The Flow of Marble. 433 



when liighly magnified, is seen to be due to an 

 extremely minute polvsynthetic twinning. The 

 chalky aspect of the deformed rock is in fact due 

 chiefly to the destruction by this repeated twinning 

 of the continuity of the cleavage surfaces of the 

 calcite individuals, thus making the reflecting surfaces 

 smaller. By this twinning, the calcite individuals are 

 enabled under the pressure to alter their shape some- 

 what, while the flattening of the grains is evidently 

 due to movements along the gliding planes of the 

 crystals. In these parts, therefore, the rock presents 

 a continuous mosaic of somewhat flattened grains. 



From a study of the thin sections it seems probable 

 that very rapid deformation tends to inicrease the 

 relative abundance of the granulated material, and 

 in this way to make the rock weaker than when the 

 deformation is slow. 



AVhen the marble is heated to 300° C. in a suitably- 

 constructed apparatus and is then subjected to defor- 

 mation under conditions which otherwise are the 

 same as before, the cataclastic structure is found to 

 be absent and the strength of the deformed marble 

 rises to 10,652 lbs. to the square inch, that is to say, 

 it is nearlv as strong as the original rock. The calcite 

 grains, which in the original rock are jD^^ctically 

 equidimensional, are now distinctly flattened, some 

 of them being three or even four times as long as 

 they are wide. Some grains can be seen to have been 

 bent around others adjacent to them, the twin lamellse 

 curving with the twisted grain. In others again of 

 these twisted lamellae, the twinning only extends to a 

 certain distance from the margin, leaving a clear 

 untwinned portion in the centre. The rock consists 

 of a imiform mosaic of deformed calcite individuals. 



