346 The American Geologist. June, i899 
LEWINSONLESSING'S CLASSIFICATION OF ROCKS 
AND DIFFERENTIATION OF MAGMAS. 
Note. -The following review of a recent important Russian contribution 
to the subject of classification and differentiation will make available to 
English-speaking geologists the views of a Russian savant whose opinions 
are otherwise nearly inaccessible. The writer has simply translated it 
from the French and hereby wishes to express to the young petrogra- 
pher his thanks for the thorough examination of the work which the re- 
view itself testifies to, and his regret that the reviewer's modesty has 
enjoint-d l\iin from ajipending the reviewer s name to the communica- 
tion. N. H. w. 
General Petrography. 
Theoretical Part. Chapter /. 
The first chapter of the work is devoted to the chemical 
classification of eruptive rocks. In the historical sketch there 
should be added, to the sketch of the views of Roth, Iddings, 
Aiichel Levy, etc., an exposition of the work of Schrocken- 
stein. published in 1886 under the title: Ausfliige auf das Feld 
der Geologic. Geologischen Sudien der Silicatgesteine. 
Schrockenstein has taken for base of his classification of erup- 
tive rocks the degree of acidity (silicatstufe )of the silicates, and 
by analogy froni these he establishes the principal types of 
eruptive rocks. 
The new classification of Lewinson-Lessing is based es- 
sentially on the same idea. However, the author states that he 
had no knowledge of the work of Schrockenstein until after he 
had completed a portion of his computations. 
In order to characterize an eruptive rock from the point 
of view of its chemical composition the ' author proposes to 
make use of the following properties : 
1. Degree or coefficient of acidity, a. Bv dividing the 
number of atoms of oxygen held by SiC);^ by the number of 
those which are contained in the other oxides, a characteristic 
number (o() is obtained. In averaging each family is found to 
Iiave its coeflficient. 
2. Formula of chemical composition. The oxides are put 
in two groups, RO* and R^Os. Taking as unity the quantity of 
that group whose content is smallest, he obtains the formula of 
*R0 equals the sum of KsO, NagO, CaO, MgO, FeO. 
