280 ELEMENTARY GEOLOGY 
the glass, producing rhombic joints. As quarrymen: 
well know, many rocks, and indeed most, are in a 
state of strain analogous to that produced by twisting. 
In many places this strain is being slowly increased, 
Fic. 160. 
Columnar joints, Isle of Staffa. 
for the rocks of the 
surface are continually 
moved and changed 
in position by contrac- 
tions of the crust? 
(see p. 283). 
So it is thought 
that this straining or. 
torsion of the strata 
may have caused 
many, and perhaps all 
joints in sedimentary 
rocks. In support of 
this view it may be 
said, that those strata 
which have been most 
twisted, are crossed 
by the greatest num- 
ber of joints, and brittle rocks are better jointed 
than the more plastic ones. 
1 In quarries a rock layer sometimes bends when the beds above it have 
been removed; and the blocks that are quarried out sometimes expand so 
that they could not be put back in the same space, 
