340 The America?! Geologist. December, 189^ 
The crystals vary in color from purplish hrown to light yel- 
lowish green; the former being the most common, and de- 
cidedly pleochroitic: purplish brown parallel to a and b- 
pale yellow parallel to c. Absorption is strong :-a> b> C- 
These crystals are rarely homogeneous; they have usually 
the well-known hour-glass structure in which the lighter col- 
ored inner part of the crystal, with the optical properties of 
diopside, is surrounded by a purplish rim of titaniferous aug- 
ite resembling optically the segyrine-augite of Rosenbusch. 
On sections parallel to the plane of synmietry this outer rim 
shows a very strong dispersion of the bisectrices (the angle 
c: C 5 > c: C V ) and a large angle of extinction (the angle 
c : C = 57^^ ) against only 51 degrees, measured in the same 
way, for the inner portion. 
The pale yellowish green variety of augite is confined ap- 
parently to dikes of somewhat andesitic character (3, 5, 6, 10, 
12, 13, 15, 16, 17, 18, 19, 21, 25), containing an abundance of 
plagioclase with but comparatively little olivine. 
This pyroxene is optically identical with the diopside cen- 
ter of the titaniferous augite. 
The augite breaks down readily into chlorite and limonite 
(6, 10, 14, 15, 16, 18, 20, 21), and in some instances it is re- 
placed by a yellowish brown mica (12, 17, 25), with strong 
pleochroism: — brownish yellow parallel to 001, pale yellow 
perpendicular to 001. 
The formation of this secondary biotite proceeds usuallv 
from the exterior inwardly along the prismatic cleavage cracks 
of the pyroxene — as is clearly demonstrated in dike 12 — but 
occasionally it is confined to the central portion of the cr}'stal 
only, leaving the outer part perfectly fresh (12). 
The plagioclase varies but little in form, and in optical 
properties — the phenocrysts being stout, taliular, formed after 
010 in the coarser grained rock, and thin lath-shajDed in the 
groundmass and in denser varieties. The crystals are fre- 
quently polysynthetically twinned in accordance with the al- 
bite law, and have on 010 sections an average extinction angle 
of 20 degrees, which, with a specific gravity of 2.70, would 
place them in the group of normal labradorites (abs an4). In 
a few coarse grained dikes of camptonitic character (1,2, 20, 
25, 30), the feldspar has a smaller angle of extinction, approxi- 
mating that of anorthoclase. 
