168  ¥F. D. ADAMS—NODULAR GRANITE FROM PINE LAKE, ONTARIO. 
grains, showing little or nothing of the tendency to develop rounded 
individuals seen in the granite itself. It often holds an abundance of 
sillimanite needles, although in it this mineral does not usually occur in 
such mats as in the feldspar. It frequently contains lines of minute 
cavities, some of which enclose moving bubbles. The muscovite occurs 
in large colorless plates, often of very irregular outline, extending in some 
cases completely across the central portion of the nodule, and holding 
many inclusions of quartz, silimanite, and other constituents. The irreg- 
ular and indented outline is quite distinct in appearance from the finely 
fretted or lace-like boundary presented by the muscovite of the granite, 
the outlines being quite sharp. It may, however, be regarded as this 
structure on a much larger scale. It has the eminent basal cleavage 
characteristic of this mineral, with the uniform extinction parallel to it. 
The sillimanite occurs in long, slender, isolated needles with transverse 
partings and extinction parallel to their length, or as bundles or mats of 
such needles, felted into nearly opaque masses. It occurs penetrating 
both the quartz and muscovite, but, as has been mentioned, is especially 
abundant in the feldspar, which is usually crowded with needles of it. 
The sillimanite individuals are usually very small, and are irregular in 
cross-section. There is, however, a tendency to develop the nearly 
square or the eight-sided prismatic forms usually seen in this species, and 
the larger grains show the usual good cleavage in the direction of one 
pinacoid. The mineral is uniaxial and positive, and c= ¢. 
In one or two instances an individual of muscovite could be seen at its 
extremity to pass into a bundle or brush of sillimanite fibers. 
The occurrence of sillimanite in granite, except along sheer zones, is, 
so far as I am aware, unknown, but the mineral often abounds in the 
quartz which occurs in veins and irregular masses in highly altered rocks 
in the vicinity of granite intrusions and elsewhere. Whether such veins 
have any genetic relation to such occurrences as those described in the 
present paper is a question for future investigation to decide. 
The feldspars are present in some nodules in a considerable amount ; 
in others they are practically absent. When present they consist in part 
of well twinned plagioclase and in part of an untwinned feldspar, prob- 
ably orthoclase. Microcline is never found in the nodules, although it 
may abound in the surrounding granite. The feldspar, especially the 
orthoclase, occurs in irregular shaped individuals having the feather-like 
forms seen in spherulites, often with an indistinct radial arrangement, 
and sometimes showing a granophyrie intergrowth with the quartz. It 
is not especially abundant in any part of the nodule, and is crowded with 
bundles and mats of sillimanite needles. ‘The iron ore, which is black 
and opaque, occurs in each nodule in the form of a few rather large 
erains, A grain or two of pyrite is also usually present, The tourma- 
