FORMATION OF VEINS 267 



change has been brought about in the following way. Pyrite, on 

 exposure to air and moisture, slowly absorbs oxygen and is con- 

 verted into the sulphate of iron (FeSo 4 ). The latter is an unstable 

 compound, and continued exposure to air and water converts it 

 into limonite, or ferric hydrate, with liberation of sulphuric acid ; 

 the gold is thus left free and the limonite stains the broken quartz 

 rusty brown or red. 



As mineral veins so frequently occupy faults, they generally are 

 inclined at high angles, and like faults may be intersected by other 

 veins. If the vein thus cut have any hade, or inclination from the 

 vertical, it will itself be dislocated and slipped, just like an ordi- 

 nary stratum. In this way we may determine which of two cross 

 veins is the older, for it will be faulted, while the newer one will 

 continue uninterruptedly across the line of intersection. 



When we attempt to determine the manner in which mineral 

 veins, and especially the metalliferous varieties, have been formed, 

 we find many facts that are extremely puzzling, and no explana- 

 tion has been devised that will cover all cases. The great economic 

 importance of metalliferous veins has caused them to be carefully 

 studied in many different parts of the world, but experience gained 

 in one region is apt to be contradicted, in one or more particu- 

 lars, by that gathered in other regions. In a general way, we may 

 be confident that the minerals were deposited from hot alkaline 

 solutions, brought up from the depths of the fissure, just as we 

 have found to be the case in certain hot springs now active (see 

 p. 130). The vein stuffs show, both by their arrangement in the 

 fissure and by their microscopic characters, that they have been 

 deposited from solution,, and such minerals as quartz are dissolved 

 in quantity only by hot alkaline waters. Both from observation 

 and experiment we learn that the alkaline sulphides are the natural 

 solvents of the heavy metallic sulphides, the form in which so 

 many of the ores occur. The materials deposited appear to be 

 derived from various depths of the fissure, and it has been noticed 

 in many instances that the contents of the vein change, as the 

 country rock changes. 



Lead deposits occur in a different fashion from the metals men- 



