56 PROCEEDINGS OF THE WASHINGTON MEETING 



intrusive bodies, the molten rock probably reached the surface, but all traces 

 of the lava have been removed by erosion. 



Petrography (by j. v. lewis) 



GENERAL CHARACTER 



Dominantly diabasic. Remarkable diversity of differentiation facies. Range 

 from coarse-grained granitic texture and pink to light or dark gray colors in 

 the larger bodies to dense black rock in thin sheets and dikes and in contact 

 facies. 



Chief constituents greenish black pyroxene and whitish to gray plagioclase, 

 the former generally preponderating. Approximately equal at many places and 

 locally the feldspar is in excess. The microscope shows plentiful magnetite 

 and minute apatite crystals and generally some quartz and orthoclase. Locally 

 in darker varieties much hypersthene or olivine or both. In lighter facies 

 quartz and orthoclase abound, chiefly in micrographic intergrowth. Here and 

 there biotite and, far less commonly, titanite. 



Pyroxene not uncommonly altered in part to uralitic amphibole or serpen- 

 tine and chlorite with granular magnetite. Corresponding alteration of feld- 

 spars yields fine scaly (apparently sericitic) aggregates and, less commonly, 

 kaolin. Epidote abundant in places. 



Typically diabasic — pyroxene filling angular interstices in a felted ground- 

 mass of slender plagioclase crystals. By coalescence of pyroxene into larger 

 areas, in which feldspars are imbedded, the texture becomes ophitic. Dense 

 varieties grade into typical basalt with- glassy ground-mass ; some with scat- 

 tered phenocrysts of pyroxene and, less commonly, feldspar and olivine. Tn 

 acid facies much quartz and orthoclase, in separate grains or micrographic 

 intergrowth, occupy angular spaces among plagioclase crystals and there is 

 much less pyroxene. 



ORDER OF CRYSTALLIZATION 



Prevailing diabasic — plagioclase completed before pyroxene. Two marked 

 exceptions: (1) Trismatic pyroxene crystals in some coarser quartz-orthoclase 

 facies with subordinate plagioclase. (2) Pyroxene phenocrysts in dense black 

 dikes, thin sheets, and contact facies, with few large feldspars. This earlier 

 crystallization, probably before intrusion, followed 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) pyrox- 

 ene, (6) micrographic quartz-orthoclase, (7) orthoclase, (S) quartz. 



VARIETIES OF DIABASE 



(1) Normal diabase, the most common pyroxene-plagioclase rock; (2) 

 feldspathic diabase, or anorthosite. chiefly plagioclase feldspar; (3) quartz 

 diabase, with abundant quartz, chiefly in micrographic intergrowth with ortho- 

 clase ; (4) viicropegmatite, mainly micrographic quartz-orthoclase; (5) aplitc, 

 essentially dense granular quartz-orthoclase rock; (6) hypersthene diabase, 

 much hypersthene, replacing in part monoclinic pyroxene; (7) olivine diabase, 

 with abundant olivine; (8) basaltic diabase, or basalt, dense black facies, in 



