1 56 ■ The Aniericatt Geologist. September, i9co 
diabase for our study. The second area borders lake Superior 
from a po'int intermediate between Duluth and Two Harbors 
to beyond Poplar river; the length of this shore line exceeds 
seventy-five miles, while the surface occupied approximates 
six hundred square miles. This region is composed chiefly of 
diabase types, which have recently been shown to belong to 
two 'different epochs. But it also includes several small masses 
of gabbro and plagioclasyte, especially at Encampment island, 
Beaver bay, and Carlton peak. The last named attains an al- 
titude of about six hundred feet above the lake ; the type of 
plagioclasyte studied came from the summit of this peak, 
where large masses of nearly pure feldspar are found. 
The geological literature of this region has become very 
voluminous within the last twenty years, and the limits of this 
paper prohibit any extensive bibliographies, which would. 
moreover, be largely superfluous since several able reviews of 
the literature already exist. \ an Hise* has published an ex- 
haustive bibliography of early Cambrian and Archean Ameri- 
can geology, with extended summaries ; Bayleyt has presented 
a very useful review of the literature of the petrography of 
gabbro rocks, especially as found in the lake Superior region ; 
Elftman^ has brought the bibliography of Minnesota gabbros 
nearly up to date, and Adams§ has given a special bibliography 
of the anorthosytes. 
The present study is from the petrographic and mineral- 
ogical points of view'. Among the more important contrib- 
utors to the petrographic knowledge of the gabbros of Min- 
nesota may be mentioned : Strang (1877), Irving (1883), Wads- 
worth (1887), N. H. Winchell (1891 and later), Grant (1893), 
and Bayley, (1893 and later). The mineralogy of the series has 
been slighted until very recently, when N. H. Winchell^ has 
made a detailed study of various minerals. 
*C. R. Van Hise: Correlation papers: Archean and Algonkian: 
U. S. G. S. Bull. 86, 1892. See also: Current Precambrian literature: 
Jour. Geol. 1893. I.; 1895, III; 1896, IV. 
fW. S. Bayley: The basic massive rocks of the Lake Superior re- 
gion: Jour. Geol. 1893, I; 1894, 11; 1895, III. 
{A. H. Elftman: The geology of the Keweenawan area in north- 
eastern Minnesota: Amer. Geol. 1898, XXI. 
§F. D. Adams: Uber das Norian oder Ober-Laurentian von Can- 
ada: N. Jahr. f. Min., etc.: 1893. VIII. 
UN. H. Winchell: Contributions to the mineralogy of Minnesota: 
Amer. Geol. 1899. XXIII. See also: Final Report: Geol. Nat. Hist. 
Siirv. Minn., 1900, V. 
