162 FIELD COLUMBIAN MusEuUM—GEOLOGY, VoL. III. 
ber of planes.. Moreover, the character of their planes is distinctive, 
the tetrahedrons being always more or less rough from etching and pit- 
ting and the cubical faces usually smooth and often more brilliant 
than. the other planes: = ie re- 
maining planes’show an angular 
ees y measurement upon the cube and 
tetrahedron corresponding to that 
of planes of the hemitetragonal 
tristetrahedron p (221), but the 
full number of planes of this form 
is never present. As a rule a 
single plane of the form occurs in 
three quadrants and two in the 
fourth. One crystal, however, 
exhibits two planes of the form in 
Fic <, Sphalerice each quadrant. It is of interest 
to note that in the pyrite de- 
scribed by Penfield from French Creek, Pennsylvania * a somewhat 
similar lack of planes occurs. Owing to the etched character of the 
tetrahedral faces on the sphalerite it is probable that the tetrahedron 
present is the positive one and the tristetrahedron is therefore 
to be regarded as negative. Fig. 5 illustrates the wdeveromicus 
exhibited by- the majority of the crystals. The crystals with 
rounded planes have as a whole more nearly the appearance of 
the tetragonal tristetrahedron than those which are more fully 
developed. The tristetrahedron may therefore be regarded as in a 
sense the fundamental form which is modified in the more fully 
developed crystals by the cube and tetrahedron. A list of the forms 
and angles follows: 
a (100) Oo (111) pb (221) 
Observed Calculated 
a \ 0 = (100) A (111) =55° 12’ (average of 11 measurements) 54° 44’ 
aA p= (loo) A (221) =48° 14’ amen Ts Ms 48° 11’ 
aA p =: (001) A (221) —Fo".007 a ick, . pote oe 
* Am. Jour. Sci., 18809 (3), 37, Pp. 200. 
