CHANGES IN THE STRATIFIED ROCKS 281 
A theory has also been suggested to the effect that the joints 
may have been formed by earthquake shocks passing through 
rocks already strained, but not enough so to cause breakage. 
In order to test this hypothesis by actual experiment, glass has 
been twisted not quite up to the breaking point. It has then 
been jarred, and a breaking has followed which has caused the 
Ea oe 
Fig. 161. 
Columnar joints of basaltic lava. Long one from Isle of Staffa, short one 
from the Palisades of the Hudson. 
glass to crack in such a way as to produce rhombic blocks, just 
as we find in the jointed rocks. 
There are two other possible causes for joints. Many of the 
sedimentary rocks have been deeply buried; and as the rate 
of increase of temperature is one degree for every fifty or sixty 
feet of descent, those that have been covered to a depth of several 
