PEEIUOTITE INTEUSIVES. 257 



with its orthorhombic pyroxene zone. Where it is in contact with tlie 

 augite it is the brown variety of hornblende, but is in optical continuity 

 with the green, which is the kind around the olivine and the orthorhombic 

 pyroxene. 



Of the remaining mineral constituents brown hornblende is the next 

 one in importance. It has in it patches of inclusions, previously described 

 as occui-ring in the hornblende of these ultrabasic rocks. It includes also 

 the augite and olivine. This brown hornblende is comparatively rarely 

 found in large plates, but usually as a rim of varying width around the 

 augite and olivine, as already described. Where it occurs in large plates it 

 is in that part of the section which is free from feldspar, and more closely 

 resembles the amphibole-peridotite phase. 



The biotite has a cream to light yellowish-brown color, and occurs in 

 irregular plates. The plagioclase feldspar is in irregular broad plates, and 

 forms the mesostasis. The feldspar contains, in not very large quantity, 

 small microlites, which by very high power are translucent and show a 

 greenish tinge. They are supposed to be hornblende microlites. 



PROCESS OF CRYSTALLIZATION. 



From the relations described as existing between the various minerals 

 it seems that the following stages may be outlined in the progress of the 

 consolidation of this rock. From the coarse even-grained character, and 

 from the fact that neither a fine-grained groundmass nor glass is present, 

 the conclusion seems to be warranted that it crystallized under high pressure 

 and must have, of course, at some time been under very hig-h temperature 

 also. 



The augite and olivine were the first and chief silicate constituents to 

 form, and crystallized out of the magma at approximately the same time. 

 The magma soon reached a condition unfavorable for further production of 

 olivine, probably on account of increasing acidity. Immediately around 

 the olivine there was formed, at this stage for a short while, the orthorhombic 

 pyroxene. The monoclinic pyroxene continued to grow during the forma- 

 tion of this orthorhombic variety. Finally, however, the condition was 

 reached when, in place of the monoclinic and orthorhombic pyroxenes, the 

 crystallization of hornblende began; 



It is not known what the conditions were which caused the formation 



MON XXXVI 17 



