176 University of California Publications in Geology [Vol. 13 



PAGE 



Petrographic summary 203 



Rock alteration 204 



Relation between the different rock types 205 



Form and type of intrusion 206 



Age of the intrusion 207 



The Rattlesnake granite 207 



Effect of the intrusion on wall rocks 208 



Basic dikes ; 209 



Pegmatite 209 



Petrography of the pegmatites 209 



Aplite 210 



Geologic structure 211 



The Friday Mine ore body 212 



Discovery and development 212 



General geologic description 212 



The rocks of the mine 213 



The massive ore body 213 



Basic dikes 214 



Pegmatite 214 



The boundaries of the ore 215 



Minerals of the massive ore 215 



Interrelations of minerals 217 



The disseminated sulphides of the Cuyamaca basic intrusion 218 



Relationships between the sulphide minerals 219 



Relation of sulphides to rock alteration 220 



Relation of sulphides to total composition of rocks 220 



The origin of the disseminated sulphides 224 



Conclusions : 224 



Relation of the massive ore body to the enclosing rocks 225 



Detailed description of gradational contact 226 



Origin of the massive ore 227 



Theories of origin of nickeliferous pyrrhotite 228 



Theories of syngenetic origin 229 



Theories of epigenetic origin, involving replacement 235 



Intrusive sulphide theory 237 



Applicability of the various theories to the Friday Mine deposit 238 



Syngenetic theories 238 



Intrusion of sulphide magma 239 



Epigenetic theories, involving replacement 239 



