360 The American Naturalist. [April, 
fuse readily in a candle flame, have a hardness of 3, and a specific 
gravity of 5.08. The index of refraction is very high, approximately 
2.07. The composition of the mineral is expressed in the formula 
PbCl,+-CuOH,O+ %AgCl or as it was written by Mallard, 3 4 PCbl 
(HO). CuCl(HO){+AgCl. The crystals are apparently cubes 
which are sometimes truncated by the octahedron or the dodecahe- 
‘dron. Cleavage is perfect parallel to (100) and less perfect parallel to 
-(111). The optical properties show clearly that the apparent cubic 
form is caused by the twinning of three tetragonal individuals so that 
their axes are all normal to one another. 
Cumengeite is much like Boleite but occurs in pyramidal crystals, 
is more violet in tone, and has hardness and specific gravity somewhat 
inferior to those of Boleite. Chemically it differs from that mineral 
in the absence of 15 AgCl, the formula being given as PbC2 (Index) 
+CuOH,O. The symmetry is tetragonal, the crystals being usually 
bounded by á (011) and m (110) and rarely also by p (001). In con- 
nection with the simple individuals of Boleite and Cumengeite are found 
very interesting twinned individuals which usually give the appearance 
which would result from attaching by its square face to each of the six 
faces of a cube the half ofasymmetrically developed octahedron. The 
face of this particular pyramid (2) has not been observed on any of 
the individuals of either mineral. Sometimes the solid angles of the 
projecting pyramids formed by / are symmetrically truncated, so that 
an apparent cube with reentrant or grooved edges is the result. The 
composition of these interesting trillings has not been fully determined, 
but they are found to be intermediate between Boleite and Cumengeite 
in their content of silver. Mallard has applied to them the name Per- 
cylite which had before been used for an undetermined cubic mineral 
of somewhat allied characters. Friedel has succeeded in producing 
artificial crystals of both Boleite aud Cumengeite by the action of 
copper chloride on lead hydrate. 
The Formula of Pseudobrookite.— The chemical composition 
of pseudobrookite has been determined by Cedarstrém on material 
from Havredal in Norway and by Rimbach on selected material from 
the Aranyerberg in Siebenbiirgen, to be represented by the formula 
'2Fe,0, 3TiO,, the two analyses agreeing very closely. The analysis of 
‘an artificial pseudobrookite from Schönebeck on the Elbe yielded Doss 
the formula Fe,O, TiO,, and led the analyst to suppose that the min- 
-eral is. isomorphous with andalusite. Frenzel’ has undertaken an 
analysis of the material from the Aranyerberg and obtained results 
5 Min. u. petrog. Mittheil., xiv, pp. 126-130, 1894. 
