PETROLOGICAL ABSTRACTS AND REVIEWS 
ALBERT JOHANNSEN 
Osann, A. “Petrochemische Untersuchungen. Th. I,’ Abh. 
Heidelberger Akad. Wiss., Math.-naturw. Kl., 1913. 4to, 163, 
pls. 8. 
In this volume the author attempts to answer two important ques- 
tions in chemical petrography. First, what are the laws of the chemical 
composition of igneous rocks, and what are the characteristics separating 
the rocks of the alkali series from those of the alkali-lime? Second, 
what are the most important chemical differences between sediments 
and igneous rocks, and how may these be made of value in determining 
the origin of crystalline rocks ? 
To answer these questions, use is made of nearly thirteen hundred 
chemical analyses. The relationships between the oxides which vary 
most in their proportions in sediments and igneous rocks and which are 
also of value in comparing the igneous rocks, have been determined. 
Molecular proportions are used and recomputed to a constant sum so 
that they may be diagrammatically compared. Unimportant and acces- 
sory constituents are omitted, but TiO, and ZrO, are added to SiO., 
BaO and SrO to CaO, and MnO to iron, which is calculated as FeO. 
The four relationships computed are: 
t. SiO.:ALO,:(Fe, Mg, Ca)O=SAIF proportions. In this series 
the alkalies are entirely omitted. In sedimentary rocks this relation- 
ship leads to a grouping in three classes; siliceous, aluminous, and cal- 
careous. 
2. ALO;:CaO:(Na,K),0=AICAlk proportions. This shows the 
more important differences between sediments and igneous rocks, and, 
in combination with the preceding, the characteristic differences between 
the alkali and alkali-lime series. 
3. Na2O:K.0=NK proportions. Recalculated to to. 
4. MgO:CaO=MC proportions. Recalculated to to. 
The last two are of importance in separating igneous rocks from 
sediments. 
The first two may be represented graphically in a triangular diagram 
after recalculation to 30. The results are plotted to the nearest o.5. A 
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