CURRENT PRE- CAMBRIAN LITER A TURE 5 2 1 



higher part of the series, suggest that in this area, near the contact 

 between the Keweenaw series and the Eastern sandstone, we are on 

 the edge of an early-Keweenawan or pre-Keweenawan basin. 



2. If the lower beds of the Keweenaw series near Portage Lake 

 rested on the sides of a basin, the later beds of the series from here 

 eastward lay at a higher altitude and, excepting those of the South 

 Trap Range, were eroded in pre-Potsdam time together, possibly, with 

 a part of the underlying Archean. 



3. The porphyries found on Keweenaw Point at the contact 

 between the Keweenaw series and the Potsdam sandstone may be in 

 part either, 



a. Marginal facies of the underlying Archean ; 



b. Intrusive in the early Keweenawan ; 



c. Early interbedded flows of the Keweenaw series ; or, 



d. Remnants of late Keweenawan intrusions by which the eastern 

 margin of the series was broken up and its degradation hastened. 



Seaman 1 gives a summary of the geological history of the Kewee- 

 nawan copper range in Michigan, Wisconsin, and Minnesota. No 

 new point on the geology of the region is added to those already 

 recorded. 



Hall 2 describes the pre-Cambrian crystalline rocks of the Minne- 

 sota river valley of southwestern Minnesota. These rocks appear in 

 numerous exposures along the river, protruding from the drift, from 

 southeast of New Ulm to Ortonville on the northwest. The great 

 bulk of the crystalline rocks are granites and gneisses. These appear 

 for the most part in the river bottoms, but stand also in a few isolated 

 knobs on the higher ground south and west of the river. There are 

 many varieties of granites and gneisses and all gradations between 

 them. They are taken as a whole to represent the Archean or Base- 

 ment complex. 



Associated with the granites and gneisses are a.much smaller number 

 of exposures of gabbros and gabbro-schists. These present many 

 varieties, all of which are believed to have resulted from the alteration 

 of two original forms and their intergradations — a hypersthene- 

 bearing gabbro and a hypersthene-free gabbro. 



1 Geology of the Mineral Range, by A. E. Seaman: First Ann. Rept. of the 

 Copper Mining Industry of Lake Superior, 1899, pp. 49-60. 



2 The Gneisses, Gabbro-Schists and Associated Rocks of Southwestern Minne- 

 sota, by C. W. Hall : Bull. U. S. Geol. Surv., No. 157, 1899, pp. 131. With geolog- 

 ical maps. 



