898 The American Naturalist. (October, 
MINERALOGY AND PETROGRAPHY: 
Anorthosites and Diabases from the Minnesota Shore of 
Lake Superior.—Along them iddle stretch of the Minnesota shore of 
Lake Superior occur several exposures of a light-colored, coarse rock, 
consisting essentially of a basie plagioclase feldspar which, according 
to Lawson,’ is sometimes bytownite; but more frequently anorthite or 
labradorite. This plagioclase is usually fresh and quite vitreous in 
appearance. It contains, as inclusions, small bleb-like masses of 
augite, plates and rods of the same mineral arranged parallel to the 
clinopinacoid, liquid enclosures, dust particles and small grains of 
hematite. In addition to the plagioclase there is also often present in 
the rock a small number of triangular augite plates between the feld- 
spars. This rock which the author calls an * anorthosite,” is found in 
knobs and bosses, and as boulders in the overlying Keweenawan erup- 
tives. The rock is evidently an eruptive which is much older than the 
volcanic flows constituting a large proportion of the Keweenawan beds. 
A second article by the same author? treats of the coarse diabase in 
“gabbro” sheets interpolated between the sedimentary beds of the 
Animikie. These are thought by the writer to be laccolitic in origin, 
i. e., to have been intruded between the sedimentaries after these had 
been solidified, and some of them even later than the time of deposition 
of the younger Keweenawan series. This conclusion is reached after a 
careful study of the contacts between the eruptives and the sedimen- 
taries, which has brought to view the existence of contact phenomena 
at both the upper and lower surfaces of the diabase. The sheets of 
eruptives have been named the “ Logan sills” in honor of Sir Wm. 
Logan, who was one of the pioneer geologists in the Lake Superior 
region. 
The Volcanic Rocks of the Andes.—In a review of Kiich’s 
volcanic rocks of the Andes, Iddings? asserts that the chemical rela- 
tions of the rocks studied indicate clearly that they all belong to the 
! Edited by Dr. W. S. Bayley, Colby v AM Me. 
en and Nat. Hist. Survey of Minn. Bull. N 
pe 
* Reiss and Stubel: Reisen in Sud-Amerika. Geologische Studien in der 
Republik y oer ci i. Pe (€ 1. Die Vulkanischen Gesteine bear- 
beitet von Richard Kuch, — d 892. 
P of Geology. Vol. 1, p. 164 
