
146 STRUCTURAL AND FIELD GEOLOGY 
direction. The width of rock between adjacent joints, belong- 
ing to the same parallel series, is very variable. In some 
cases it may be many feet or even yards; in other cases it 
may be considerably less than a foot. In certain thin-bedded 
strata, for example, so closely set are the joints that the rock 
breaks up readily into small cubes and parallelopipeds. In _ 
addition to the more or less regularly intersecting main joints, 
numerous subordinate joints may traverse a rock in many 
different directions ; and when such is the case, the system of 
main joints becomes obscured or unrecognisable—the irregu- 
larly fissured rock breaking up into a rubble of larger and 
smaller angular fragments (see Plates XX XIII, XXXIV.). 
When bedded rocks are inclined, the intersecting main 
joints are known as dzp-joints and strike-joints respectively— 
the former running in the direction of the dip or inclination 
of the beds, while the latter cross these at approximately 
right angles. Strike-joints are usually more pronounced than 
dip-joints. To this rule, however, there are many exceptions— 
sometimes the latter being more conspicuous than the former, 
while not infrequently the one set is hardly better developed 
than the other. 
The joints that traverse a succession of alternating strata 
of limestone, shale, sandstone, etc., are often interrupted and 
sharply shifted to the side as they pass from one bed to another. 
Often, indeed, certain beds in a series are more regularly 
divided than the immediately overlying and underlying strata 
the jointing of which may be very irregular, and more 
abundant or less so than that of the intermediate strata. 
In such cases the divisional planes of the several beds appear 
to be more or less independent. 
While regular jointing may be met with in all kinds of derivative 
rocks, it is best displayed, as a rule, in fine-grained deposits of homo- 
geneous composition, as in limestones, freestones, flagstones, coal, etc. 
Common household coal, for example, is divided by three sets of planes 
disposed at right angles to each other: namely (@) planes of bedding, 
with a dull sooty surface, and (4) and (c¢) joint-planes with bright surfaces. 
One set of joints (4) runs in the direction of the inclination or dip of the 
bed, and is termed by miners the “end of the coal,” the other set (c) 
traverses the bed at right angles to the “end,” and is known as the “‘face 
or cleat of the coal.” The “face,” therefore, coincides with the sévzke of 
the strata. In a coal-mine cross-galleries are driven in the direction of 
