8 



ECONOMIC GEOLOGY 



waters charged with magnesium carbonate in solution a stratum 

 of calcium carbonate should be encountered, a part of the cal- 

 cium would be exchanged for the less soluble magnesium. In 

 time there would occur a shrinkage of the rock mass amounting 

 approximately to 12 per cent. Such a shrinkage would necessi- 

 tate the shattering of the rock mass and the formation of cavities 

 that might be subsequently filled with mineral matter. 



(3) Cavities are caused by fracturing. Fracturing may be 

 produced by a shrinkage as the direct result of the cooling. 

 These fractures may be brought into the zone of vision through 

 the erosion of large masses of superincumbent strata. If rocks 

 are igneous in origin, these rocks must cool. If they cool, 

 they must shrink. If they shrink, they must fracture. 



FIG. 4. Section of a fault formed during a Japanese earthquake. (After 



Koto.} 



(4) Cavities are produced in rock masses by earthquakes. 

 These may arise through the intrusion of igneous dikes or by 

 volcanic eruptions. (See Fig. 4.) 



(5) Cavities may form in the sheared zone of intrusives. 

 Massive diorites pass into diabases and then into hornblende 

 schists or amphibolites. These amphibolites may metamorphose 

 into serpentine. In the shearing and the serpentinization the 

 crushed areas become favorable places for the formation of ore 

 bodies. 



(6) Cavities may be formed by the faulting of the strata 

 without the formation of mountains. This may arise through 

 lateral pressure. The displacement, however small it may be, 

 affords a channel for the transference of solutions and the deposi- 



