229] J. T. Singeiuald 31 



separate them entirely from the true segregations, for there 

 has been a concentration of sulphides not only in the rock 

 but at its contact, and they appear to him to be a close 

 parent to contact metamorphic deposits which are formed 

 when the wallrock is a limestone. They represent a type of 

 ore deposition in which mineralizers are more abundant and 

 active than in the preceding. The very close relation postu- 

 lated between these deposits and typical contact metamorphic 

 deposits indicates clearly that he does not look upon them 

 as representing a crystallization from a molten state. The 

 mineralization of the last group is analogous to that of the 

 fourth, the types of deposits included under it being formed 

 where the country rock is other than a carbonate rock. It 

 includes most of the deposits classified by Beyschlag, Krusch 

 and Vogt as injected sulphide deposits. There are no sharp 

 lines of demarcation between these groups, as DeLaunay 

 recognizes a complete transition from purely igneous deposits 

 to hydrothermal veins and that in some instances it is difficult 

 to decide between fusion and solution. 



In an attempt to settle some of the doubtful points con- 

 cerning the mode of formation of the sulphidic ores usually 

 classed as magmatic segregations, C. F. Tolman, Jr. and A. 

 F. Eogers of Stanford University have just published the 

 results of a very comprehensive petrographic and metallo- 

 graphic investigation of those ores as a monograph entitled 

 " A Study of the Magmatic Sulfid Ores." They formulate a 

 number of statements which they find applicable to all of the 

 deposits studied, the most significant of which are : 



1. The first minerals to form are olivine, the pyroxenes 

 and the feldspars. 



2. Magmatic alteration of the silicates often takes place 

 prior to the formation of the ore minerals. The most com- 

 mon change is that of pyroxene to hornblende, but easily 

 distinguishable from the hydrothermal process of uralitiza- 

 tion. 



3. The ores replace the silicate minerals but without re- 

 action rims. 



