Lezvitiso/i-Lessuigs Classification of Rocks. 
347 
the type: ;//RO, R^Oa, //SiO^. Generally each family and each 
type has a characteristic formula. 
3. The relation between R-^O and RO in molecular propor- 
tions. 
4. The relation of Na^O to K-O in alkaline rocks. 
In order to obtain the formulas and the numbers of the 
table the author computed the results of 345 analyses of rocks 
taken principally from the following works: 
F. Zirkel: Lehrbuch der Petrographie, 1894. 
H. Rosenbusch: Ueber die chemischen Berichtungen dc-r 
Eruptivgesteine. 
C. Brogger: Die Eruptivgesteine des Kristianiagebietes. 
W. Cross. The laccolitic mountain groups of Colorado, 
Utah and Arizona. U. S. G. .S., 1894, etc. [Table is on pp. 
350-351-] 
Without direct reference to these works, an analysis made 
by the author niay be taken as an illustration of his method of 
calculation. M. Lewinson-Lessing states that the chemical 
composition of the rock, expressed in the percentages of silica 
and of the different oxides, is not well characterized. In or- 
der to obtain the four properties mentioned (above) which in 
his opinion are characteristic he proceeds as follows: He di- 
vides the quantity, expressed in percentage of each oxide (SiC):; 
R-O, RO, RiO-i) which is given by the analyses, by the cor- 
responding molecular weight; the quotient, called by him the 
"molecular proportion," or equivalent quantity, is a character- 
istic number. 
Let us take as an example the diabase of Assa (Caucasus), 
p. 374 and 385 of the text. 
SiOs AI3O3 1 FegOg FeO 
CaO 
MgO K^O Na^O 
Total 
Per Cent 
49.19 
16.83 
1.96 
8.15 
12.38 
7-50 
trace 
3-24 
99-25 
Molecu- 
lar 
Weight 
60 
102 
160 
72 
56 
40 
62 
Quotient 
of 1st by 
2d 
.828 
.166 
.012 
.114 
.222 
.189 
.050 
Hence: as RO equals R.,0-j-RO equals Fet)-|-Ca(3-(- 
MgO + KaO + Na.O, and 
R2O3 equals AL^Og + Fe^Oa, 
