225] J. T. Singewald 27 



it was early recognized that in part at least the sulphides are 

 distinctly later than the rock-forming silicates, and the rock 

 itself has frequently undergone considerable alteration. 

 Many geologists have consequently insisted on .regarding 

 them as hydrothermal deposits. A most interesting feature 

 of the controversy over the genetic position of these sulphidic 

 ores has been that the largest and most important example, 

 the nickel deposits of Sudbury, Ontario, which has been cited 

 by the advocates of the magmatic origin as a typical illustra- 

 tion of that type, is one to which most serious objection has 

 been raised by those contending for a hydrothermal origin. 



In view of the departures manifested by these deposits 

 from the conceptions based on purely petrographic pheno- 

 mena and concepts, it is interesting to see how the problem 

 has been handled in four of the leading recent textbooks on 

 ore deposits. The four selected are: R. Beck, Die Erzlager- 

 statten, 1909 (3rd edition) ; Beyschlag-Krusch-Vogt, Die 

 Lag erstdtten der nutzbaren Mineralien und Gesteine, 1910; 

 W. Lindgren, Mineral Deposits, 1913; L. DeLaunay, Giles 

 Mineraux et Metalliferes, 1913. 



Beck, in denning magmatic segregations, says : " In many 

 instances there took place in the rock either before or during 

 solidification from the molten state a concentration of the 

 ores into irregular masses. ... In spite of the concentra- 

 tion into a limited space, the ores of such deposits remain, 

 what they as scattered particles in the rocks in question are, 

 namely accessory constituents." Commenting on Vogt's ob- 

 servation that in certain of the Swedish and Norwegian titani- 

 ferous iron ore deposits the silicates formed first and then 

 the titanif erous magnetite, he says, " these are departures 

 from the rule otherwise prevailing for eruptive rocks that the 

 iron ores belong to the earliest minerals to separate." In 

 all chrome deposits for which he cites the sequence of crys- 

 tallization, the chromite is the earliest constituent. Of the 

 sulphidic deposits, on the other hand he says, " The strict 

 proof of segregation from a molten magma cannot always be 

 established with the same degree of sharpness. . . . For a 



