386 The American Geologist. ueceiubcr. 1901 
with the course of investigation of the geology of the Lake Su- 
perior region. 
"The loitgoing fapcrs* render it unnecessary for me here to take 
up the general stratigraphy of the lake Superior region. However, our 
worl; nortli of hike Superior, in northeastern Minnesota and Canada, 
has upon two points modified our pubHshed conclusions as to succes- 
sion and correlation. Those who compare this paper with other pa- 
pers will note two important modifications. First, the Archean has 
heretofore been supposed to be composed wholly of igneous rocks, no 
sediments ha\ing been recognized in this divsion of the pre-Cambrian. 
The north-shore work, however, makes it very probable that certain 
of the sedimentp.ry iron-bearing formations must be included in the 
Archean. As examples of such are the productive iron formatiofi? of 
the Vermilion and Michipicotcn districts. Thi's modification is im- 
portant from a theoretical point of view, since it will make changes nec- 
essary in my general definition of the Archean and of the Algonkian. 
The Algonkian has been defined to include all pre-Cambrian sediment- 
ary rocks. The Archean has been defined to include all pre-Algonkian 
rocks, and has been supposed to contain igneous rocks only. These 
definitions must be modified so as to include in the Algonkian all pre- 
Cambrian series which are dominantly of sedimentary origin or ec|uiv- 
alent in age with those which are dominantly of sedimentary origin. 
Tho Archean must be defined to comprise the rocks older than the 
Algonkian which are dominantly of igneous origin, but which may in- 
clude subordinate amounts of sediments. Recent work in northwest- 
ern Europe, and especially in Scotland. Scandinavia and Finland, where 
the ancient rocks are best exposed in Europe, shows that these modi- 
fications in the definitions of the Archean and Algonkian are also there 
applicable. The changes are quite in line with what might be expected, 
for in recent years no one feature in geological advance has been more 
significant that the sweeping away of sharp dividing lines between the 
various periods. 
"Second, the iron-bearing formations of the Vermilion and similar 
districts I have heretofore regarded as Lower Huronian. In placing: 
these formations in the Archean I recognize three series in which pro- 
ductive ore formations are found, the Upper Huronian, the Lower Hur- 
onian, and the Archean. 
"The evidence upon which these modifications of my opinion con- 
cerning the Lake Superior stratigraphy are based cannot be here pre- 
sented in detail. It will be fully given in a monograph on the Vermil- 
ion district to be published later. In general it may, however, be stated 
that our work in the Vermilion district of Minnesota and on the Canad- 
ian side of the international boundary has convinced us that bands of 
sedimentary iron-bearing formation are interstratified with the upper 
part of the oldest series of the Lake Supeirior region, composed of 
greenstones, greenstone schists and tuffs, although the thick productive 
beds of the Archean appear to rest upon the greenstones and green- 
stone schists." ******* 
* A list of the publications of the United States Geological Survey respeet- 
the formations and iron ores ot lake Superior, [n. h. w.] 
