32 TRANSACTIONS OF THE [ocr. 28, 
The plagioclase makes up somewhat more than half the rock 
and is largely idiomorphic, often also with a well-marked zonal 
structure. The different individuals occur in very varying 
stages of decomposition in the same slide, and those with the 
zonal structure have almost invariably the centre more decom- 
posed than the periphery. In the zonal crystals the outer rim 
never shows a greater extinction angle than 4° in individuals 
whose twinning lamelle give nearly symmetrical extinction, 
while the centre of the same individuals frequently gives an 
angle of 20° to 22°, indicating a range from oligoclase to a basic 
andesine or acid labradorite. Inclusions of all the other constit- 
uents of the rock except quartz are common. 
Along with the undoubted plagioclase in the slides are seen 
numerous individuals, always so far gone into decomposition 
that little can be learned from their study concerning their 
original condition, but which inso far as they retain any ‘indica- 
tions of twinning whatever, show only that after the Carls- 
bad law. They are regarded, somewhat doubtfully, as repre- 
senting an original contest of orthoclase in the rock. The un- 
doubted plagioclase always far exceeds this uncertain feldspar 
in amount. 
Next to the plagioclase hornblende is the most abundant con- 
stituent. It isa dark green hornblende with very strong ab- 
sorption—c < ) <<a—and pleochroism of the usual character in 
diorite—a greenish, § dark greenish brown and ¢ dark bluish 
green. The smaller individuals possess sharp crystal bound- 
aries in the prism zone, but the larger individuals are irregularly 
bounded and ev ery where poikilitic, mottled with inclusions of 
both feldspar and quartz. The maximum extinction angle found 
is 17°. Inclusions of apatite and the iron oxides are numerous. 
The ordinary decomposition is to chlorite, but in a few instances 
epidote also appears. The formation of a light bluish green 
hornblende with only slight pleochroism, resembling actinolite 
seems to be an initial stage in the process. Both the feldspar 
and the hornblende show admirable evidence of the dynamic 
action to which the rock has been subjected. In addition to 
widespread undulatory extinction and sliding of portions of 
crystals along cleavage planes, are many beautiful examples of 
crystals bent into a curve, extinguishing gradually from one 
end to the other, the difference amounting to 20° in some cases. 
Quartz is present in all the slides and nearly equals the horn- 
blende in quantity. It is totally allotriomorphic and contains 
frequent inclusions of the other minerals as well as a multitude 
of minute black inclusions of uncertain nature. A few small 
patches of granophyric appearance are present. 
