MICROSCOPICAL CHARACTER OF GRANITE AND NODULES. 167 
granite porphyries. Highly pleochroic biotite is present in considerable 
amount in the usual lath-shaped forms. A small amount of muscovite, 
often intimately associated or intergrown with the biotite, is also present. 
This mineral also occurs enclosed in the feldspars, either as well defined 
individuals or in grains having the peculiar feathery, fretted, or lace-like 
margins exhibited by it in certain other granites, and in such cases it 
often has an edging of quartz surrounding a portion of it. Im some cases 
it was even observed enclosed in quartz. A few grains of iron ore and 
apatite complete the list of minerals found in the rock. 
MicroscopicAL CHARACTER OF THE NODULES 
Six nodules were selected for microscopical study, and eight sections, 
each comprising a whole nodule, were prepared. Three of these nodules, 
when cut across, showed a rather distinct concentric structure, the outer 
and inner portions differing somewhat in color, the former consisting 
chiefly of quartz and sillimanite and the latter chiefly of quartz and mus- 
covite. The outer and inner portions, however, are not separated by a 
sharp line, but fade into one another, so that no satisfactory separation 
of them for purposes of chemical analysis could be effected. Many 
nodules also, as has been mentioned, when broken across, show a small 
sponge-like mass of black tourmaline toward the center. In the case of 
the other three nodules this zonal arrangement was merely suggested, 
the nodules being essentially uniform throughout. One of them, how- 
ever, contained the tourmaline sponge, before referred to, near the center, 
and in two of them there was an indistinct tendency to a radial arrange- 
ment on the part of some of the constituents. 
The absence of pronounced concentric or radial structure is one feature 
in which these nodules differ in a marked manner from the basic con- 
cretions, nodules, and varioles described in other granitic rocks. On pass- 
ing from the granite to the nodule there is seen under the microscope an 
abrupt change both in grain and composition. The regular mosaic of 
the granite is replaced by a coarser grained and sometimes indistinctly 
radial or sheaf-like arrangement of the constituents; the biotite and 
microcline disappear entirely, while the quartz and muscovite, especially 
the former, become more abundant, and sillimanite makes its appear- 
ance, usually in large amount. 
Quartz, muscovite, and sillimanite are the chief constituents of the nod- 
ules. Plagioclase and an untwinned feldspar, probably orthoclase, which 
are present in some nodules in considerable amount, but in others in very 
small quantity, with tourmaline in some cases and a few grains of iron 
ore and pyrite, complete the list of constituent minerals. 
The quartz is uniaxial and forms a well defined mosaic of polygonal 
