130 EDSON S. BASTIN 



it, but they are nowhere observed to continue into adjacent areas 

 of feldspar, hornblende, or pyrrhotite. 



Chalcopyrite.— The analysis shows this mineral to constitute 

 1.03 per cent, by weight of the rock. In the hand specimens and 

 slides it is seen to be associated irregularly and in small amounts, as 

 was pyrite, with the areas of pyrrhotite. 



Pyrite. — Small amounts of pyrite are associated irregularly with the 

 pyrrhotite. 



Biotite. — This mineral occurs as a few scattered plates seldom 

 over 0.7"^"^ in greatest dimension, which in some sections are entirely 

 surrounded by olivine. In other cases the biotite occurs in long narrow 

 areas between olivine and pyrrhotite. In one case, which is shown 

 in Fig. 2, a narrow biotite blade lies at right angles to the contact be- 

 tween perfectly fresh olivine and pyrrhotite, penetrating some distance 

 into each of the crystals. In this occurrence at least it is undoubtedly 

 primary, though some of the plates may be of secondary origin. 



Spinel. — A few grains of a dark green isotropic mineral occur in 

 certain portions of the rock in irregular association with pyrrotite and 

 hornblende or as inclusions in the latter mineral. Its grains are without 

 regularity in form and range in size up to 0.4™™ though mostly 

 under 0.2™"^. The mineral is probably pleonaste or chlorospinel. 



SECONDARY MINERALS 



Serpentine. — This mineral is developed in short, closely aggregated 

 fibers as an alteration product of olivine. Most of it is colorless in 

 the thin section ; but rarely light to deep yellow colors occur. Between 

 crossed nicols it is, in some cases, nearly isotropic, while in other 

 cases showing blue-gray interference colors. The serpentinization 

 begins in the normal manner at the borders of the olivine grains 

 and proceeds inwards along irregular cracks. The degree of serpen- 

 tinization varies greatly in different portions of the rock, some olivine 

 grains showing only a narrow border of serpentine not exceeding 

 QQj^mm \^ width along their contact with pyrrhotite and chalcopyrite, 

 while in other cases the olivine grains have been almost entirely 

 serpentinized. 



Amphibole. — In addition to original hornblende, whose occurrence 

 has already been described, amphibole occurs as reaction rims between 



