104 
CONTRIBUTIONS TO MINERALOGY. 
[BULL. 262. 
The quartz occurs in allotriomorphic grains, which are so minute and 
so mixed with the andalusite and muscovite that recourse was had to ! 
chemical analysis to verify their determination as quartz. 
The dumortierite occurs in small spherulites scattered throughout 
the rock and occasionally bunched together to form a large patch of 
blue. On an average they reach a diameter of from one-half to 1 mm. 
Their shape, while in general circular, is often elliptical and mal 
become very irregular in places. They consist of fibers radially 
arranged and show the optical phenomena of "spherulites." The 
dumortierite has parallel extinction, and its birefringence is somewhat 
higher than that of the quartz and also slightly more than that of! 
the andalusite, though the 
difference between that of! 
> the latter two minerals is 
very small. 
The intensity of the ple- 
ochroism of the dumortier- 
ite varies, so that in some 
spherules there are concen- 
tric bands of fibers differing 
greatly in the depth of their 
color. Some spherules are 
almost colorless, and it was 
at lirst thought they might 
be parallel growths of anda- 
lusite with the dumortierite, 
but such a conclusion could 
not be verified. 
The libers are not always 
perfectly radial. They are at times gathered into "brushes," and a 
number of these put together may form a spherulite. The libers are 
thus more thickly crowded in some places than elsewhere, and this, 
frequently results in intensifying the pleochroism, so that in some 
spherulites there are numerous blotches of blue of much deeper color 
than the rest. Muscovite is frequently plentiful in a spherulite, being 
formed between the libers, and is probably an alteration product of 
the dumortierite. Frequently a mass of dumortierite will be almost 
completely changed to mica, leaving but small fragments of the 
original mineral behind. 
The fibers of dumortierite, while usually arranged radially, some- 
times assume different shapes, and some of the masses of dumortierite 
seen under the microscope are reproduced in fig. 7. 
Fig. 7 shows variations from typical spherulitic form that the libers 
of dumortierite assume, a is four prisms irregularly joined at the 
lumortierite. 
