1895.] Petrograp hy. 363 
the igneous rocks of the region with special reference to the lavas 
whose studies led Hague to the proposal of the theory that the various 
types of rocks in the Eureka district are differentiated portions of one 
magma, which split up into two, one yielding feldspathic acid rocks 
and the other pyroxene basic ones. Among the intrusive rocks of the 
regiou Iddings mentions only granites, granite-porphyries and quartz- 
porphyries. The volcanic rocks include hornblende-andesite, horn- 
blende-mica-andesite, dacite and rhyolite, which are the types derived 
from the more acid portion of the original magma, and pyroxene- 
andesites and basalts derived from the basic portion. The pyroxene- 
andesite contains anorthite, hypersthene, augite, hornblende, a little 
biotite and an occasional quartz grain, in a glassy groundmass with a 
felt-like structure produced by labradorite and augite-microlites. The 
hornblende-mica-andesites are more acid. They contain labradorite, 
hornblende, biotite and a little quartz as porphyritic crystals in a 
micro-crystalline groundmass of lath-shaped plagioclases and inter- 
growths of feldspar and quartz. The dacites are rare. They possess 
macroscopic quartz-phenocrysts together with hornblende, hypersthene, 
a little augite, biotite, labradorite, anorthite, and possibly orthoclase 
in a pumiceous glass base, which also often contains many beautifully 
crystallized zircons. The rhyolites met with present few characters of 
special interest. They vary in the texture of their groundmass from 
micro-crystalline to glassy varieties. Their sanidine phenocrysts have 
the plane of their optical axes sometimes in the plane of symmetry 
and sometimes perpendicular thereto. Occasionally the rock possesses 
also phenocrysts of hypersthene. The basalts are poor in olivine, and 
this mineral when present is often changed into serpentine or into the 
reddish-brown substance to which Lawson has given the name iddings- 
ite. Hypersthene is present in some of the sections, and in others are 
a few grains of quartz surrounded by augite borders. 
Notes from Minnesota.—In a preliminary report of a season’s 
field work in northeastern Minnesota, Elftman‘ refers to the gabbro of 
the region as producing contact metamorphism in the slates and schists 
to the north of it. He describes more particularly the actinolite-mag- 
netite slates, from near Birch Lake, that are believed to have origina- 
ted in a fragmental rock whose nature, however, is not fully set forth. 
The gabbro is an olivinitic variety. In it are great masses of anorth- 
osite regarded by the author as phases of the gabbro. This is the rock 
492d Ann. Rep. Geol. & Nat. Hist. Survey of Minn., p. 141. 
