SHRINKAGE FISSURES OLDER ROCKS. 25 



caused by a part of a bed or beds, swelling or expand- 

 ing, and thereby raising and breaking the superin- 

 cumbent strata. This latter phenomenon can be 

 studied on a small scale, in the following simple 

 experiment : Mix some lime into mortar, and after it 

 is made, introduce pieces and fragments of unslacked 

 lime, then spread out the mass, and level it with 

 either a roller or trowel. As the introduced pieces 

 slack, they expand and raise the mortar over them, 

 sometimes forming regular systems of faults (a main 

 fault with transverse branches) ; at other times a dome 

 will first form, but this, if the expansion is great 

 enough, will eventually split, and caught-up portions 

 of the mortar are often elevated, just as we find 

 masses of an older igneous rock raised on a newer. 

 The boundary of these raised portions may be most 

 irregular, yet on examination it seems invariably 

 to be a combination of straight lines. 



The widening of master joints, and the consequent 

 formation of fissures, seems sometimes to have gone 

 on gradually, while in many other cases they would 

 appear to have opened at different periods with 

 intervals of rest between each. This seems suggested 

 by finding in many mineral lodes layers in duplicates, 

 each set differing in texture, aspect, and composition, 

 as if each opening had been formed and filled prior 

 to the subsequent expansion. Jukes, however, points 

 out that the layers of different minerals in some lodes 



