STRUCTURES COMMON TO ALL ROCKS. 229 



concentric shells, producing domes ; in eruptivcs, of many 

 shapes, rough cubic, ball-like, regular columnar, tilelike. 



FIG. 135. Regular jointing of limestone. 



For this reason a cliff, especially of stratified rock, looks 

 like a wall of titanic masonry without mortar. 



Cause. These cracks are supposed to have been formed 

 by the shrinkage of the rocks ; in stratified rocks, in con- 

 solidating from sediments ; in igneous and metamorphic 

 rocks, in cooling from a state of fusion or semi-fusion. 

 In stratified rocks they are usually confined to the stra- 

 tum, though some larger joints (master-joints) run 

 through several strata. They are mentioned mainly that 

 the student should not confound them with other kinds 

 of structure. 



Great Fissures. 



Joints are probably shrinkage-cracks. Fissures are 

 fractures by crust-movements. Joints are cracks of the 

 individual strata ; fissures are fractures of the earth's 

 crust, extending through many formations, and continu- 

 ing for many miles. 



Cause. We shall see hereafter that the earth's crust 

 is subjected to a powerful horizontal pressure, by which 



