1070 The American Naturalist. [November, 
MINERALOGY AND PETROGRAPHY.! 
Petrographical News.—A most important contribution to the 
study of the origin of the crystalline schists has lately been made by 
Van Hise,? through the medium of the Bulletin of the recently or- 
ganized Geological Society of America. It will be remembered that 
only a short time ago this writer? showed that certain mica-schists of 
the Penokee-Gogebic region in Wisconsin and Michigan are nothing 
less than sediments, in which secondary mineral changes have taken 
place. He now goes further, and shows that under the influence of 
. pressure, and probably heat, the pre-Cambrian slates and conglomerates 
of the Black Hills, Dakota, have been changed into schistose rocks, 
among which are gneisses. The reasons given for this conclusion are: 
(1) The gradation of the slates into schists, with loss of slaty cleavage, 
and the development of a foliation, usually oblique to the cleavage, and 
sometimes even perpendicular to it ; (2) the concentric arrangement of 
the schists around granitic areas in such a way that the strike of their 
foliation is always parallel to the boundaries of the eruptive rock, and 
the dip always inclined away from them; (3) the clear evidence 
afforded by the microscope to the effect that the rocks intermediate 
between the schists and slates have all suffered squeezing to such an 
extent that their various constituents, more particularly the quartz, have 
been flattened, cracked, and even broken, so that their different parts 
extinguish differently ; and finally (4) the certainty that much of the 
material of the schists is of secondary origin. The new minerals pro- 
duced by the forces at work are silica in different forms, biotite, mus- 
covite, and feldspar, and sometimes hornblende, garnet, tourmaline, and 
staurolite. In the less schistose varieties the grains of the original 
slates can be distinguished, as they are outlined by a layer of ferrite 
deposited upon them before they had lost their characteristic shapes. 
The quartz grains are flattened in the direction of the line of supposed 
pressure, and are broken. The cracks are often filled with particles of 
iron oxides, and sometimes are marked by lines of fluid inclusions. The 
deposition of silica around the fractured quartz grains and the produc- 
tion of secondary mica and feldspar are regarded as abundantly able 
to change a slate into a schist, especially when foliation has been 
! Edited by Dr. W. S. Bayley, Colby University, Waterville, Me. 
.? Bull. Geol. Soc. of Am., Vol. I., p. 203. 
3 AMERICAN NATURALIST, Aug. 1886, p. 723. 
