ALLEN: ORE DEPOSITION 171 



and this characteristic of the mineral may be taken as an index of 

 the temperature which prevailed in its genesis. In a similar way, 

 the composition of original solutions has to a slight extent been 

 determined, since it has been observed that crystal form is some- 

 times determined by the composition of the solution in which 

 the crystal grew. For the determination of pressure limits, 

 there is at present no method. 



In drawing inferences regarding the genesis of natural minerals 

 from experiments in mineral synthesis, one must of course be 

 constantly on guard lest he mistake secondary for essential con- 

 ditions. It is therefore necessary for the chemist who works in 

 this field to keep in constant touch with the geologists. 



The Sulphides of Iron. — In this paper we shall consider the 

 application of some of the above principles to the mineral sulphides 

 of iron which form one of the most important classes of the sul- 

 phide ores. The frequent association of these minerals, pyrite, 

 marcasite and pyrrhotite, with other valuable minerals of the 

 sulphide group would indicate that the knowledge of the condi- 

 tions of the genesis of the former might be applied to the latter 

 as well. Also the association of the different sulphides of iron 

 with one another have suggested several interesting problems of 

 more particular nature which will here be considered. 



Pyrite and Marcasite. — Pyrite and marcasite,, the disulphides 

 of iron, often occur in such geological formations as to lead to the 

 belief that they were precipitated from cold surface solutions. 

 A typical case of this character is the deposit in the Mississippi 

 Valley. Geologists have reasoned that these and similar deposits 

 have been precipitated from sulphate solutions because surface 

 waters have frequently been observed by them to carry ferrous 

 and ferric sulphates which were formed by the oxidation of older 

 deposits of pyrite or marcasite thru the action of atmospheric 

 influences: 



FeS 2 +7 O + H 2 = FeS0 4 + H 2 S0 4 . 



Observation has gone a step farther. Recent pyrite has been 

 observed in wooden conduits, and on the twigs of trees which 

 have fallen into hot springs, and the fact that the same mineral 



