PETROGRAPHY 631 



The chief constituents are a greenish black pyroxene and a whitish to 

 gray plagioclase, largely andesine-labradorite. The pyroxene generally 

 preponderates, but at many places feldspar is approximately equal in 

 amount and locally it is in excess. The microscope shows plentiful magne- 

 tite, minute apatite crystals, and generally some quartz and orthoclase — 

 these latter very commonly in micrographic intergrowth. Locally in the 

 darker varieties there is much hypersthene or olivine or both, while in the 

 lighter facies quartz and orthoclase abound. Here and there biotite is 

 observed and, far less commonly, titanite. 



Not uncommonly the pyroxene is altered in part to uralitic amphibole 

 or to serpentine and chlorite with granular magnetite. The correspond- 

 ing alteration of the feldspars yields fine scaly (apparently sericitic) 

 aggregates and, less commonly, kaolin. In places epidote is abundant. 



The texture is typically diabasic — the pyroxene filling angular inter- 

 stices in a felted ground-mass of slender plagioclase feldspar crystals. By 

 the coalescence of the pyroxene into larger areas, in which the feldspars 

 are imbedded, the texture becomes ophitic. Dense varieties grade into 

 typical basalt with glassy ground-mass — some with scattered phenocrysts 

 of pyroxene and, in places, feldspar and olivine. In the more acid facies 

 of the rock there is much quartz and orthoclase, either in separate grains 

 or in micrographic intergrowth or both. These occupy angular spaces 

 among the plagioclase crystals and there is much less pyroxene. 



The order of crystallization is prevailingly diabasic — that is, the plagio- 

 clase was completed before the pyroxene — but there are two marked ex- 

 ceptions: (1) Pyroxene forms prismatic crystals in some of the coarser 

 quartz-orthoclase facies of the rock, in which plagioclase is a subordinate 

 constituent; (2) there are pyroxene phenocrysts in the dense black dikes, 

 thin sheets, and contact facies, in which feldspar phenocrysts are few. 

 Thus the earlier crystallization, probably antedating the intrusion, fol- 

 lowed the usual order in plutonic rocks and would have produced a gabbro. 

 In the normal diabase the order has been: (1) Apatite, (2) magnetite, 

 (3) olivine, (4) plagioclase, (5) pyroxene, (6) micrographic quartz and 

 orthoclase, (7) orthoclase, (8) quartz. 



The varieties described are: (1) Normal diabase, the most common 

 pyroxene-plagioclase rock; (2) f elds path ic diabase, or anorthosite, chiefly 

 plagioclase feldspar; (3) quartz . diabase , with abundant quartz, much of 

 it in micrographic intergrowth with orthoclase; (4) micropegmatite, 

 consisting in the main of micrographic quartz and orthoclase; (5) aplite, 

 essentially a dense .granular quartz-orthcclase rock; (6) hypersthene dia- 

 base, with much hypersthene, replacing in part monoclinic pyroxene; (7) 

 olivine diabase, with abundant olivine; (8) basaltic diabase, or basalt, 



