476 PETROLOGICAL ABSTRACTS AND REVIEWS 
no resistance to stress. Ordinarily, a substance is said to be plastic 
when it can be continuously deformed without rupture. Experiment 
shows that all substances are probably latently plastic depending on 
pressure, temperature, and time. Gypsum, stibnite, halite, calcite, ice, 
galena, cyanite, fluorite, apatite, anhydrite, bismuthunite, vivianite, 
lorandite, graphite, molybdenite, brucite, mica, and sylvite are found 
to be plastic at room temperature when subjected to pressure in one 
direction, as shown by the work of Brewster, Reusch, Bauer, Averbach, 
and Miigge. They are found to be more plastic in certain directions 
than in others. Miigge’s experiments seem to indicate that plasticity 
in crystals is conditioned by “planes of translation,” along which move- 
ment takes place when the crystal is deformed, and that crystals having 
“translation planes”? are plastic under all physical conditions when 
adequate differential force is applied. About 77 crystal species have 
been tabulated by Vernadsky which show gliding planes, among them 
hornblende, topaz, dolomite, corundum, beryl, tourmaline, and epidote. 
The conclusion seems justified that all substances, even those which 
have not been shown to be plastic by experiment, are plastic under the 
conditions which prevail in the deep parts of the earth, since the degree 
of plasticity increases with temperature and pressure. H. Tressa in 
1864 showed that the plasticity of lead, aluminum, and ice was greatly 
increased under pressure. By means of pressure, W. Spring (1880) 
welded various metal powders and caused them to flow through an 
aperture. In 1902 and 1903 G. Tamman proved that flowage in these 
cases was due to internal friction and not to temporary melting. 
After A. Heim had appealed to plasticity as the means by which 
mountain deformation is accomplished, confirmatory experiments 
seemed desirable. Pfaff’s and Giimbel’s experiments (1879 and 1880) 
were futile because they did not prevent fracture by supplying adequate 
pressure on all sides. In 1886 O. Miigge succeeded in getting plastic 
deformation of diopside, galena, and anhydrite by first imbedding them 
in lead or zinc and then subjecting them to pressures on all sides. In 
1892, G. Kick imbedded test materials in molten alum, sulphur, and 
shellac inside a copper tube, and compressed them between the plates 
of a hydraulic press. He succeeded in getting plastic deformation of 
halite and marble. F. Rinne (1903), amplifying Kick’s experiments, 
flattened calcite rhombohedra under a pressure of 1,200 Kg/qcem, but 
they partly broke into powder. Halite and sylvite, however, were 
deformed without fracture, loss of strength, or clearness. F. D..Adams 
(1910) continued Kick’s experiments, and found that minerals were 
plastic in an inverse ratio to their hardness. Minerals less than 5 in 
