
JOINTS 147 
the “end” (ze. with the dip or inclination of the strata), while the 
working-galleries are driven along the “face” (z.e. the strike), and they 
necessarily follow, therefore, a level-course. So long as the inclination 
of the strata remains constant in direction, the working-galleries must 
follow a straight line at right angles to the dip, but with any change in 
the direction of the latter there will necessarily be a corresponding change 
in the direction of the “‘level-course.” 
Excellent examples of regular jointing are exhibited by the thick 
Carboniferous Limestones of Ireland and England—the main or master- 
joints crossing each other nearly at right angles, and preserving their 
direction for long distances. The Old Red Sandstone strata of N.E. 
Scotland, which are so finely displayed in numerous coast-sections, afford 
equally good illustrations of the same structure. 
Joints in Igneous Rocks are seldom as regularly 
arranged as those of sedimentary strata; indeed, they are 
frequently so very irregular that no system or arrangement 
can be recognised. In other cases, however, the division- 
planes show a modified regularity, following determinate 
directions like the master-joints described above; while in 
yet other cases they are so symmetrically disposed as to 
confer a prismatic columnar structure on the rock they 
divide. 
The joints in granite are often wonderfully regular—two 
sets of vertical or steeply inclined division-planes intersecting 
at various angles, which often do not depart widely from 
right angles. The rock thus tends to be divided into 
columnar masses that vary in shape according to the 
character of the joints (which may be straight or curved), 
and the angle at which they intersect. Sometimes the 
joints are widely separated, so that large monoliths can be 
_. obtained; at other times they are so closely set that the 
rock breaks up into a rubble of small fragments. The main 
vertical joints are often accompanied by minor irregular 
joints the presence of which necessarily prevents the 
extraction of large blocks. In addition to its vertical 
_ division-planes granite often exhibits a set of cross-joints, 
arranged at approximately right angles to the others. These 
cross-joints may be horizontal or inclined, and often give the 
rock a kind of bedded appearance, at least towards the sur- 
face (Plate XXXV.). They are seldom, however, so even as 
planes of bedding. Usually they are somewhat undulating, 
