226 University of California Publications in Geology [Vol. 13 



It is easy to understand how such a narrow transition zone could 

 be erased by slight slipping, and the conclusion, based on lack of 

 gouge, that the bounding slip planes of the ore body were the result 

 of only minor movement, is confirmed. 



Detailed description of gradational contact. — The following obser- 

 vations are based on the examination of a suite of thin sections of the 

 different stages of the transition zone, and of a polished surface cut 

 so as to show several centimeters on either side of the contact. 



The norite from 1.5 to 2.0 cm. from the contact is an olivine norite 

 with hypersthene, monoelinic pyroxene, and brown hornblende. The 

 plagioelase is labradorite-bytownite. Some spinel and pyrrhotite, pent- 

 landite, and chalcopyrite are present. 



The rock at 0.5 to 1.5 cm. from contact is similar to the above 

 except that the feldspar is anorthite. Sulphides occur in consider- 

 able amount, probably 5 per cent of the whole mass. The textural 

 relations of the pyrrhotite to the silicates is no different from those 

 seen in all norites. Rounded grains of pyrrhotite within fresh hypers- 

 thene crystals are particularly abundant. On approaching closer to 

 the contact the spinel and sulphides increase in amount. The brown- 

 hornblende also becomes more abundant while the hypersthene seems 

 to remain constant in amount and the feldspar decreases. A few 

 millimeters from the contact a large angular mass of pyrrhotite, con- 

 taining chalcopyrite and polydymite ( was noted within fresh sili- 

 cates. The actual contact is an extremely irregular surface along 

 which the ore and norite penetrate one another for distances up to 

 1 cm. from a median plane. The ore adjacent to the contact consists 

 of pyrrhotite, polydymite (?), and chalcopyrite with bluish-green 

 hornblende in somewhat rounded prisms and confused aggregates of 

 actinolite. A small amount of calcite is found in minute veinlets and 

 in pseudomorphs after primary minerals. Two minute rounded masses 

 of a bright yellow substance, identified as serpentine, were found in 

 one of the thin sections. 



The ore adjacent to the norite is no different from that found in 

 other portions of the ore body, with one exception. There is a yellow- 

 ish material of high metallic luster that bears the same textural re- 

 lations to both silicates and polydymite ( ?) and chalcopyrite as does 

 the pyrrhotite. It shows a peculiar concentric banding which seems 

 related to tiny veinlets of calcite which penetrate the ore. Micro- 

 chemical tests did not establish the identity of this material, except 

 to show the presence of irregular patches of pyrrhotite within its 



