344 



University of California Publications. 



[Geology 



MINETTE. 



In the underground work at the Ruth Mine there have been 

 brought to light two occurrences of lamprophyres neither of 

 which were detected at the surface of the ground. Both of these 

 have the mineralogical characters of minette. The first occurs 

 as a small dyke a few feet in width in No. 1 level east. The 

 dimensions and extent of the dyke are difficult to determine since 

 it has been affected by the various shearing movements in the 

 porphyry, which it cuts. Its exposed width is about 3 or 4 feet. 

 It strikes N. 45 E. and dips to the N.W. at 30° and conforms to 

 the direction of the slip planes in this part of the porphyry. It 

 is best seen at the first cross-cut east of the station on No. 1 level, 

 but may be traced for perhaps 50 feet beyond this point. 



The rock of the dyke is fine grained and has a dark pepper- 

 and-salt gray appearance. It is traversed by quartz stringers 

 in much the same way that the porphyry is but is not notably 

 mineralized. Under the microscope it is readily seen to be com- 

 posed of about equal proportions of brown biotite and feldspar 

 in an allotriomorphic granular aggregate. The feldspar is un- 

 twinned and in all sections has a lower refractive power than the 

 balsam. 



The second occurrence of the minette is a finer grained, com- 

 pact, bluish green rock, which, in the zone of oxidation, decom- 

 poses to a soft yellowish or brownish mass of almost cheesy con- 

 sistency. The thickness of this mass is not known since the mine 

 workings have not pierced it, and, owing doubtless to its soft 

 decomposed condition, it can not be traced at the surface. It 

 seems, however, to form a somewhat irregular wall limiting the 

 ore-bearing porphyry to the northeast, thus forming an impor- 

 tant element in the geology of the copper deposit. All of the 

 mine workings in the copper-bearing porphyry lie to the south- 

 west of it ; but the main shaft of the mine and the upper levels to 

 the east have penetrated it for some distance. 



In thin section the rock appears as a microcrystalline mosaic 

 of untwinned feldspar having a lower refractive power than 

 balsam, throughout which are abundant shreds of brown biotite 

 and numerous areas of secondary white mica. Besides this there 



