THE SULPHIDE ORES. 1105 



pyrite, marcasite, pyrrhotite, and other forms — exceed in quantity many 

 fold all other sulphides. Standing next in amount is the group of zinc, 

 lead, and copper sulphides, the order of abundance probably being that 

 named. Nickel, arsenic, and antimony sulphides, also occur in important 

 amounts. Mercury and silver sulphides, while important ores of these metals, 

 are insignificant as compared with those previously mentioned so far as 

 absolute quantity is concerned. As to the original source of the sulphides, 

 it is well known that sulphide of iron occurs as an original constituent of 

 igneous rocks. Probably the same is true of the other sulphides, but as their 

 quantity is very much less they have been little noticed. Even if sulphides 

 had not been observed to be original constituents of the igneous rocks, the 

 large amount of sulphur compounds issuing from the interior of the earth, 

 in connection with volcanism, would lead to the conclusion that sulphides 

 must exist in the igneous rocks. This makes it highly probable that sulphur 

 as sulphide is or was present in sufficient quantity in the original rocks to 

 fully account for all of the sulphur compounds of the ore deposits. 



While it is doubtless true that the original sources of all the sulphide 

 ores are the sulphides of the igneous rocks, it "does not follow that the 

 igneous rocks are the immediate source of existing sulphide deposits. 

 Indeed, in many instances this is not the case. For instance, it will be 

 seen (p. 1146) that the sulphides of the lead and zinc deposits of the Missis- 

 sippi Valley are segregated from sulphides in limestones. These sulphides 

 were derived from the sulphides of earlier rocks which were probably trans- 

 formed to sulphates, transported to the sea, and in the sea, as shown by 

 Chamberlin, precipitated as sulphides at the same time the limestone was 

 formed. In other cases, the sulphides of a particular ore deposit may be 

 derived from a metamorphic rock. In many instances the sulphides of an 

 ore deposit have their immediate source very largely in sulphides which 

 have undergone one or more cycles of segregation by the processes which 

 are given below. In many cases the sulphides of a given ore deposit are 

 not derived from any single source or rock, but from the various sidphides 

 in all the rocks through which the particular system of underground 

 circulation producing the ore deposit passed. 

 mon xlvii — 04 70 



