IIo MINERALOGY AND LITHOLOGY. 
Magnesia, : : - ‘| i z 7 e 2.19 1.64 
Ignition, ‘ 3 é e ‘ a 27 -68 
101.81 99.81 
This very great difference in the two analyses is explained by the cir- 
cumstance that the staurolite with a high percentage of silica is rendered 
impure by the enclosure of quartz, as has been proved by a number of 
investigators; and these irregular varieties, when purified from the quartz 
by treatment with hydrofluoric acid, as was done by Rammelsberg, give, 
on analysis, a composition which corresponds also with the formula. 
This admixture of quartz is apparent in the specific gravity which sinks 
from 3.76 in the purer variety from Franconia to 3.41 in the impure 
variety from Lisbon. Sections of the Lisbon staurolite show the quartz, 
which is present as large clear grains scattered through the interior of 
the crystal. 
Staurolite forms macles, or tessellated crystals like andalusite ; but this 
is a rare occurrence, and I am not aware of such having been observed 
save those noticed by Jackson at Charlestown, in our state. These ma- 
cles of staurolite are made in the same manner as the andalusite macles 
by the symmetrical arrangement of pure and impure material. Fig. 8 on 
Pl. 2 represents the base of one of these crystals. They are found in 
the mica slate, which Jackson states gradually passes into an argillite, 
and with it the character of the crystals changes till they become anda- 
lusite macles in the argillite. 
This staurolite is not in twin crystals; a rare occurrence, since stau- 
rolites which are apparently simple usually prove to be compound when 
cut and examined. These peculiar macles have been examined by Peters 
and Rosenbusch, and by Jackson. Fig. 8 is taken from Jackson's article. 
Fig. 8a is from Rosenbusch’s Mzkroscopische Physiographie, by which it 
is shown that the macle is not produced by twinning, since the cleavage 
lines are undisturbed, but that the case is one where a core is surrounded 
by another crystal, giving to the whole a laminated structure, and the 
macle results from the regular arrangement of impurities and cavities, 
which are most abundant between the outer and the inner crystal. 
The microscopic characters of minute crystals are usually the same 
as those of the large ones. In them more or less quartz is found, and 
apparently simple crystals, with polarized light, are seen to be twins; 
