384 /• MORGAN CLEMENTS 



a narrow zone around the olivine. The hornblende is predom- 

 inately a brown variety, showing strong pleochroism ; a is light 

 cream yellow, C is yellowish-brown, and fo is reddish-brown. 

 b>C>8. Patton z has already called attention to the exceptional 

 pleochroism of this hornblende, in which the brownish color is 

 that of rays vibrating parallel to the orthodiagonal axis. The 

 brown hornblende is accompanied by a small quantity of green 

 hornblende, which is in crystalline continuity with the brown, 

 and is apparently original. The biotite began to crystallize 

 before the hornblende had ceased growing, as we find it in 

 ragged plates included by it, especially upon the periphery of 

 the individuals. It is normally the least well-developed mineral 

 present. Feldspar is present at times in small quantity, and 

 forms the mesostasis. The olivine possesses a certain interest, 

 as it is surrounded by zones 1 of different minerals; first, ortho- 

 rhombic pyroxene, surrounded in its turn by green compact 

 hornblende, which is in optical continuity with the predominant 

 brown hornblende of the rock. This green hornblende lies next 

 to the feldspar, and is traversed by anastomosing tabular feld- 

 spar growths. 



From the relations described as existing between the various 

 minerals composing the peridotite, 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 pres- 

 ent, the conclusion is warranted that it consolidated very slowly, 

 and must have, of course, at some time been under very high 

 temperature. The olivine and augite were the first of the 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. There was then formed 

 the orthorhombic pyroxene occurring in a zone surround- 

 ing the olivine. The monoclinic pyroxene continued to grow 

 during the formation of this orthorhombic variety, as it is 



1 Loc. cit., p. 186. 



