150 STRUCTURAL AND FIELD GEOLOGY 
Joints in Schistose Rocks.—As schistose or foliated 
rocks differ much in composition and structure, they might 
have been expected to show considerable variety in the 
character of their jointing. Those of them in which the 
foliated structure is well developed are occasionally divided 
by vertical or steeply inclined joints, but these are very rarely 
arranged symmetrically, and no system of intersecting joints 
like those of sedimentary strata can be detected. Now and 
again, however, granitoid gneiss is crossed by division-planes 
which have a general resemblance to the joints of granite. 
But, as a rule, the jointing of schistose rocks is irregular and 
capricious. 
Slickensides (Plate XX XVI.).—The surfaces of joints in 
all kinds of rock are often smoothed and marked by parallel 
ruts and strie, as if the opposite rock faces had been ground 
and rubbed against each other. These s/zckenszdes, as they are 
termed, are frequently coated with a skin of mineral matter, 
which naturally shows a cast of the opposite joint face, and 
thus has the appearance of having itself been smoothed and 
striated. In opening up a joint occupied by a thin vein of 
mineral matter, it is often possible to detach complete 
segments of the vein, which yield a cast of both joint faces. 
While slickensides are not confined to any particular areas, 
they yet tend to be most abundantly developed in regions 
where considerable crustal movement or rock-displacement 
has taken place. They often occur, for example, in the joints 
of rocks near lines of fracture and dislocation of the crust, 
and, as we shall learn presently, the walls or faces of such 
dislocations themselves frequently exhibit the same smoothed 
and striated appearance. 
Origin of Joints.—No one cause suffices to explain all the phenomena 
of joints. In many cases we can hardly doubt that these superinduced 
division-planes are simply fissures of retreat, formed during the consolida- 
tion and solidification of the rocks in which they occur. Some joints, 
again, are of such a character as to show that considerable force was 
required for their production, and these are suggestive, therefore, of 
crustal movements of some kind. The precise origin of others, however, 
is still obscure. The more obvious causes which have led to the jointed 
structure of rocks may be considered under the headings of Contraction, 
Expansion, and Crustal Movements. 
CONTRACTION.—Any moist and plastic rock, such as clay, necessarily 

