7 
erystals attached to one another unsymmetrically. Even, how-— 
ever, where there is bending, the relative orientation of the in- 
dividuals will preserve its symmetrical character, because the 
distortion of the structure of one individual of the nucleus will be 
accompanied by the similar distortion of that of the other. i 
For comparison with examples of the first kind, in which there 
is no bending, we may mention the Carlsbad twin of orthoclase,’ 
and for comparison with the results of twinning accompanied by 
bending of the nucleus, the anorthic twins of pericline and those 
of anorthite. 
644 Scientific Proceedings, Royal Dublin Society. 
Secondary twinning. 
A word may appropriately be said here about an important 
property of some assemblages whose parts are competent by a mere 
linear distortion to pass to a different symmetry. This property 
may be described as follows :— 
Besides the kinds of twinning just referred to, another kind of 
twinning which is also produced by a shear is conceivable, in 
which, however, the origin of the disturbance of the original 
equilibrium-arrangement, instead of being found in a dimorphous 
change, 7.¢., in an alteration of the relations subsisting between 
the parts, originates in some external deforming agency, the kind 
of internal symmetry towards which the system tends in obedience 
to the principle of closest-packing continuing the same after the 
disturbance. 
This secondary twinning can occur in an assemblage whose 
parts are so related as to be geometrically competent by a linear 
distortion, or simple shear, to pass to a different order of symmetry, 
in the cases where this different symmetry has a plane of symmetry not 
found in the undistorted assemblage. 
For where this is the case the return distortion of the derived 
symmetry, which would eliminate the plane of symmetry and 
produce the initial arrangement, must be inclined to this plane, 
and must, therefore, owing to the presence of the latter, be one of 
a pair of enantiomorphously related distortions equally possible, 
and the angle between the directions of which is bisected by the 
1 See Maskelyne’s ‘‘ Morphology of Crystals,’’ p. 176. 
