812 A TREATISE ON METAMORPHISM. 



calcite grains, which in the original rock are practically equidimensional, 

 are now often 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 lamellae and the extinction 

 curving with the twisted grain. In other twisted individuals the twin 

 lamellae only extend in to a certain distance from the margin of the grain, 

 leaving a clear untwinned portion in the' center; and other crystals again 

 show not only the fibrous structure due to twinning in one direction, but 

 broader lamella?, crossing this obliquely. As the twinning in all cases is 

 probably parallel to — iR — , this is due to the appearance of a set of twin 

 lines parallel to a second face of the rhombohedron." " ..... "There 

 has been no breaking — the rock has not been crushed in the ordinary 

 sense of the term. The movement has been brought about partly by 

 twinning, but chiefly by a deformation of the grains due to a slipping on 

 their gliding planes."" (PI. VI, C.) 



Third, the dry marble was deformed during eight and one-fourth 

 hours — much more rapidly than in the second case — at a temperature of 

 400° C. The result in this case was very similar to that in the previous 

 case. From this experiment it was concluded that "quick deformation at a 

 high temperature shows therefore that calcite has freer movement in its 

 gliding planes at a high temperature and breaks less readily than when 

 cold." c (PI. VI, D.) 



Fourth, the marble was slowly deformed during a period of fifty-four 

 days, at a temperature of 300° C, in the presence of water gas. This 

 rock after deformation "is not weaker, but actually stronger, than the 

 original rock.'"* The thin sections of the rock showed substantially the 

 same textures and structures as in the case of the dry marbles deformed at 

 300° or 400° C. The "granulated material is so tiivial in amount that 

 the deformation may be said to be due exclusively to movements on the 

 gliding planes of the calcite, accompanied by polysynthetic twinning. It 

 is thus identical in character with that seen in the case of the marble when 



deformed while dry, either at 300° C. or 400° C." "There are 



no signs of solution and redeposition of calcium carbonate even in this 

 iron-stained portion of the rock. The presence of water, therefore, did 

 not influence the character of the deformation. It is just possible, however, 



« Adams and Nicolson, cit., p. 379. & Op. cit., p. 380. <■• Op. cit, p. 382. <i Op. cit, p. 385. 



