354 WHITMAN CROSS 
were recognized by chemical analysis, largely through the 
researches of Gustav Rose, a further mistake was made. It was 
for a time thought that orthoclase (or potash feldspar), albite, 
oligoclase, labradorite, and anorthite were the only feldspar 
species entering into rocks, and that they did not occur to any 
great extent in association. Hence, after setting up a great 
division of feldspathic rocks, the logical method for subdivision 
was according to the kinds of feldspar present. This was 
thought to be the more surely a correct principle, since its 
application seemed to produce groups nearly coinciding with 
those previously recognized, but less sharply defined. 
Perhaps the first to advocate the use of feldspars in this way 
was Hermann Abich, in 1841.7 Abich viewed magmas as basic, 
neutral, or acid silicate solutions of a great range in specific 
gravity. As the known feldspar series presented also a wide 
range in silica and in specific gravity, it seemed to him certain 
that there must be a definite relation between the silica content 
of a given magma and the feldspar which could crystallize out 
of it. It therefore seemed to him possible to use the various 
feldspars for the classification of endogenous (or eruptive) rocks 
very much as fossil remains were used in sedimentary rocks. For 
the next twenty-five years a sharp division according to feldspars 
was advocated, in systems to be reviewed ; and it required the 
application of the theory of isomorphous mixtures to feldspars, 
the polarizing microscope, and modern methods of mechanical 
and chemical analysis, to demonstrate the true distribution of 
feldspars in rocks, and the thoroughly artificial nature of this 
arrangement. 
Influence of genetic hypotheses——The connection of chemical 
composition of igneous rocks with theories as to the origin of 
the differences in composition of magmas and the use of the 
genetic conception in classification forms an interesting chapter 
in the history of systematic petrography. Yet the application 
of this genetic idea led at first only to a broad and ill-defined 
tH. Apicu, Geologische Beobachtungen tiber die vulkanische Erscheinungen und 
Bildungen in Unter- und Mittel-[talien, Braunschweig, 1841. 
