278 ELEMENTARY GEOLOGY 
ment of dislocation. The rocks are simply split or 
cleaved, as if they had been cut by a saw (Fig. 159); 
and these smooth-sided joints often extend over con- 
siderable areas with nearly the same direction. Usually 
there are two sets of joint planes, reaching nearly ver- 
tically into the earth, and cutting one another almost 
at right angles. This, with the bedding planes, causes 
the rock to be broken naturally into blocks which 
are often nearly perfect rhombs. Sometimes the joint 
planes are close together, but more commonly several 
SS 
Fic. 158. 
Diagram to illustrate origin of concretions. Layer on left contains substances 
disseminated (indicated by dots), which in right-hand figure are accumulated 
in bunches or concretions. 
feet apart, though even in the same rock we may 
see the two extremes side by side. 
In Igneous Rocks. Not only are sedimentary strata 
thus cut by jomt planes, but in igneous and meta- 
morphic rocks the same is seen. In some igneous 
rocks, particularly basaltic lavas, like those of Fingal’s 
Cave on the Isle of Staffa (Fig. 160), off the Irish 
coast, and in the Palisades of the Hudson, there is a 
remarkable form of jointing, which cuts the rocks 
into columns with a variable number of sides, though 
prevailingly with six (Fig. 161). The rock looks like 
