71 
It is usually hypidiomorphic, but is frequently idio- 
morphic, showing a distinct cleavage parallel to the 
pinacoids, but usually none parallel to the prismatic 
faces. It belongs to the variety of diopside-like augites 
which occur in rocks of this class. The extinction is high, 
reaching 45°. 3 
Biotite is deep brown, almost identical in colour 
with the hornblende and is strongly pleochroic. It 
occurs intimately associated with the hornblende and 
augite, and also frequently as a border around the iron 
ore. While usually present in comparatively small amount, 
in the finer grained essexite forming the summit of the 
mountain it is much more abundant than the hornblende. 
In this variety of the essexite both the mica and the 
hornblende often possess a poikilitic structure owing to 
the presence of numerous inclusions of plagioclase. The 
plagioclase also often penetrates the individuals of biotite 
and hornblende in the form of well developed crystals. 
Olivine is found in the finer grained variety of the 
essexite at the summit of the mountain, and was also 
observed in the thin sections from the essexite at one 
point on the east side of the mountain not far from the 
summit. It is very pale green in colour and occurs as 
little grains inclosed in the biotite and pyroxene. 
The plagioclase in the rock has well-developed, lath- 
like forms and is, almost without exception, excellently 
twinned according to the albite law. Twinning according 
to the carlsbad and pericline laws is also very common, 
occurring in the same individuals which show the albite 
twinning. The laths of plagioclase can in a few cases be 
seen to be distinctly twisted, evidently owing to pressure 
exerted upon them by other crystals during the consolida- 
tion of the rock, since the rock was subjected to no dynamic 
action subsequent to its crystallization. 
As before mentioned, all the plagioclase individuals 
are not of the same dimensions. There are larger laths 
associated with the large hornblende crystals, and between 
these are smaller laths. The two sets are not, however, 
sufficiently well marked to cause the resulting structure 
to be classed as porphyritic. The plagioclase in the rock 
is not all of the same composition, but varies somewhat 
even in the same hand specimen, ranging from an extremely 
acid labradorite to an oligoclase. It, however, is chiefly 
andesine. 
