Mineralogy and Petrography. 249 
contact phenomena, a late paper by Hussak,' on the origin of the 
spots in altered sedimentary rocks (Knoten-and Frucht-schiefer) is 
worthy of close study. Rosenbusch regards these spots as due to the 
aggregation of the pigment, which was distributed evenly through- 
out the fragmental rocks before their intrusion by the eruptives. 
In certain cases he supposes these aggregations to pass by recrystal- 
lization into chiastolite and andalusite. Ward regards them as un- 
developed andalusites. Other writers have described them as pos- 
sessing characteristics which indicate their close connection with 
other mineral species. Hussak, upon examining the spotted schists. 
from Tirpersdorf, Saxony and Hlinsko, Bohemia, found that the 
darker portions of the altered schists possess properties which can 
only be explained by supposing them to be the remains of altered 
cordierite, in the case of the Saxon rocks, and of altered chiastolites 
and andalusites in the rocks from Bohemia. 
number of new forms were detected, which ad 
known make a total of seventy-five thus far identified on the 
pendent mineral species distinct from manganite. It is orthorhom- 
bic, with an axial ratio: 1 : 10513; 1: 0.63177. Finally, he re- 
Opportunity to determine its optical constants. The tiny crystals 
are bounded by the planes œ P= O° 
. Pæ 
color is orange brown. They are weakly 
P and 4 Po 
pleochroic. The plane 
5 Min. u. Petrog. Mitth. ix., 1887, p: 47. 
