of Mineral Species of the Trimeiric Sysiem. 37 
twin-composition gives importance to its indications, and there- 
fore similarity in modes of composition suggests identical or ho- 
mologous relations between the planes of composition in different 
species, and vice versa. Thus when we observe different species, 
as Aragonite, Cerusite, etc., affording stellate twins and hexago- 
nal forms by composition parallel to the faces of a prism nearly 
-120° in angle, we infer that the prisms are homologous; an 
when similar prisms occur in Chrysoberyl or Copper Glance, we 
conclude that the prism of 119° in these species, parallel to faces 
of which the composition takes place, is the true vertical prism, 
as in Aragonite. The fact that 120° x3 or 60° x6 equals 360°, 
is evidently the fundamental reason for the occurrence of such 
twins; and hence in other species a like angle for the vertical 
prism, especially if the prisms are alike in their other dimensions, 
would be likely to produce the same result. 
Hence we conclude that the sulphates (RO, SO*), although 
affording in one direction a prism near 120° in angle, have not. 
this prism as the fundamental vertical prism, for stellate composi- 
tion, does not occur parallel to it; the true vertical prism is the 
one usually so assumed—that of 101° to 
Bournonite affords another illustration of this subject. G. 
ose has assumed its homeomorphism with Aragonite, on the 
ground that it has a vertical prism of 115° 58’. But this species, 
instead of forming twins parallel to the faces of this prism, actu- 
ally affords cruciform twins parallel to a prism of 93° 40’, the 
one usually taken as the fundamental prism. The prism of 
115° 58’ is i3(a P 3) and there is no reason for regarding it as 
other than a secondary prism. f 
Chrysoberyl has been placed near chrysolite by the author, 
and also by M. Scacchi, of Naples. In a certain position the re- 
semblance in angle exists. But still the species are rather widely 
remote, inasmuch as the twins, like those of Aragonite, parallel 
to faces of the prism of 119° 46’, show that this is the funda- 
mental prism. Chrysolite affords no such twins; the angle of its 
Vertical prism is 94° 3’, and it belongs to a different zone. Chryso- 
beryl is actually near Aragonite in angle; it has a brachydome of 
108° 26’, and Aragonite one of 109° 39’ 
Monoclinic prisms near 120° in angle, never present stellate 
twins like trimetric prisms. Such twins in oblique forms appear 
to be impossible, since they require a regular symmetrical charac- 
ter in the molecule above and below the middle section. This 
remark appears to apply also to hemihedral forms of the trimetric 
System, like those of datholite. : : 
_ 3. General Habit of Crystals.—A resemblance in general habit 
is often to be detected between species related in crystallization. 
‘as Brookite, ured in this Journal, vol. xvii, p. 86, resem- 
bles Columbite in the general arrangement of its planes; and we 
