Geology and Mineralogy. 403 



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 individ- 

 uals were enabled under the pi-essure to alter their shape some- 

 what, while the flattening of the grains was evidently due to 

 movements along the gliding planes of the crystals. In these 

 parts, therefore, the rock presented a continuous mosaic of some- 

 what flattened grains. From a study of the thin sections it 

 seemed 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 deformation was carried out at 300° C, or better at 

 400° C, the cataclastic structure described above, was not 

 developed, and the whole movement was found to be due to 

 changes in the shape of the component calcite crystals by twin- 

 ning and gliding. 



In the experiments where moisture was present, the deforma- 

 tion of the marble taking place at 300°, no change in the charac- 

 ter of the deformation was detected. In other words the presence 

 of water appeared to exert no influence, the marble was deformed 

 as when dry at 300° by twinning and gliding alone without cata- 

 clastic action. 



In order to ascertain whether the structures exhibited by the 

 deformed marble were those possessed by the limestones and mar- 

 bles of contorted districts of the earth's crust, a series of forty- 

 two specimens of limestones and marbles from such districts in 

 various parts of the world were selected and carefully studied. 

 Of these, sixteen were found to exhibit the structures seen in the 

 artificially-deformed marble. In these cases the movements had 

 been identical with those developed in the Carrara marble. In 

 six other cases the structures bore certain analogies to those in 

 the deformed rock but were of doubtful origin, while in the 

 remaining twenty the structure was different. 



It appears, therefore, that by submitting limestone or marble to 

 differential pressures exceeding the elastic limit of the rock and 

 under the conditions described in this paper, permanent deforma- 

 tion can be produced. This deformation, when carried out at 

 ordinary temperatures, is due in part to a cataclastic structure 

 and in part to twinning and gliding movements in the individual 

 crystals comprising the rock. Both of these structures are seen 

 in contorted limestones and marbles in nature. 



At elevated temperatures only the changes of the second kind, 

 i. e. by twinning and gliding, take place. This latter movement 

 is identical with that produced in metals by squeezing or ham- 

 mering, a movement which in metals, as a general rule, as in 

 marble, is facilitated by increase of temperature. There is there- 

 fore a flow of marble just as there is a flow of metals, under 

 suitable conditions of pressure. The movement is also identical 

 with that seen in glacial ice, although in the latter case the move- 

 ment may not be entirely of this character. 



