808 The American Naturalist. [September 
ing to criticisms recently made by Brögger. This author declares that 
the order of primary differentiation can be learned only from a study 
of large bodies of plutonic rocks, and that conclusions with respect to 
this subject based on the study of extensive masses are not reliable. 
He further states that the order of succession in eruptions is from basic 
to acid magmas, often ending with basic ones, and not from interme- 
diate magmas to greater and greater extremes. After describing at 
some length the general distribution of the igneous rocks in Idaho, 
Montana and Wyoming, and comparing the great volume of the effu- 
sive rocks erupted in this volcanie district with the relatively small 
(though actually great) volume of intrusive rocks, Iddings states that 
he cannot but believe that the differentiation which gave rise to the 
former must have been more fundamental than that which gave rise to 
the intrusive rocks, and hence its products reveal the true character of 
the primary differentiated of a molten magma at considerable depths 
beneath the surface. Moreover, the sequence of the intrusive rocks is 
practically the same as that of the effusive in the Electric Peak dis- 
trict, viz., from intermediate through acid to basic rocks. 
Granites of Pyramid Peak District, California.—Lindgrew’ 
describes the rocks of the Pyramid Peak district in the Sierra Nevadas 
as consisting of an older series of slates, tuffs, schists, porphyrites and 
granitic rocks overlain by Tertiary andesites, rhyolites and basalts. 
The granites are intrusive in the old series, metamorphosing the latter 
for a distance of several miles from their contacts with them. The clay 
slates in their most metamorphosed forms are nificaceous schists or 
gneisses. At a greater distance from the granite they are ‘ knoten 
schiefer,’ often carrying andalusite. The granitic rocks include aplites, 
granites, granite-diorites, diorites and gabbro. The highest ridges of 
the district are composed of granitite. Granodiorite is the predomi- 
nant rock. It consists of quartz, an acid plagioclase, biotite, hor 
blende and a little sphene and magnetite. The rock is intermediate 
_in composition between quartz-mica-diorite and Brogger’s quartz-mon” 
zonite. While not always easily distinguished from the former rock, 
the author would restrict the name granodiorite to rocks containing 
59 per cent.69 per cent. SiO,, 14 per cent.-17 per cent. Al,O» wae 3 
cent.-2} per cent. Fe,O,, 1} per cent—4t per cent. FeO, 3 per cent. s 
per cent. CaO, 1 per cent.—24 per cent. MgO, 1 per cent.—34 per pega” 
K,O and 24 per cent.-44 per cent. Na,O. Analyses of the granite a 
and of the grano-diorite (II) follow : 
ë Amer. Jour. Sci., Vol. III, 1897, p. 301. 
