1922] Hudson: Geology of the Cuycmaca Region of California 229 



or during the solidification of the magma. The proponents of the 

 non-magmatic theories conceive of the introduction of the sulphide 

 minerals into the igneous rock, after its consolidation, by either hydro- 

 thermal or contact metamorphic processes. 



Tolman and Rogers in their recent paper conclude that the ore 

 minerals were introduced by mineralizers during a "late magmatic 

 stage," and replace the previously formed silicate minerals. They 

 apply the term magmatic to this process. Following their complete 

 theory we must admit that the mineralizers, which they believe effected 

 the replacement of silicates by sulphides, would be magmatic in ulti- 

 mate origin. Their application of the term magmatic seems unfortu- 

 nate, however, as the rocks in which the deposits occur were, accord- 

 ing to their conception, solid rocks and not magmas at the time the 

 ores were introduced. 



In order to avoid confusion the terms syngenetic and epigenetic 

 will be used here. Syngenetic implies that the materials of the ore 

 bodies were essential constituents of the magma before its consolida- 

 tion and that the ore bodies were formed from material derived from 

 the adjacent magma before or during its consolidation. Epigenetic 

 implies that the ore minerals were introduced after the consolidation 

 of the igneous rocks. The magmatic theories of most authors postu- 

 late a syngenetic origin of the ore bodies. 



Theories of Syngenetic Origin 



In 1890 Dr. Robert Bell announced, as a result of his study of 

 the Sudbury deposits, that the ores were syngenetic with the inclosing 

 igneous rocks. The following is quoted from his paper : 



The ore bodies .... do not appear to have accumulated like ordinary metal- 

 liferous veins from mineral matter in aqueous solution, but to have resulted 

 from igneous fusion. The fact that they are always associated with diorite, 

 which has been left in its present positions in a molten state, points in this 

 direction. As the diorite and the sulphides fuse at about the same temperature, 

 they would naturally accompany each other when in the fluid condition. 26 The 

 bodies of molten diorite and the sulphides, being large, would remain fluid for 

 a sufficient time to allow the diffused sulphuretted metals to gather themselves 

 together at certain centers by their mutual attractions and by concretionary 

 action. In the case of great irrupted masses of diorite, the bodies of ore which 

 had formed near enough to the solid walls cooled and lodged with a mixture 

 of the broken wall rocks where we now find them, while larger quantities, re- 

 maining fluid, probably sank slowly back through the liquid diorite to unknown 

 depths 27 



26 The diorite and part of the greenstone of the earlier investigators of the 

 Sudbury district are norites. (F. S. H.) 



27 Bell, Robert, Bull. Geol. Soc. Am., vol. 2 (1891), p. 135. 



