432 J. D. Dana on some points in Lithology. 
these and other rocks. They are throughout lithology a 
source of difficulty in characterizing kinds of rocks, as already 
s ut they do not set aside the fact that the division 
between the mica and potash-feldspar series and the hornblende 
and potash-feldspar series is the most reasonable on mineral- 
ogical and chemical grounds. 
ecies through isomorphous substitutions of the 
tions of these % 
tersilicate and bisilicate (the amount of sodium present deter- 
because, in several cases, mechanical mixtures of one species 
with another had been ascertained to exist in crystals. Now 
that Des Cloizeaux has proved, by optical investigations, that 
several of the species of triclinic feldspars are really species, 
other departments of chemistry, and that there are not indefinite 
blendings, the term “ plagioclase” has become merely a synonym 
for ‘triclinic feldspar.” 
