370 WHITMAN CROSS 
Mineral composition was regarded by von Richthofen as 
d 
‘essentially dependent on the chemical composition,” and was 
used by him as ‘‘more articulate” than the latter, in the con- 
struction of his scheme. The outline of von Richthofen’s sys- 
tematic arrangement is as follows: | 
Eruptive Rocks: 
ClassI. Granitic Rocks. 
Orders: 1, Granite; 2, Syenite; 3, Diorite ; 4, Diabase. 
Class II. Porphyritic Rocks. 
Orders: 1, Felsitic Porphyry; 2, Porphyrite; 3, Melaphyr; 4, Augitic 
Porphyry. 
Class III. Volcanic Rocks. 
Orders: 1,7) Rhyolite’;|.2,; Drachyte; 3, “Propylites 43) Andesite: 
5, Basalt. : 
The further mineralogical variation of rocks was expressed 
in families under each order. 
In this system three principles were applied: (1) Mode of 
origin; (2) A supposed fact of correlation between age and 
texture; (3) Chemical composition as represented in mineral 
composition, and practically expressing the author’s belief in 
the Bunsen law. 
While von Richthofen’s system was not followed in its details 
by petrographers, some space has been given to it here because 
of its influence upon many geologists, perhaps especially in the 
United States, and because it illustrates so clearly the perils of 
introducing genetic ideas into the systematic classification of 
igneous rocks, even when those ideas are believed to be estab- 
lished as laws. 
It may be mentioned here that von Richthofen enunciated 
practically the same views in his Fiihrer fiir Forschungsretsende, 
issued in 1886. 
Cordier’s system and its influence-—Turning now to France, 
before taking up the essays at classification made during the 
era of the microscope, it appears at first thought strange that 
for thirty years after Brongniart’s classic work no important 
