183 
spar. It is conspicuous in the felspar on account of its 
bright polarization colours and its higher refractive index, 
but is perfectly clear and colourless. Though scattered 
through che whole of the large grains of the enclosing mine- 
ral, it is partieularly abundant, forming a selvage around 
their edges, especially where the fineness of the quartz mosaic 
indicates a point of intense compression. "These features in- 
dieate very plainly its secondary origin. All the muscovite 
is not, however, of secondary origin. In marked contrast to 
these very minute flakes there are comparatively large rag- 
ged plates of the same substance fairlv abundantly distri- 
buted through the section. Their optical characters call for 
no special remark. Like all the other constituents they have 
suffered in the processes of rock movement, the results being 
bending and fraying, and the presence of pseudo-inclusions of 
quartz and felspar. True inclusions are very scarce, and in- 
clude only an occasional minute prism of apatite. The indi- 
vidual sections of biotite are all small, but their occurrence 
in “nests” accounts for the apparently large individuals when 
they are observed macroscopically. In no case is biotite inter- 
grown with muscovite. The colour is light yellowish-brown : 
the pleochroism, though strong in yellow and dark brown 
tints, is not so powerful as is often the case with granitic 
biotite, the absorption never approaching extinction. Even 
basal sections are just noticeably pleochroie: in convergent 
light they give a dark cross which breaks up into hyperbolic 
brushes. The optic axial augle is very small. The usual de- 
composition into moderately pleochroic, bright green chlorite, 
with very weak double refraction, is observable. "The only 
inclusions are a few small prisms, apparently zircon, but 
these are not surrounded by pleochroic halos. Тһе biotite 
does not show strain effects. 
lron ores of various kinds appear as accessory constituents. 
Some of the opaque material seems to be magnetite, some is 
certainly ilmenite. The composition of most of it is pro- 
bably that of titaniferous magnetite. Some leucoxene. has 
been developed at the expense of these ores. A very occa- 
sional grain of pyrite is met with. 
Traces of calcite occur amongst the thickest aggregates of 
secondary muscovite. Apatite is sparingly distributed 
throughout the rock. 
Unfortunately, 1 am not in a position to have chemical 
analyses made, and the time and means at my disposal do not 
permit me to carry them out myself. As a substitute I have 
calculated the analysis from a quantitative determination of 
the mineral composition by the method of Rosiwal.* 
* Cross, Iddings, Pirsson & Washington: ‘‘Quantitative Classifi- 
cation of Igneous Rocks,’ Univ. of Chicago Press, 1903, p. 204. 
