
CHAP Piva. 
JOINTS 
Joints, Close and Gaping. Joints in Bedded Rocks—Master-joints, Dip- 
and Strike-joints. Joints in Igneous Rocks—in Granitoid Rocks, 
Prismatic Joints. Joints in Schistose Rocks. Slickensides. Origin 
of Joints—Contraction, Expansion, Crustal Movements. 
JOINTS are superinduced divisional planes which traverse 
rocks in different directions and at various angles, so as to 
allow of their ready separation into larger and smaller blocks 
and fragments of regular or irregular shape. The faces of a 
joint are generally smooth and flat, but in certain cases (as in 
many crystalline igneous rocks), they are often somewhat 
curved. In fresh, unweathered rocks, joints are usually 
inconspicuous, the faces being sometimes in such close 
apposition that the fissure can hardly be detected. The 
presence of even the closest joints, however, is often betrayed 
by the alteration of the rock induced by percolating water— 
the degree of alteration naturally depending to a large extent 
upon the character of the rock. In the case of red sandstone, 
for example, the position of the joints is frequently indicated 
by more or less vertical lines and bands of bleached rock. 
In limestone, again, the joints tend to gape, as might have 
been expected from the ease with which that rock is dis- 
solved by acidulated water. ‘The faces of joints are frequently 
coated with a pellicle of brown or yellow iron-oxide; or with 
other depositions from aqueous solution, such as calcite, 
barytes, quartz, chalcedony, etc. Gaping joints are in like 
manner often filled with similar products—and are then 
described as veins, which may vary in width from less than an 
inch up to many feet. The phenomena of mineral veins, 
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