CHAPTER II. 



SHRINKAGE FISSURES OLDER ROCKS. 



WE have endeavoured to point out what can be 

 learned from the study of the formation of cracks or 

 joints in recent deposits; and if from them the 

 student turns to the observation of rock masses, he 

 will find that nearly similar laws appear to have been 

 followed in the jointing of all rocks of derivate origin, 

 no matter to what geological period or age they 

 belong. One marked difference, however, ought to be 

 pointed out ; namely, that in the older rocks, gene- 

 rally the joints are more regular and systematic. 

 There are, however, exceptions to this general rule. 



It seems probable that all rocks, when deep-seated, 

 are more or less charged with moisture. This they 

 lose after being subjected to a drying process, whether 

 by atmospheric influences or internal heat. Rocks, 

 when dried, contract in a greater or less degree, accord- 

 ing to the composition of the rock; the contraction 

 being rarely equal in the vertical and in the horizontal 



