34^ The American Geologist. Juno, \mi 
SiOj equals 0.83; RO equals 0.575 (equals 0.58) and 
R.^O, equals .178 (equals 0.18). 
Multiplx in^- the coefficients by 10 we obtain the formula : 
5.8RO, i.SRoO'g, 8.3SiO, 
Taking the quantity of the smallest group (1.8) and divid- 
ing the coefficients we get the final formula of the diabase, viz. : 
3.2RO, R2O3, 4.61 SiO.. 
The coefficient of acidit\', a, equals 
O of SiO., , Q.22 , 
— r\ — FTu n: — — ^A equals — ^ equals 1.49 
O of the other oxides ^ 6.2 
Oxygen of SiOg equals 4.61 X2 equals 9.22 
Oxygen of the otheroxides equals (3.2 xO +3 equals 6.2 
equals 1.49. 
RO (properl\) equals FeO + CaO + MgO equals 0.525. 
The relation of R.^OrRO equals 0.052^0.525 equals ,\, 
Lewinson-Lessing has added at the end of his book the 
tables showing the quotients, as above obtained, of 345 analy- 
ses. If comparison be made of the formulas of the granites, 
for example, with the formula RO, RXl:., 7.7810^ of the table of 
classification, it becomes apparent that it is a medium formula. 
It is the same also for the coefficient of acidity. The author 
says that the error does not exceed from o.i to 0.2 for a 
Water is not considered amongst the oxides of RO. 
Characteristics of the pri/icipal groups. 
The author thus characterises the principal groups: 
I. Hypo or ultra basic rocks: Predominance of monosili- 
cates; total absence (or in great part) of the feldspars; absence 
of free silica; .'t>i.4. He finds that the place assigned to lim- 
burgyte in the classification, i. e., between peridotyte and basalt 
with nepheline and leucite, satisfies equally mineralogical and 
geological petrographers. 
II. Basic rocks: Importance of the bisilicates; presence 
of the plagioclases, of leucite or of nepheline; absence cf free 
silica; frequent presence of olivine; according to the author the 
coefiticient of acidity a approaches that of the phonol\tes and 
of the syenites with eleolite, and of the basalts, which explains 
the presence of olivine in certain eleolite syenites. It is the 
same type of magma as the basaltic. The onlv difference is 
