634 STOSE AND LEWIS TRIASSIC IGNEOUS ROCKS 



before that of the pyroxene. To this general rule there are, however, two 

 exceptions: (1) In some of the coarser grained feldspathic and quartzose 

 facies of the rock, in which orthoclase is the chief feldspar and plagio- 

 clase is much less abundant, the pyroxene has a well developed prismatic 

 form and has evidently preceded the bulk of the feldspar in crystalliza- 

 tion. (2) The common occurrence of pyroxene phenocrysts in the fine- 

 grained contact facies of the larger intrusives and in the dense black 

 dikes and thin sheets, Avith only here and there a large feldspar, indicates 

 that the conditions of earlier crystallization, probably antedating intru- 

 sion, would have led to the completion of the pyroxene before the feldspar 

 and the production of a rock of gabbroic texture. 



Apatite and magnetite show hy their crystalline form and their indif- 

 ference to the other minerals that they were among the first products of 

 crystallization. Here and there an inclusion of apatite in magnetite 

 shows that the former mineral preceded the latter. Olivine, where it oc- 

 curs, came before both the pyroxene and the feldspars, in both of which 

 it forms inclusions. Orthoclase and quartz were the last constituents to 

 crystallize — first in micrographic intergrowth, where this was formed, 

 and then as separate grains of the two minerals, with quartz alone filling 

 the last interstices. The prevailing order of crystallization in these rocks 

 may be enumerated therefore as follows : (1) Apatite, (2) magnetite, (3) 

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

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



VARIETIES 



General description. — -Differentiation of the diabase magma has given 

 rise to numerous well defined varieties in which the minerals occur in 

 widely varying proportions. Thus (1) normal diabase is the prevalent 

 pyroxene-feldspar aggregate already described; (2) feldspathic diabase, 

 or anorthosite, consists chiefly of plagioclase feldspar; (3) quartz diabase 

 contains abundant quartz, largely in micrographic intergrowth with or- 

 thoclase; (4) micropegmatite consists essentially of the graphic inter- 

 growth of quartz and orthoclase; (5) aplite is a dense granular quartz - 

 orthoclase rock; (6) liyperstlicne diabase carries much hypersthene, which 

 replaces in part the common monoclinic pyroxene; (7) olivine diabase 

 has abundant olivine; (8) basaltic diabase, or basalt, is the dense black 

 facies, some of which is vesicular and has a glassy ground-mass; (9) 

 olivine basalt is the dense black variety, with abundant olivine. 



Normal diabase. — Except near the contacts, the rock of the larger 

 bodies, both sheets and cross-cutting masses, is for the most part medium 

 fine to coarse grained and ranges in color from light gray and pinkish to 



