1887] - Mineralogy and Petrography. 175 
group. This disposition Rosenbusch accepts as the correct one 
for many of the rocks heretofore denominated teschnites, an 
among them that from Teschen, Moravia, named by Hohen- 
egger teschnite, in which Zirkel, in 1868, and Tschermak, in 1869, 
thought they had found nepheline. Since, therefore, the original 
teschnite is probably only a variety of diabase, it was thought 
best to drop the name as descriptive of intrusive nepheline-plagi- 
lase rocks, and to substitute for it the name theralite (@j;e¢-— 
seek diligently). 
is name is intended to cover all intrusive rocks containing 
nepheline and plagioclase as prime constituents. That such 
rocks occur has not yet been positively proved, although the 
recent work of Wolff in Montana and the earlier work o 
Hawes and of Harrington in the neighborhood of Montreal, 
Canada, render their existence probable. 
The gabbros and the norites have been included together in 
one family, the former comprehending those plagioclase rocks in 
which a monoclinic augite (of a composition approaching that of 
diallage) occurs as the predominant bisilicate constituent, and the 
latter those in which this augitic constituent is orthorhombic. 
Every gradation between the typical gabbro and the typical 
norite is recognized as possible. 
The diabases are regarded as occupying a peculiar position 
characteristics of thin magmas which cooled at the surface, 
such as the possession of amygdaloidal upper surfaces, the asso- 
ciation with them of tufas, etc. The greater mass of diabase, 
however, Rosenbusch describes as intrusive (in sheets or dykes), 
description of this rock chlorite also was considered as essential. 
This mineral is now regarded as merely secondary, the last pro- 
duct in the alteration of the augite. In this connection occasion 
is taken to remark that uralitizatine is not a paramorphism of 
hornblende after augite, but that the change is probably due to 
loss of calcium,—the hornblende is an apomorph. The frequent 
association of epidote with chlorite would seem to confirm this 
ew 
scription of Dathe’s, which this writer himself acknowledges to 
be erroneous in some of its essential particulars. : 
The second great class is that of the dyke rocks. These are 
not as well characterized as the intrusive class. The conditions 
: American Naturalist, Notes, May, 1885, p. 499. 
VOL. XXI.—NO. 2. 12 
