CHANGES IN THE STRATIFIED ROCKS 279 
a honeycomb. ‘These igneous rock joints appear to 
be due to the contraction of the rock in cooling. 
When they first solidify, the lavas are very hot; 
then, as they cool, they begin to contract, and this 
causes the rock to split into hexagonal columns. 
Joints of cooling of a somewhat different type are 
Fie. 159. 
Joint planes on the shore of Lake Cayuga, N. Y. 
also seen in granite and other igneous rocks (Fig. 
162). 
Cause of Joint Planes in Sedimentary Rocks. It is 
more difficult to account for the joints in the sedi- 
mentary rocks. By experiments with glass, it has 
been shown that a twist, or torsion, will finally snap 
