70 PROCEEDINGS OF THE GEOLOGICAL SOCIEIY. 



Recent researches have shown that every crystal possesses a 

 number of planes, all of which are related to its peculiar symmetry, 

 along which the several physical forces operate in a marked manner 

 to produce changes in the physical and chemical properties of the 

 crystal. These planes have been called the " structure-planes 99 of 

 the crystal. 



By far the most obvious of these structure-planes of crystals 

 are those of cleavage. When crystals are subjected to the 

 action of mechanical force they break up along one, two, or three 

 definite planes, with varying degrees of ease. In some cases 

 when this separation cannot be readily effected by percussion or 

 pressure, it may be brought about by the unequal expansion and 

 contraction in a crystal resulting from alternate heating and cooling. 

 We cannot arrive at the limit of this liability of a crystal to 

 separate along its cleavage-planes ; if we powder a calcite-crystal 

 and examine the fine dust under a microscope, each minute grain 

 will be seen to have the form of a cleavage-rhomb ! 



Xow the exquisite molecular structure of a crystal, of which 

 this wonderful property of cleavage is the outcome, is borne 

 witness to, not only by the perfection of the cleavage-surfaces — 

 presenting, as they do, a lustre which no artificial polish can imi- 

 tate — but by the fact that each particular set of cleavage-surfaces 

 presents definite characteristics, analogous to those seen in the 

 actual faces of crystals. Each exhibits striking peculiarities in its 

 mode of reflecting light ; each yields in varying degrees to a hard 

 point drawn across it in different directions ; and each, when treated 

 with appropriate solvents, is attacked in a characteristic fashion, 

 giving rise to the geometrical forms known as the etched-figures. 

 Wonderful as these cleavage-surfaces are, however, it must be re- 

 membered that the power of cleavage is one that, under ordinary 

 circumstances, remains altogether latent in crystals. 



Cleavage-planes, however, are not the only latent structure-planes 

 in crystals. Long ago it was shown by Brewster, Beusch, and 

 Pfaff that when minerals are subjected to pressure in certain direc- 

 tions, their molecules appear to glide over one another along certain 

 definite planes within the crystal ; and if we examine optically a 

 crystal which has been treated in this manner, it is actually found 

 to exhibit a series of twin -lamellae arranged parallel to the so-called 

 " gliding-planes." It thus appears that in the movements set up 

 within a crystal by the application of force from without, certain of 

 the molecules of which the crystal is built up, lying in bands 



