233] J. T. Singewald 35 



cates that concentration of the metallic content of a magma 

 to the extent necessary to form important ore bodies takes 

 place only when the necessary migration of the metals is 

 aided by the presence of mineralizers. Eor the only group 

 recognized by him in which mineralizers did not participate 

 in ore deposition, his gites d' inclusions, contain no deposits 

 of economic importance; and it is only in his next group, 

 in which mineralizers begin to play a part, that important 

 ore deposits begin to be represented. There have been two 

 lines of thought seeking to explain ore genesis, the one repre- 

 sented by the French school which has always emphasized 

 the role of mineralizers, and the other by the American and 

 German economic geologists who have tended to draw the 

 sharp line of demarcation between the magmatic deposits and 

 the non-magmatic. The participation of the latter group has 

 so greatly preponderated over that of the former during the 

 last quarter century in the development of the science of 

 economic geology that the views of the French school have 

 often been completely overshadowed and have not received 

 the attention they merit. The monograph by Tolman and 

 Rogers will serve to establish among American economic geol- 

 ogists the ideas embodied in the conceptions of the French 

 school. It is hoped that this survey of the problem and the 

 corroborative evidence contributed in the case of the iron 

 ores will serve the same purpose. One cannot help but feel 

 that a new study of the chrome ores with this interpretation in 

 mind would place them in harmony with it. As their case 

 now stands, they seem to be an exception and to represent a 

 direct segregation as the first product of crystallization from 

 a molten magma. 



