EXPERIMENTS ON FLOW OF ROCKS 459 



synthetic twinning. The chalky aspect of the deformed rock is, in fact, due chieifly 

 to the destruction by this repeated twinning of the continuity of the cleavage sur- 

 faces 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 movements 

 along the gliding planes of the crystals. In these parts, therefore, the rock pre- 

 sents 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. 



AVhen the marble is heated to 300 degrees centigrade 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 be absent and the 

 strength of the deformed marble rises to 10,652 pounds 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 lamellae curv- 

 ing 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 center. The rock consists of a uniform mosaic of deformed and 

 for the most part highly twinned calcite individuals. 



Plate 43, flgure 1, shows a microphotograph of a thin-section of the Carrara 

 marble used in the experiments. The individual grains have very nearly the 

 same diameter in every direction, although differing somewhat in size among 

 themselves. Twinning is seen only in two or three grains, and in these is repre- 

 sented by a few broad lamellae. The section was photographed in ordinary light 

 and magnified 50 diameters. 



Plate 43, figure 2, shows a microphotograph of the same rock after having been 

 slowly deformed during 124 days at a temperature of 300 degrees centigrade. The 

 individual grains can be seen to be distinctly fiattened, giving a certain foliation 

 to the rock, and also to possess the fibrous appearance referred to as resulting from 

 polysynthetic twinning. It was photographed between crossed nicols and magni- 

 fied 50 diameters. 



When the deformation is carried out at 400 degrees centigrade, no trace of cata- 

 clastic structure is seen. 



An experiment was then made in which the marble was deformed at 300 degrees 

 centigrade, but in the presence of moisture, water being forced through the rock 

 under a pressure of 460 pounds per square inch during the deformation, which ex- 

 tended over a period of fifty-four days, or nearly two months. Under these con- 

 ditions the marble yielded in the same manner as when deformed at 300 degrees 

 centigrade, in the absence of moisture, that is by movements on gliding planes 

 and by twinning, but without cataclastic action. The deformed marble, however, 

 when tested in compression, was found actually to be slightly stronger than a piece 

 of the original marble of the same shape. The structure developed was identical 

 with that of the marble deformed at 300 degrees centigrade in the absence of water. 

 The presence of water, therefore, did not influence the character of the deforma- 

 tion. It is quite possible, however, that there may have been a deposition, of in- 

 finitesimal amount, of calcium carbonate along very minute cracks or fissures, 



