220 J.J. H. TEALL—-PETROLOGICAL NOTES ON 
(1) as fairly large (‘55 mm. x *-75mm.), and more or less perfectly 
developed crystals of the common form; and (2) as minute crystals 
or crystalline granules (‘05 mm.). The former show fairly good 
cleavages, which appear in the sections as lines running parallel to 
each other, or else belonging to two series which intersect at various 
angles according to the direction of the crystal section, up to about 
90°. They are not numerous, but in the slides in my possession 
may be recognized the eight-sided, six-sided, and four-sided forms 
characteristic of sections more or less at right angles to the prin- 
cipal axis, parallel to the ortho-pinakoid, and parallel to the clino- 
pinakoid respectively. By far the larger portion of the augite 
occurs in the form of minute crystals and crystalline grains. A 
green chloritic substance, having a fibrous appearance in thin sec- 
tions, sometimes wholly or partially replaces the augite. In some 
of the sections from the Barwick quarry the augite crystals are 
entirely replaced by calcite pseudomorphs, giving aggregate polari- 
zation. These are especially interesting, as the forms of the replaced 
augite were evidently very perfect. Augite does not, at any rate 
as a rule, occur as inclusions in the large porphyritic felspars. 
Many of the very small augite grains when examined with a mag- 
nifying power of 500 diameters, are seen to have their edges ill 
defined and ragged like the felspar microlites, and seem to be 
connected with the brownish substance which appears as indistinct 
granules and fibres in the interstitial matter. This very interesting 
feature will be again referred to in describing the microscopic 
character of the interstitial matter. 
Magnetite (and Ilmenite?).—This occurs in the form of fairly well- 
developed crystals, and also as skeleton crystals and small granules. 
The crystals show the usual quadratic, triangular, and hexagonal 
forms. Now and then a lath-shaped section occurs, such as might 
be formed by cutting a tabular crystal of ilmenite at right angles 
to the basal plane. The occurrence of this latter mineral must, 
however, be regarded as doubtful; for the microscopic evidence is not 
clear, and the small quantity of titanic acid revealed by analysis 
may be contained in the magnetite. On heating a thin section for 
some time with hot hydrochloric acid, the black opaque mineral 
entirely disappears. 
Biotite——This mineral is not constantly present. It has been 
observed in specimens from Armathwaite and Ayton. It shows a 
tendency to alteration, and is frequently represented by green chlo- 
ritic fibres. 
Quartz.—This may be recognized in most of the slides, either in 
the form of definite crystals or as erytalline grains, which usually 
show a polysynthetic structure. It may, in part, be an original 
constituent; but from its frequent association with calcite, I am 
inclined to class it under the secondary minerals. 
Calcite——This mineral occurs in irregular crystalline plates, 
giving definite extinctions, and is also scattered throughout the 
slide in the condition of crystalline powder. The last-mentioned 
mode of occurrence indicates, however, a very advanced stage of 
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