642 Scientific Proceedings, Royal Dublin Society. 
alike the symmetry and unsymmetry which characterize the — 
nucleus. ; = 
Examples of pseudo-symmetry in which crystals twin in such — 
a way as to ape a higher symmetry than that which they possess, — 
and which strictly resemble the kind of twinning of homogeneous 
assemblages just shown to be producible, are numerous. ‘The — 
pseudo-hexagonal twin form of cassiterite, and the pseudo-tetra- — 
gonal form of staurolite may be instanced. 
The aping of a higher symmetry by a lower will be likely to © 
be most conspicuous in the case of the existence of two dimorphous — 
forms of the same assemblage in which the distribution of the 
constituents is much the same,’ for at some stage of the change of 
conditions which produces the dimorphism the two compared as- 
semblages may probably have been of nearly or quite the same — 
form. a 
This reminds us of the familiar case of dimorphism of calcium 
carbonate which crystallizes in the rhombohedral system as calcite, 
and in the orthorhombic as aragonite. ‘The prism-angle of the — 
latter is 116° 13’,,and when the prism is accompanied by the 
brachypinacoid the combination has much the same appearance as 
a hexagonal prism of calcite, of which the angle is 120°. And 
in this instance the mimicry of the higher symmetry is frequently 
increased by complicated twinning upon the primary prism. 
In some other cases where two dimorphous forms of the same 
substance are obtainable, a still more remarkable relation exists. 
Thus the prism angle {70° 32’) of claudetite, the monoclinic form — 
of arsenic trioxide, has the same value as a cubic octahedron 
angle, 7.e., of that angle of arsenolite, the cubic form of the same 
substance. And a similar relation is presented by the forms of 
antimony trioxide, which is isodimorphous with arsenic trioxide.” 
In the preceding cases of twinning of homogeneous assem- 
blages from dimorphous change, a boundary surface separating 
= > = 
time at similar surfaces. And to compare with this we have evidence, ¢.g., in embedded 
twins such as the Karlsbad twin of orthoclase, that larger faces often grow faster than 
the smaller ones of the same kind. 
1 See p. 576. 
2 See Pope’s Translation of Fock’s ‘‘ Chemische Krystallographie,’’ p. 152, where 
other similar instances are given. 
