
1891.] Mineralogy and Petrography. 825 
those rocks acid that contain an excess of the latter,—/. e., more than 
enough to saturate the bases present. This excess shows itself as free 
quartz, Thus it is not the percentage of silica in a rock-mass that carries 
it into the acid, neutral or basic class, but it is the lack or excess of silica 
as compared with the bases. Several empirical formulas represent the au- 
thor’s types. They are based on the determined ratio between bases and 
acid in neutral rocks, in which the amount of silica necessary to saturate 
the bases (in percentage) is represented by formula SiO, = 2(R,O+ RO) 
+R,0,. In neutral rocks this relation exists, consequently the for- 
mula represents the composition of such rocks. The amount of silica 
in basic rocks is represented by SiO, = R,O+RO-+R,O,, and in acid 
rocks SiO,= 2(R,O0+RO)+R,0, jase é., the percentage of 
silica in thésé i is greater than twice the sum of the percentages of R,O 
and RO, plus R,O,, by as much as there is quartz in the rock. The 
author then uses these formulas, with others deduced from them, as a 
basis for the classication of eruptives. His paper is interesting reading, 
and if facts justify his formulas the conclusions reached by the author 
will prove of value to petrographers. Some recent discussions on 
the pressure alteration of basic eruptives have added quite a little to 
our information regarding this phenomenon. Welch ê has described the 
alteration of diabases into schists in the region of the Soonwalde, in 
the Taunus, on, the left side of the Rhine. Well-developed diabases 
exist near Rauenthal. Under the influence of pressure they have given 
rise to schistose rocks containing actinolite, and others in which chlo- 
rite and epodite have replaced the original augite. Other schists, whose 
relationship to diabase can be determined only by a microscopical 
examination, have been called hornblende-sericite-schists, augite- 
schists, and sericite-calc-phyllites. The author has made a careful 
study of all these, which has resulted in their separation into schists 
composed of actinolite and epidote, those containing a blue amphi- 
bole, and finally those made up principally of chlorite. In some of 
these derived rocks augite may still be detected, in others the diabase 
structure is still visible, while in a third class no traces of the original 
constituents nor of their arrangement are recognizable. All show 
evidence of pressure in the shapes of their components and in their 
nature. In the first class the epidote and chlorite have been derived 
from augite. The second class contains, in addition to glaucophane, a 
bluish-green actinolite, sericite, and biotite. Epidote is entirely want- 
ing. The rocks of the third group are combinations of chlorite, 
quartz, sericite, and generally calcite. In each the structure is schis- 

6 Zeits. d. deuts. geol. Ges., XLI., 1890, p. 394- 


