128 EDSON S. BASTIN 



As shown in Fig 2, the pyrrhotite is normally completely allotrio- 

 morphic with respect to olivine grains, its relations in this respect 

 being similar in every way to those exhibited by the feldspar. The 

 pyrrhotite is also in contact with fresh massive hornblende and with 

 almost unaltered plagioclase feldspar. The allotriomorphic relation 

 of nearly all the pyrrhotite to unaltered grains of the original mineral 

 olivine is considered to be conclusive evidence that practically all 

 the pyrrhotite is an original crystallization from the magma and is 

 essentially contemporaneous with the other principal constituents 

 of the rock. In a few narrow zones traversing the rock, pyrrhotite 

 occurs in angular areas more or less connected with each other and 

 associated with chlorite and hornblende. These very limited portions 

 may be a secondary crystallization. 



Feldspar. — In the hand specimens this mineral forms dull-gray 

 areas with irregular outlines, never over one-fourth of an inch and 

 seldom over one-eighth of an inch across. Under the microscope 

 it is found to occupy irregular areas between the rounded olivine 

 grains, with respect to which the feldspar is entirely allotriomorphic. 

 It shows albite and occasionally carlsbad twinning, the angles of 

 the former running up to thirty degrees. The index of refraction 

 as determined on powdered material by the immersion method 

 approximates 1.55. The angle in the section perpendicular to the 

 negative bisectrix between M and the plane of the optic axes is twenty- 

 nine degrees. These results show that the feldspar is andesine- 

 labradorite with approximately the composition Ab^Anj. These 

 are also the exact proportions in which albite and anorthite are present 

 in the norm as calculated from the analysis. 



With the exception of the reaction rims of amphibole described 

 in the section on secondary minerals, the feldspar is in general very 

 fresh though some of it is clouded with aggregates of small, highly 

 refracting plates and flakes. Most of these have indices of refraction 

 which are only slightly greater than that of the feldspar, and are 

 probably muscovite. Others of higher index are probably an amphi- 

 bole similar to that developed between the feldspar and the olivine. 



Hornblende. — The mineral next in importance is hornblende, 

 which, in some parts of the rock, is almost wholly absent, but in other 

 parts is present in large crystals, which are allotriomorphic with 



