996 The American Naturalist. [ November, 
eruptive has been marbleized. In it are pyroxenes and garnets, the 
latter often in large numbers. This diorite has heretofore been re- 
garded as a metamorphosed sediment, but, from the evidence at hand, 
the author concludes that it is a true irruptive. The gabbro of the 
region is confined to two small knobs. In one, the rock grades from 
an anorthosite into a peridotite. In the latter phase olivine consti- 
tutes nearly half of its mass. Hypersthene is abundant, while augite, 
plagioclase, and the usual accessories, spinel and magnetite, are present 
in small quantities. Reactionary rims always surround the olivines 
when in contact with plagioclase. These are composed of three zones, 
an inner one of hypersthene which is continuous with the large hyper- 
sthene components ; a middle one, composed of fine needles of uralitic 
amphibole, and an outer zone consisting of uralite and a deep green, 
highly refracting substance in grains, probably a spinel. The contact 
rim is supposed to be secondary. The various phases of the rock are 
usually much altered into actinolitic varieties. 
South American Volcanics.—The collection of Argentine 
volcanic rocks belonging to Berlin University has been investigated 
by Siepert. The collection embraces quartz-porphyries, porphy- 
rites, diabases, augite-porphyrites, melophyres and an epidiorite- 
porphyrite. In the quartz-porphyries quartz grains are often sur- 
rounded by aureoles of the same substance, whose optical orientation 
coincides with that of the surrounded particles. Many of the grains 
show undulous extinction, which the author regards as secondary. In 
some of the specimens the granophyric structure, in others the micro- 
granitic, and in still others the felsophyric structure predominates. In 
many instances the granophyric structure is unquestionably secondary. 
The porphyrites include diorite-porphy rite, eustatite-porphyrite and 
epidiorite-porphyrite. In one of tha latter a feldspar granule was seen 
to be surrounded by a feldspar aureole. The other rocks examined 
present no unusual features. 
Specimens of the younger voleanic rocks gathered by Sapper in 
Guatemala were submitted to Bergeat® for study. They comprise 
trachytes, rhyolites, dacites, andesites and basalts. The trachytes, 
though of the “ Drachenfels” type, contain about 66 % of silicia, and 
are thus closely related to the rhyolites. The andesites are the most 
abundant types. They include pyroxene, hornblendic and mica horn: 
blende varieties. Some of the pyroxenic andesites contain two pyrox- 
t Neues Jahrb. f. Min., ete., B. B., ix, p. 393. 
* Zeits. d. deutsch. geol. Ges. xlvi, I, p. 126. 
