1922] Hudson: Geology of the Cuyamaca Region of California 233 



the diorite was first separated, then the copper as copper pyrites, and the iron as 

 pyrrhotite containing some nickel, and finally, in those portions remaining longest 

 molten the nickel separated as a true nickel mineral.so 



Vogt's latest ideas, published as a part of Beyschlag, Krusch and 

 Vogt's textbook on ore deposits, 51 depart but little from those in his 

 earlier publications. He appeals to the theory of limited miscibility 

 to explain the separation of sulphides from a sulphide-rich silicate 

 melt. This is derived from Harker's original statement of the "theory 

 of limited miscibility in rock magmas.'"' 2 Soret's principle is still 

 believed to be the explanation of many of the peripheral ore bodies, 

 while the idea of gravitative settling is employed to explain the Sud- 

 bury and certain Norwegian occurrences. 



As a result of detailed petrographic work on rocks from the Sud- 

 bury "nickel-eruptive," Dresser 53 concludes that the larger masses of 

 ore are syngenetic and that their segregation was due to their im- 

 miscibility in molten norite. In addition to this, he believes that the 

 partially consolidated norite contained liquid sulphides and "acid 

 mother liquor," which, as a result of dynamic action, may have been 

 "filter pressed" to regions of less pressure. This theory is used to 

 explain the presence of sulphides found high up in the norite and the 

 quartz and pegmatite of the lower part of the norite. 



This section of the report would be incomplete without reference 

 to an important paper by Knopf, descriptive of "A magmatic sulphide 

 ore body at Elkhorn, Montana." 54 The ore here consists of pyrrho- 

 tite, containing no nickel, augite, and a minor amount of chalco- 

 pyrite. Brown hornblende, biotite, plagioclase, and quartz occur 

 sparingly. 



The pyrrhotite and chalcopyrite are closely associated, the ehaleopyrite, as 

 seen both with the unaided eye and with the metallographic microscope, forming 

 small, separate and distinct, solid particles surrounded by pyrrhotite. The avail- 

 able evidence appears to show that the two sulphides are essentially contem- 

 poraneous in origin. They occur either as interstitial masses between the augite 

 grains, or as irregular intergrowths with them. It is noteworthy that the augite, 

 although invariably anhedral where in contact with other grains of augite, 

 shows a closer approximation to its idiomorphic outlines where it is joined or 

 surrounded by sulphides. Characteristic quadratic cross-sections with truncated 

 corners are occasionally found. The grains of augite, where enveloped by 



so Browne, D. H., Col. Univ. Sch. Mines, Quart., vol. 16 (1895), p. 311. 

 si Beyschlag-Kruseh-Vogt, Die Lagerstatten der nutzbaren Mineralien und 

 Gesteine (Stuttgart, 1914), vol. 1, pp. 300-311. 



52 Harker, A., The natural history of igneous rocks (New York, 1909), pp. 

 196-200 (Harker's statement is much clearer than Vogt's). 



53 Dresser, M. A., Econ. Geol., vol. 12 (1917), pp. 563-580. 



54 Knopf, Adolph, Econ. Geol., vol. 8 (1913), pp. 323-336. 



