376 WHITMAN CROSS 
conditions in that country up to 1870 may be summarized by 
the statements of Archibald Geikie, in 1872, in the Students’ 
Manual of Geology. We there says: ‘‘There is as yet no good 
English treatise on petrography, or the classification and descrip- 
tion of rocks.’ In this work all rocks are first classified ‘‘under 
the four great heads of igneous, aqueous, aerial, and metamor- 
phic, according to the nature of the agencies by which they have 
been brought into their present state and position.” ‘Igneous 
rocks without exception are composed of minerals which are 
silicates. These minerals may be said to belong to two great 
classes: silicates of magnesia and silicates of alumina,” each 
combined with other bases. ‘‘The felspars are the bases of all 
truly igneous rocks, those in which no felspar or mineral of 
that type is present being very few and unimportant, even if 
they exist at all.” Here we see the proposition made by Zirkel 
stated in extravagantly positive terms. 
Igneous rocks are divided into volcanic, trappean, and gran- 
itic, with crystalline and fragmental subdivisions under the first 
two. The trappean class is vague and “of convenience only.” 
In the volcanic class the law of Bunsen is practically recognized 
and two groups established: ‘the trachytes, or felspathic or 
acidic group,” and “the dolerites, or pyroxenic or basic group.” 
Similar groups are formed under the trappean class. The 
granitic class embraces only granite and syenite. 
WHITMAN Cross. 
(To be continued.) 
