Bartow—A Mechanical Cause of Homogeneity of Crystals. 645 
direction of this plane. And these two distortions will, it is 
evident, owing to the symmetry, affect similarly the system of 
points lying in the plane of symmetry. 
Consequently if one half of an ideal assemblage possessing the 
derived higher symmetry, lying on one side of the acquired plane 
of symmetry, experiences one distortion, while the other half ex- 
periences the distortion enantiomorphous to it, the points lying 
in the plane of symmetry will be able to obey both distortions at 
the same time, 7.e., they will be distorted in precisely the same 
way by each of them. 
Therefore, since all planes of points parallel to one another are 
similarly affected by any uniform linear distortion of an assem- 
blage, every plane of points parallel to the plane of symmetry 
will experience the same change whichever of the two enantio- 
morphously related distortions it is subjected to. 
Therefore, finally, if those planes of points composing the 
original assemblage whose plane direction would become that of 
the plane of symmetry if a distortion to the derived symmetry 
took place, are capable of sliding on one another, 7. ¢., of under- 
taking a simple shear in any direction, it follows that twinning of 
the assemblage can be produced by such a shear if it affects the 
part of the assemblage lying on one side of some one of the planes 
of points referred to, and not the part on the other side. 
Or, if the movement is accompanied by a slight temporary 
increase of the distances separating the moving layers, it can 
equally well take place if the plane separating the hali of the 
assemblage affected by the shear from the other half has instead a 
certain direction which, when distortion to the derived symmetry 
has taken place, would be perpendicular to the plane of symmetry. 
This will be easier to follow in the particular case treated of 
below. 
The shifted portion of the assemblage will be the enantio- 
morph of the unaltered portion unless these two portions are 
identical with their own mirror-image, and therefore with one 
another, when they will merely occupy enantiomorphously similar 
positions with respect to the separating plane. 
The following is an example of a rhombohedral assemblage 
possessed of the property referred to. 
