232 S'n.i 1 . F. I*. A.lams ami Dr. .1. T. tfi 



unaltc Thi- turbid apj>earance is most marked along a series 



of n-tirulating lines running through the sections, which when highly 

 magnified are seen to consist of lines or bands of minute calcite 

 granules. They are lines along which shearing has taken place. The 

 cal.-ite individuals along these lines have broken down, and the frag- 

 ments so produced have moved over and past one another, and remain 

 as a compact mass after the movement ceased. In this granulated 

 material are enclosed great numbers of irregular fragments and shreds 

 of calcite crystals, bent and twisted, which have been carried along in 

 the moving mass of granulated calcite as the shearing progressed. 

 This structure is therefore cataclastic, and is identical with that seen 

 in the felspars of many gneisses. 



Between these lines of granulated material the marble shows move- 

 ments of another sort. Most of the calcite individuals in these posi- 

 tions can be seen to have been squeezed against one another and 

 in many cases a distinct flattening of the grains has resulted, with 

 marked strain shadows, indicating that they have been bent or 

 twisted. They show, moreover, a finely fibrous structure in most 

 cases, which, when highly magnified, is seen to be due to an extremely 

 minute polysynthetic 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 

 somewhat, while the flattening of the grains is evidently due to move- 

 ments along the gliding planes of the crystals. In these parts, there- 

 fore, 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 increase the relative abundance of the granulated 

 material, and in this way to make the rock weaker than when the 

 deformation is slow. 



When the marble is heated to 300 C. in a suitably-constructed 

 apparatus and is then subjected to deformation under conditions which 

 otherwise are the same as before, the cataclastic structure is found to 

 l)e absent and the strength of the deformed marble rises to 10,651' ll>s. 

 to the square inch, that is to say, it is nearly as strong as the original 

 rock. The calcite grains, which in the original rock are practically 

 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 lamella- 

 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 uniform mosaic of deformed calcite individuals. 



