1895. | NEW YORK ACADEMY OF SCIENCES. 211 
spars and some interstitial quartz, with phenocrysts chiefly feld- 
spar, sometimes hornblende, occasionally quartz. They are 
holocrystalline, rather fine grained, and’ distinguishable macro- 
scopically from diabases by their paler gray color, granular tex- 
ture, and often by the short hornblende rods which stand out 
on weathered surfaces. The texture is mostly uniform, but in 
two cases is extremely irregular in different parts of the same 
dyke. They are singularly like the basic segregations in the 
granite, which latter they cut in several places, and are perhaps 
to be connected with it as the last member of the intruded series, 
injected after the rest had solidified. 
Under the microscope they show numerous well formed crys- 
tals of hornblende, varying in size according to the width of the 
dyke, with prismatic faces always well developed, often also with 
good terminal faces. The prismatic faces are m, usually }, rarely 
a. The terminal faces are not safely determinable, but apparently 
r is present, and also a steep pyramid or clino-dome. In the 
less altered specimens (293, 294, 603, 608) the hornblende is 
brown, showing a strong pleochroism : 
a—brownish-yellow. 
§—brown. 
c—greenish-brown. 
The extinction angle is high, not less than 17°, and the colors 
are scarcely so deep or so red tinted as those of basaltic horn- 
blende. It appears therefore to be a brown variety of the com- 
mon kind. 
Green hornblende sometimes appears as a secondary rim at 
the edges of the brown crystals, more commonly asa paramorph 
after them ; all stages of the change can be seen in different crys- 
tals, the alteration beginning at the edges or in cracks. In 
either case the orientation of the two varieties is the same, and 
their extinction identical. 
In the more altered dykes (90,225,284 and 336), the horn- 
blende is entirely green, its pleochroism being 
a—pale brownish yellow, 
h—green, 
c—bluish green. 
The outlines tend to become less distinct with increasing 
metamorphism. 
In spec. 284, from a small dyke, there are, instead of the usual 
corroded feldspar phenocrysts, abundant long rods of green 
hornblende, from 0.2 to 0.56 mm. diameter, and several millime- 
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