274 PETROLOGICAL ABSTRACTS AND REVIEWS 
pressure, and by recrystallization. According to A. Heim, rocks which 
are subjected to pressures far in excess of their strength are in a condition 
of latent plasticity (1878). In great depths, where stresses are trans- 
mitted equally in all directions, rock particles are under hydrostatic 
pressure, and therefore are deformed without fracture. Where deforma- 
tion takes place without fracture, shearing on gliding planes as shown 
by O. Miigge and the increase in plasticity of crystals with rise of 
temperature as shown by L. Milch on halite are very important. 
R. Brauns pointed out in 1896 that, wherever pressure, heat, and hot 
water act on a rock, alterations in the rock take place until equilibrium 
is reached. 
In 1898 Van Hise divided the earth into a zone of fracture and a zone 
of flow. The crystalline schists develop in the latter, recrystallization 
and mineral parallelism being effected by water which causes the transfer 
of material by solution and deposition from areas of maximum pressure 
toward areas of minimum pressure. Van Hise also divided the earth’s 
crust into two physical chemical zones, the upper characterized by reac- 
tions which liberate heat and the lower by reactions which absorb heat. 
L. Milch and others believe that the ‘law of volume,” according 
to which those minerals which occupy minimum volume tend to develop, 
is counteracted by high temperature, which tends to bring about the 
opposite result. L. Milch therefore separates the crust into two zones, 
an upper in which the “volume law” controls, characterized by the 
development of hydrous minerals, and a lower zone in which temperature 
is the controlling factor and practically prevents the development of 
hydrous minerals. U. Grubenmann designates the transition between 
these two regions a third zone. The type minerals of the lower zone 
are pyroxene, garnet, biotite, lime plagioclase, potash feldspar, silli- 
manite, cordierite, olivine; those of the upper zone, zoisite, epidote, 
muscovite, albite, and chloritoid. Hornblende, quartz, tourmaline, 
staurolite, titanite, and rutile are common to both. Milch also believes 
that the “Kristalloblastic” texture of schists is determined by the 
crystallizing force of the contemporaneous minerals. Accordingly those 
minerals assume crystal form which have the densest aggregation of 
molecules, and preferably those crystal surfaces develop which have the 
closest spacing of molecules, particularly cleavage faces. Mineral 
parallelism is less due to mechanical plasticity than to solution and 
deposition according to the principle of Riecke. The following tables 
of U. Grubenmann show the relation of the three zones to the rocks 
which are characteristic of each. 
