396 Transactions of the South African Pliilosoijhical Society. 



In Baumhauer's experiment, if one takes a rhombohedron of 

 calcite and presses a knife across one of the terminal edges, a slight 

 sideward pressure being at the same time given to the blade towards 

 the summit of the rhombohedron, the following effect is produced : — 

 as the knife enters the crystal, successive layers, all parallel to the 

 twinning-plane, glide along each other, and take up a position com- 

 plementary to the original crystal, and form a twin by reflection." 

 This twinning-plane here is also a gliding-plane, but it is more than 

 merely a gliding-plane in the ordinary sense. Along this plane, 

 provided a particular pressure is applied in a particular direction, 

 the crystal substance will move without breaking. The ultimate 

 nature of a crystal is still very imperfectly known, but it seems that 

 the molecules have a regular swing in a particular order, the rhythm 

 of which is expressed outwardly by hard and fast crystal shapes. 

 When the knife is pressed in a certain direction in the above 

 experiment, it affects all the molecules in the portion twinned 

 which are swinging in unison ; hence all the molecules are aflected 

 equally by the disturbance, and their mutual arrangement is un- 

 disturbed before and after the experiment. In the slab of marble, 

 the top layer is under pressure. In this layer there are thousands 

 of grains, some of which lie in the right direction for the pressure to 

 effect a movement along the plane of gliding, but as static pressure 

 alone is not sufficient to cause any motion, the slab has to wait till 

 some trembling of the earth occurs to start off the gliding. When 

 the first set of crystal grains have accommodated themselves to the 

 pressure, the distribution of stress over the surface of the layer is 

 altered, and a new lot of grains come into the right position for the 

 twin-gliding, and these also have to wait for an adequate shock 

 before they adjust themsevles, and the process goes on slowly till 

 the whole slab becomes markedly deformed in the course of years. 



A great many crystals of rock-forming minerals have similar 

 winning-planes, the felspars of dolerite, for instance, being very 

 similar in this respect to ' calcite, but the experimental testing 

 whether they will adjust themselves to pressure in the same way as 

 calcite has not been done. Some crystals like quartz show no signs 

 of similar lamellar twinning, nevertheless, even in quartz there is 

 a variety known as " twisted " or warped, which Tschermak regards 

 as due to a similar gliding along twin-planes, f 



The ultimate molecular state of all crystals must be similar to that 

 of calcite ; the molecules must have symmetric movement, and there 



* H. Miers, Mineralog]i, 1902, p. 95. 



t The experiments now being carried on by Prof. F. D. Adams at McGill 

 College, Montreal, however, may throw light on this subject. 



