354 The American Geologist. May, isas 
Glacial ix^-iod and thereforo by necessary inference to some earlier time, 
cither preglacial or interglacial, when the absence of the ice-sheets per- 
mitted migration. 
Tht Oeoltxjicdl and NaturdI Historu Survey of Minnesota, Tiventieth 
Annual Report, for the year 1891. N. H. Winchell, State Geologist, 
Minneapolis, 1893, pp. 344. In this volume, besides the summary state- 
ment of the progress of the survey and records of additions to its mus- 
eum and library, the state geologist discusses the structure and origin 
of the crystalline rocks of northern Minnesota; U.S.Grant presents notes 
of field observations on certain granitic areas in the northeastern part 
of the state, along the Kawishiwi river and in the neighborhood of Snow- 
bank, Kekequabic, and Saganaga lakes, illustrated by a map plate and 
several figures in the text. H.V. Winchell and A. C. Lawson contribute 
the papers next reviewed in these pages; and Benjamin W. Thomas and 
Prof. Hamilton L. Smith write of fresh water diatomacese in an inter- 
glacial peat bed of Blue Earth County in southern Minnesota, giving a 
list of one hundred species, all of which, excepting one or two, are now 
found living in the great lakes of the St. Lawrence or in their tributa- 
ries. The peat occurs between deposits of till, being overlain by a thick- 
ness of twenty to thirty feet. In the till, both below and above the peat, 
careful examination reveals no diatoms, but only foraminifera, radiolaria, 
and other marine microscopic organisms, which were derived from Cre- 
taceous beds eroded by the ice-sheet. 
Professor Winchell defines the principal rock terranes of the region 
northwest of Lake Superior as follows, in descending order: 
1. The Nipigon or Keweenawan series of alternating fragmental and 
eruptive beds, which south of Lake Superior are overlain by the St. 
Croix sandstone. 
2. The upper part of the Animikie series, comprising thin-bedded 
slates, silicious and actinolitic schists, magnetitic jaspers,andquartzytes, 
interbedded with sheets of eruptive rocks and of tufaceous sediments. 
3. Irruptive gabbro, bearing large quantities of titaniferous magnet- 
ite, and sometimes enclosing considerable masses of the next older 
quartzyte strata. Intimately associated with the gabbro are also acidic 
irruptives, as red felsytes, quartz porphyries, and reddish granites. 
4. Basal Animikie beds of Pewabic quartzyte, associated with iron 
ores and cherts, including the important iron deposits of the Mesabi 
range. This formation also encloses sheets of volcanic outfiows and 
tuffs, and is often conglomeritic with debris from the underlying 
Architan I'ocks. 
All the foregoing are grouped togetJier by Prof. Winchell as the 
Taconic system, which is divided from the Archaean by unconformity and 
a great interval of erosion. The three members of the Archaean 
group or fundamental crystalline complex are together, like the four of 
the Taconic, regarded as a grand unit in their origin and history. 
5. The Keewatin volcanic formation, consisting mainly of eruptive 
rocks and a great thickness of tuffs with more or less evidence of aque- 
ous sedimentation. These rocks are mostly graywackes, sericitic schists, 
agglomerates, conglomerates, and very fine-grained serpentinous schists- 
