OF (till-: 



lead one to the conclusion that it is an original mineral. In 

 the peridotite belt that stretches from Alabama to Nova Scotia, 

 in North Carolina especially, corundum occurs in considerable 

 abundance as an original mineral and emery is found in Massa- 

 chusetts. The former has the formula Al 2 Oj and the latter 

 contains hematite or magnetite intimately mixed with corun- 

 dum. These original minerals, surpassed in hardness only by the 

 diamond, are worthy of mention here for their use as abrasives. 

 (3) The third class as worked out by Vogt was that of the 

 metallic sulphides. The segregation of metallic sulphides is 

 characteristic of several types of igneous rocks as a result of dif- 

 ferentiation either before or after intrusion. They are common 



Fia. 21. Section to illustrate the mode of occurrence of sulphidic segre- 

 gations connected with norite intrusions. 1, Norite; 2, pyrite; 3, gneiss. 

 (After Thomas and MacAlister's Geology of Ore Deposits.) 



products of segregation from magmas rich in ferro magnesian 

 minerals. The sulphides commonly occurring with igneous origin 

 are copper, iron, cobalt and nickel. (See Fig. 21.) Associated 

 with these are the arsenides of a few metals. These are less im- 

 portant and need not be further considered. The sulphide of 

 copper most commonly segregating is chalcopyrite, GuFeSj, 

 Bornite and chalcocite may also occur as primary minerals. 



Pneumatolysis. Pneumatolysis demands the formation of ore 

 bodies from both acid and basic intrusives through the agency of 

 gases dissolved in the magmas. Because the gases were present 

 at the time of the intrusion, they were under high pressure and 

 above their critical temperature. The magmatic vapors and 



