REVIEWS 299 
chlorite, talc, zoisite, and epidote. These minerals, with quartz and cal- 
cite, form the characteristic rocks of this zone—namely, sericite-phyllite, 
sericite-quartzite, lime-phyllite, albite-phyllite, talc, and chlorite-schists, etc. 
The conditions named for the middle zone are higher temperature and 
intense stress, stronger hydrostatic pressure, and positive and negative 
heat-toning. The predominant effect of pressure is recrystallization and 
remineralization with decrease in volume. The minerals assigned to this 
zone as characteristic are biotite, zoisite, epidote, hornblende, staurolite, 
garnet, disthene, titanite, magnetite, ilmenite, muscovite, microcline, 
albite, and oligoclase. The typical rocks of this zone constitute the great 
mass of the crystalline schists: mica-schists, amphibolites, garnet, and 
staurolite-schists. 
The specified conditions of the lowest zone are enormous hydrostatic 
pressure and high temperature, very feeble stress, and negative heat- 
toning. The reactions are inferred to take place with volume increase, 
since the minerals of this zone nearly all belong to Lowinson-Lessing’s 
“plus” group. (‘‘The molecular volume of the minerals of the plus group 
exceeds the molecular volumes of the oxides which compose them.”) Dr. 
Grubenmann argues from this that the reactions are probably controlled by 
temperature and not by pressure. The minerals which are described as 
peculiar to this zone are cordierite, magnetite, sillimanite, ilmenite, ortho- 
clase, plagioclase, biotite, augite, olivine, and garnet. The gneisses consti- 
tute the predominant rocks. 
A number of mineral and rock alterations are cited as peculiar to each 
zone. Olivine of the lowest zone becomes hornblende or garnet in the 
middle, and serpentine in the upper zone. Granulation of grain, chloriti- 
zation, and sericitization occur in the upper zone. An aluminous sediment 
composed of kaolinite, quartz, mica, feldspar, iron oxide, and lime becomes 
a phyllite in the upper zone, a mica schist in the middle, and a gneiss con- 
taining feldspar, sillimanite, and other anhydrous minerals, in the lower 
zone. 
Dr. Grubenmann’s threefold division is not intended to apply to all 
the lithosphere with regard to metamorphic processes, since it is concerned 
with the development of the crystalline schists only. In a broad way, the 
three zones collectively correspond to Van Hise’s zone of anamorphism 
and rock-flowage. Anyone who has made a field study of the crystalline 
schists appreciates the difficulty of classifying them according to the meta- 
morphic processes to which they owe their origin, since no direct obser- 
vational evidence can be obtained to form the basis of such a classification. 
Although Dr. Grubenmann refers to the occurrence of schist zones in 
