818 THEE JOURNAE, OF AGHOLOG Ve 
description of the volcanic rocks of the Michigamme district, 
that in this district we find well preserved evidence of true vol- 
canic activity, in the presence of amygdaloidal lava flows and 
volcanic breccias and tuffs. These are comparable in every way 
in their external appearance to the ejectamenta of Tertiary and 
recent volcanoes. Like these, also, the rocks vary from very 
basic (basalts) to those containing a high percentage of silica 
(rhyolites). 
That they have undergone more or less alteration as a result 
of their age alone was to be expected, but it is remarkable that 
in spite of their age and the dynamic action to which they must 
also have been exposed—since we now find the flows on edge— 
they should remain so fresh and retain so perfectly their char- 
acteristics as volcanic rocks. The age of these deposits has 
been determined to be Huronian. 
This adds one more to the localities in America in which the 
occurrence of associated basic and acid volcanics of pre-Cambrian 
age have been described microscopically.* 
That such pre-Cambrian volcanics are by no means rare in 
™M. E. WADSwoRTH: Notes on the Min. and Pet. of Boston and Vicinity. Proc. 
Bos. Soc., Nat. Hist., Vol. XIX., pp. 217-237. 1877. Geology of the Iron and Copper 
District. Bull. Mus. Comp. Zool., Vol. VII. (Geol. Ser. 1), No. I, p.157. 1880. 
Did not observe the felsitic porphyries interbedded with the basic rock on Keweenaw 
Point, and described later by Irving, but described the conglomerates made up of 
trachytic and rhyolitic fragments (p. 120). 
J. S. DILLER: The Felsites and their Associated Rocks North of Boston. Bull. 
Mus. Comp. Zool., Vol. VII. (Geol. Ser. 1), No. 2; and Proc. Boston Soc. Nat. Hist.,. 
Vol. XX., pp. 355-368. January 1880. 
R. D. Invinc: The Copper-bearing Rocks of Lake Superior. Mon. V., U.S. 
G.S. 1883. 
ANDREW C. Lawson: Report on the Geology of the Lake of the Woods Region.. 
Geol. and Nat. Hist. Sur. of Can. (New Ser.), Vol. 1, CC. 1885; and Geology of the 
Rainy Lake Region. Geol. Sur. of Can., 1887-8, F., p. 182. 
E. HawortTH: A contribution to the Archean Geol. of Missouri. Am. Geol.,. 
Vol. I., pp. 280-297, 363-382. 1888. 
T. G. BonNEy: Notes on a part of the Huronian Series in the Neighborhood of 
Sudbury, Can. QO. J.G.S., Vol. XLIV., pp. 32-45. 1888. 
G. H. Witiiams: In Bell: Onthe Sudbury Mining District. Geol. Sur. of 
Can., 1890-1, F. App. L, pp. 55-82. The Greenstone-schist Areas of the Menominee 
and Marquette Regions of Mich. Bull. 62, U. S. G. S., 1890. The Volcanic Rocks of 
South Mountain in Pennsylvania and Maryland. Am. Journ. of Sci. 3d Ser., Vol. 
XLIV., pp. 482-496. December 1892. 
