80 



PHYSICAL GEOLOGY 



Removed \ \ bu Erosion 

 \ i 



copper, which happen to be dissolved by the waters elsewhere, are 

 deposited in veins. A fissure with a valuable metalliferous filling 

 of this sort is an ore vein (Fig. 61). 



Thus it appears that the zone of fracture may be divided 

 into two fairly distinct belts according to the nature of the 



changes which affect the 

 rocks. Near the surface, 

 and especially above the top 

 of the body of underground 

 water, the rocks are parti- 

 ally dissolved and tend to 

 decay. This is the belt of 

 weathering. At greater 

 depths, and chiefly in the 

 region where the rocks are 

 saturated with water, pores 

 and cracks are gradually 

 filled and the whole mass 

 becomes cemented. This is 

 the explanation of the sin- 

 Fro. 61. -Diagram of a copper vein. &}** fact that man y dee P 

 The entire vein was once rather lean mines are dry J there are no 



P ' ' lonjrer anv Dassasewavs 



copper ore removed soaked down into through which water may 



flow. In this belt of cementa- 

 tion, marked chemical 

 changes in the minerals themselves are in progress, resulting 

 in the formation of new minerals out of old ones. Thus gar- 

 net may change to chlorite, and augite may become horn- 

 blende. 1 



Alterations in the zone of flowage. With the pressure so 

 great as it must be beneath 5 to 6 miles of solid rock, it 



1 When a mineral is thus changed gradually into one of different com- 

 position, without altering the original form, the result is a pseudomorph. 

 Petrified wood, although not originally a mineral, is a pseudomorph in this 

 sense. 



Unenriched Vein 



the vein below and there formed a 

 particularly rich deposit (solid black). 



