1922] Hudson: Geology of the Cuyamaca Region of California 237 



We conclude that the ores are later than the silicates, for the reason that 

 all the silicates indiscriminately occur as relicts in a groundmass of ore. The 

 ore-minerals surround the silicates, enter along the contacts between them, cut 

 them, and penetrate easily cleavable minerals such as biotite. In some cases 

 they cut the silicates in well defined veinlets. These relations are explained, 

 in part, by those favoring an early magmatic origin of the ores as follows: 

 The sulfid ores remain in a molten condition during the formation of the pri- 

 mary silicates (we add: during the formation of the late magmatic hornblende), 

 and then solidify. ' 



From the latter part of the foregoing statement one would think 

 that Tolman and Rogers believe that the sulphides existed at one time 

 as molten substances, and one might conclude that their theory of 

 origin differed from that of Vogt only with regard to the precise time 

 when the sulphides solidified. That such is not the case is shown by 

 the following quotation in which the authors clearly state that the ores 

 came from without, and replaced solid silicate minerals: 



The process, however, is not one of corrosion, but of replacement. If the 

 ores were molten, corrosion should produce metallic silicates by reaction. No 

 such metal-bearing slag is found. The phenomena are those of ordinary replace- 

 ment, and the agency that brought in the sulfids removed the dissolved silicates, 

 all of which indicates active mineralizers.ss 



Intrusive Sulphide Theory 



From a petrographic study of rock from the Frood Mine, Howe 

 was unable to conclude as to whether or not the ore and silicate min- 

 erals were contemporaneous. 89 On the other hand, he concluded 

 from both field observations and laboratory study that the Creighton 

 deposit resulted from an intrusion of pyrrhotite into already solidified 

 norite, the differentiation of sulphides and silicates having been ef- 

 fected, not in the place where the ores are now found, but in the 

 magma chamber from which the norite originally came. 70 He accepts 

 the statement of Campbell and Knight that in the ore pyrrhotite is 

 cut by pentlandite and these two in turn by chalcopyrite, but believes 

 that the relations can be better explained by the ' ' nearly simultaneous 

 cooling of the different sulphides that had previously separated as 

 distinct mineral compounds, non-miscible, though still molten." 71 



Bateman has presented an hypothesis which is a modified or am- 

 plified form of that advanced by Howe. In brief it is that the Sud- 

 bury deposits are to a minor extent due to magmatic segregation in 

 place ; in greater part to intrusions of pyrrhotite, as postulated by 

 Howe, and in minor part to hydrothermal action. 72 



Ibid., p. 15. oo Howe, E., Econ. Geol., vol. 9 (1914), p. 514. 



to Ibid., p. 521, 71 IMd., p. 522. 



"Bateman, A. M., Econ. Geol., vol. 12 (1917), p. 426. 



