1893.] Mineralogy and Petrography. 1003 
MINERALOGY AND PETROGRAPHY:' 
The Trachytes and Andesites of the Siebengebirge.—In 
the course of a discussion on the geological relations of the trachyte 
and andesite of the Siebengebirge, Grosser’? describes the various 
occurrences of these rocks and gives an outline of their petrographical 
characteristics. The trachytes he separates into typical, andesitic and 
aegerine varieties, and the andesites into trachytic and basaltic kinds. 
In the typical trachytes hornblende phenocrysts are frequent, but 
crystals of this mineral in the groundmass are unknown. Among the 
andesites the trachytic variety is noted for the absence of dark com- 
ponents from the groundmass and their rarity among the rock’s pheno- 
crysis. The basaltic andesite is rich in iron minerals, both as pheno- 
crysts and as constituents of the groundmass. The order of eruption 
was trachyte, andesite, basalt. 
A Variolitic Dyke in Ireland.—A variolitie dyke from Anna- 
long, County Down, Ireland, resembles in the hand-specimen the vario- 
lites from Mt. Genévre. Cole’ mentions it as consisting of devitrified 
glass, often containing skeleton crystals of magnetite, augite and plagio- 
clase, and enclosing spherulites that are much larger toward the cen- 
ter than at the edge of the dyke. Thin selvages, 1 cm. in thickness, 
with very small spherulites scattered through them, exist on the sides 
of the dyke. Beyond these there is an abrupt transition to material 
containing the large spherulites. The selvages evidently cooled and 
lined the walls of the crevice now occupied by the dyke, before the in- 
terior filling consolidated ; for not only is the transition between the 
substances of the two portions sharp, but the spherulites of the interior 
mass have in some cases grown from the line separating the two por- 
tions. 
The Chemical Nature of Eruptive Rocks.—Lang‘ has 
returned to his study? of the chemical nature of eruptives. After a criti- 
cal examination of many fresh specimens, the author concludes that 
the mineralogical nature of igneous rocks cannot be determined from 
x oe ba Dr. W. S. Bayley, Colby University, Waterville, Me. 
* Min. u. Petrog. Mitth., xiii, p.115. 
* Cf. AMERICAN NATURALIST, 1892, p. 334. 
