240 BULLETIN OF THE 



and a fine groundmass. Among the phenocrysts angite greatly exceeds 

 hornblende, biotite is rare. The hornblende crystals are brown and 

 strongly plechroic ; they have lost their crystal angles and show rounded 

 resorption outlines. The same is the case with the few pieces of biotite 

 seen. With the angite, however, it is different ; this mineral appears 

 commonly with complete or partial crystal outlines, though some show 

 the rounded resorption form. It is evident that most of the angite 

 crystals were formerly rounded and have gained the angular form by 

 a later growth in the magma, for irregularly rounded cores can be dis- 

 tinguished surrounded by portions having different extinction angles. 

 The structure thus given closely resembles zonal structure ; zonal 

 structure does sometimes occur in these secondary borders. Tlie older 

 portions of the crystals are pale green in color, the borders are pinkish; 

 in one case the pink border is pleochroic, pink to greenish yellow. 



The phenocrysts of hornblende and angite may each contain rounded 

 inclusions of the other mineral, inclusions of the angite in the hornblende 

 being much more common. Thus there seem to have been five stages 

 in the crystallization of this magma: first, a time when hornblende and 

 angite formed in good-sized crystals ; second, these crystals were resorbed 

 until they became rounded grains ; third, another separation of large 

 crystals of angite and hornblende, which often enclosed indiscriminately 

 and without crystallographic relation the grains above mentioned; fourth, 

 this second generation of angite and hornblende underwent resorption 

 sufficient to destroy the ci-ystal outlines ; fifth, a final separation of 

 angite, renewing tlie crystalline form of the augite phenocrysts, and 

 pi'obably taking place at the same time as the crystallization of tlie 

 groundmass. The inclusion of hornblende crystals in augite, as well as 

 the converse, has been noted before,-^ but the peculiar association above 

 noted is perhaps new. The accompanying plate shows the essentiul 

 points above mentioned. Both varieties of phenocryst also contain 

 inclusions of caleite ; the frequent occurrence of iron oxide with tliis 

 calcite may indicate tliat the caleite fills cavities left by the solution of 

 the iron oxide. 



The groundmass is composed chiefly of minute augite crystals of a 

 pale pink or green color, closely compacted together. ^Magnetite in fine 

 crystals is abundant, perhaps composing a third u{ the bulk of the 

 groundmass. Small ciystals of brown hornblende are common, but 

 compose no considerable percentage of the mass. 



1 J. F. Kemp and V. F. Marsters, Amer. Geo!., August, 1889. Also J. F. Kemp, 

 Amer. Geol., ^Lirch, 1890. 



