610 The American Naturalist. [June, 
MINERALOGY AND PETROGRAPHY.' 
The Basalt of Stempel.—Bauer’s’ description of the basalt of 
Stempel, near Marburg, and its concretions and inclusions is one of 
the most excellent pieces of petrographical work that has appeared in 
a long time. A favorable opportunity has enabled the author to secure 
a splendid suite of specimens of this rock so noted for its beautiful 
zeolites. It consists of the usual constituents of basalt, viz.: plagio- 
clase, augite and olivine in a groundmass of augite and feldspar 
microlites in a base of glass. The plagioclase is andesine without 
peculiar characteristics. The augite is also without special features 
except that it is frequently zonally developed, with a dark-green ker- 
nel and brown-colored coats, in which the extinction decreases from 48° 
to 36°. The olivine is so well bounded by crystal planes that the rela- 
tions of the shapes of the cross-sections to the crystallographic axes 
have been well worked out. Twins parallel to PX are not uncommon. 
The liquid inclusions, upon careful study, are found to differ from 
those of the olivine of the concretions (Knollen), and the glass inclu- 
sions are learned to have a different composition from the glass form- 
ing the groundmass of the rock. One of the most interesting features 
of the rock is the occurrence of amygdaloidal cavities, coated within 
by a layer of glass, whose limits are sharply defined. Sometimes a — 
partition of this glass divides a cavity into two, and occasionally sev- 
eral concentric partitions give rise to a series of chambers that are 
strikingly like the chambers in Idding’s lithophysae. The olivine 
bombs included in the rock consist largely of bronzite and chrome- 
diopside grains cemented by olivine substance. The bronzite is pres- 
ent in two varieties, one an almost opaque greenish-brown kind, and 
the other a transparent olive-green variety. Picotite is also present 
quite abundantly in grains and aggregates of grains in most of the 
bombs. The effect of the action of the rock magma upon its inclu- 
sions is seen in the granulation of the pyroxenes, and the effect of the 
material of the bombs upon the magma is shown in the presence of micro- 
lites of hypersthene in the veins of the rock that ramify the bombs. 
Since the minerals of the bombs contain characteristic inclusions not 
common to lherzolitic rocks, and since, moreover, the olivine and bron- 
zite are sometimes found in forms never seen in lherzolite, the author 
1Edited by Dr. W. S. Bayley, Colby University, Waterville, Me. 
2Neues Jahrb, f. Min., etc., 1891, ii, p. 156. 
