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502 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IV. 
in profile, appear marked off into so many wall-like masses. But this 
symmetry often gives place to a more or less tortuous course with 
lateral joints in various random directions, more especially where the 
different strata vary considerably in lithological characters. A single 
joint may be traced for many yards, sometimes, it is said, for several 
miles, more particularly when the rock is fine-grained, as in limestone. 
But where the texture is coarse and unequal, the joints, though 
abundant, run into each other in such a way that no one in particular 
can be identified for more than a limited distance. The number of 
joints in a mass of stratified rock varies within wide limits. Among 
strata which have undergone little disturbance the joints may be 
separated from each other by intervals of several yards. But in 
other cases where terrestrial movement has been considerable, the 
rocks are so jointed as to have acquired therefrom a fissile character 
that has nearly or wholly obliterated their tendency to split along 
the lines of bedding. 
An important feature in the joints of stratified rocks is the 
direction in which they intersect each other. In general they 
have two dominant trends, one coincident, on the whole, with the 
direction in which the strata are inclined from the horizon, and the 
other running transversely at a right angle or nearly so. The former 
set is known as dip-joints, because they run with the dip or inclina- 
tion of the rocks; the latter is termed strike-joints, inasmuch as they 
conform to the strike or general outcrop. It is owing to the existence 
of this double series of joints that ordinary quarrying operations can 
be carried on. Large quadrangular blocks can be wedged off, which 
would be shattered if exposed to the risk of blasting. A quarry is 
usually worked to the dip of a rock; hence the strike-joints form 
clean-cut faces in front of the workmen as they advance. These are 
known as “backs,” and the dip-jomts which traverse them as_ 
“cutters.” The way in which this double set of joints occurs in a 
quarry may be seen in Fig. 218, where the close parallel lines 
traversing the shaded and unshaded faces mark the planes of strati- 
fication, which here are inclined from the spectator. The steep faces 
in light are defined by the strike joints or “backs.” The faces in 
shadow have been quarried out along dip-joints or “cutters.” It will 
be observed that the long face in sunlight is cut by parallel lines of 
dip-joints not yet opened in quarrying, while in like manner the 
shaded face of dip-joint is traversed by parallel lines of strike- 
joint. 
Ordinary household coal presents a remarkably well developed 
system of joints. -A block of such coal may be observed to be 
traversed by fine laminz, the surfaces of many of which are soft and 
soil the fingers. ‘These are the planes of stratification. Perpendicular 
to them run divisional planes, which cut each other at right angles or 
nearly so, and thus divide the mineral into cubical fragments, One | 
of these sets of joints makes clean sharply defined surfaces, and is 
known as the face, slyne, cleat, or bord; the other has rougher, less 
