Acid Peg maty te in Diabase, — Jaggar. 209 
for a crystal near the periphery of the section on the side re- 
mote from the inchision ; the extinctions were lower at a point 
(the only one in the slide, see figure i) where the augite zone 
is interrupted, and a feldspar half penetrates within the zone, 
coming into immediate contact with an acid feldspar of the 
inner zone; this crystal showed a more acid composition, near 
the andesine group. The feldspars occupy, in the thin sec- 
tion, an area greater than all the other minerals of the diabase 
taken together. The feldspar extinctions are frequently wavy, 
and the mineral is kaolinized only about the border of the tri- 
angular interspaces. The rock is otherwise quite fresh, and 
the other minerals arc distributed in charaeteristic ophitic 
fashion; the triangular interspaces among the feldspars are 
tilled with augite and secondary uralite. with magnetite, ilme- 
nite and leucoxene, biotite and secondary chlorite, large apa- 
tite crystals and some calcite. In the hand-specimen biotite 
is quite abundant in flakes two or three millimetres in diame- 
ter. 
ZONE OF DIABASE FELDSPAR (DF,). Next to 
tlie band of augites, the feldspars of the diabase form a con- 
fused mass of finer texture than in the normal rock; they form 
a zone, averaging one millimetre in width, of short rectangular 
prisms, allotriomorphic to the distal terminations of the next 
inner zone of augite prisms. Scattered magnetite and ai)atite 
crystals, the only other minerals in this band of small basic 
feldspars, are most abundant along the irregular contact line 
of the augites, interpenetrating both these and the feldspars, 
and hence of earlier generation than either. 
ZONE OF AUGITE CRY ST A IS. (A). This corre- 
sponds in all respects with the normal augite wreath of quartz 
inclusions (see preceding page). The outer border of the augites 
is very irregular and appears to be the terminations of a 
crowded row of prisms of varied length, attached on the inner 
or (relative to the inclusion) proximal side to a smooth and 
gently sinuous surface, their terminations forming- on that 
side an uninterrupted contour in section. The general ar- 
rangement of the prisms is in clusters radiating from the con- 
vexities of the surface of attachment. The idiomorphic distal 
terminations represent the usual pyramid faces (iii), and be- 
tween crossed nicols occasional twimiing is seen, parallel to 
