REVIEWS 3 301 
centage weights of TiO, and P,O, are converted into their equivalent 
weights of SiO, and added to the latter. In the same way Fe,O; and MnO 
are added to FeO, and BaO and SrO are united with CaO. H,0O is neg- 
lected. The seven oxides resulting from this elimination are recalculated 
on a basis of 100, and then converted into molecular percentages. The 
molecular percentages are then distributed as follows: 
S=molecules of SiOz. 
A=sum of molecules of K,0+Na.O united with Al.O; in the ratio of 1:1. 
C=CaO united to Al,O; in the ratio of 1:1. 
FE =sum of FeO and MgO. If Al.O; is deficient, the remaining CaO is added 
to F. 
M=CaO added to F. 
T=excess of Al,O; after satisfying CaO and the alkalies in the ratio of 1:1. 
Ke= Sio 
GA-F2C Es 
These seven so-called group values, S, A, C, F, M, T, and K, are the 
‘‘empirically derived chemical values” which delimit the groups. Besides 
these values, Dr. Grubenmann has adapted? Osann’s? projection values, 
a, c, f, to the chemical classification of the crystalline schists. 
The determination of the texture, structure, and mineral content of the 
schist completes the examination necessary for the complete identification 
of a’schist according to Dr. Grubenmann’s system. 
Dr. Grubenmann does not claim to have produceda perfect system of 
classification. ‘The zonal conceptions, he admits, are not entirely free from 
well-founded objections. A separation into two zones, he believes, would 
in many cases be more in accord with the facts of occurrence. 
Die kristallinen Schiefer may be regarded as an excellent summary of 
known facts relating to the crystalline schists. It also contains new mate- 
rial. With present incomplete knowledge it is not possible to say whether 
the proposed classification is warranted and supported by the facts in the 
field. Perhaps its greatest service to the science consists in proposing a 
new theory which points out the necessity of a far more detailed investiga- 
tion of the crystalline schists than has been made thus far. 
EDWARD STEIDTMANN 
UNIVERSITY OF WISCONSIN 
t See Tschermark’s minerologische und petrographische Mittheilungen, Vol. XIX, 
P= 352- 
20A 20C 20F 
2 ECR? (SRE CER  (SALCLE- 
