1922] Hudson: Geology of the Cuyamaca Region of California 227 



mass. The tests, however, are suggestive of marcasite and the material 

 is probably then a partial replacement of pyrrhotite by that mineral. 



Origin of the Massive Ore 



The close genetic relationship of the massive ore body to the norite 

 is shown by the following facts : 



(1) The primary gangue minerals of the ore are ferromagnesian 

 silicates commonly found in the norite. 



(2) The ore minerals of the massive ore body are the same as 

 those found as disseminated particles in the norite, save that polydy- 

 mite ( ?) occurs in place of pentlandite. 



(3) The ore was formed before the cessation of activity of the 

 basic magma. This is shown by the fact that dikes of pyrrhotite- 

 bearing, hornblende norite, cut not only the massive norite but also 

 the ore body. The rock of these dikes is no different from the ordinary 

 norite save that it is finer grained. 



That the ore was not introduced from without after the solidi- 

 fication of the rocks which now inclose it is shown by (1) lack of 

 either large or small scale veins of sulphides in the norite, (2) grada- 

 tional contact between ore body and norite, (3) lack of replacement 

 of the silicate by ore minerals. 



The massive ore body is thought to have accumulated as an ultra 

 basic differentiate of the norite magma, before or during the consoli- 

 dation of the norite which now forms its walls, for the following 

 reasons : 



(1) The molten norite was a competent source, as the norite now 

 carries disseminated sulphides which were normal constituents of the 

 magma. 



(2) The rock surrounding the ore body is more basic as regards 

 its silicate constitution than is the whole mass of the Cuyamaca Basic 

 Intrusion, and its content of sulphides is greater than is that of the 

 whole mass. If differentiation from a gabbro-norite with labradorite- 

 bytownite feldspar, and 0.7 per cent sulphides, to an olivine norite, 

 with bytownite feldspar and 1.1 per cent sulphides, can take place, 

 there is every reason to suppose that further action could produce a 

 rock made up of pyrrhotite, nickel and copper sulphides, hornblende, 

 and augite. 



It was shown, in the case of the disseminated ore particles in 

 norite, that the sulphides were probably still in liquid condition after 

 the solidification of the silicate minerals. It is probable that this 



