AGE OF THE SERIES 371 



Noting the characters of the less altered phases of the Virginia Kee- 

 watin, Spurr, on lithologic grounds, correlates these with the Cloquet 

 rocks because almost every phase can be duplicated, the only difference 

 being the presence in the latter of a minor transverse cleavage, while the 

 resemblance of the Stony Brook exposures in section 27, township 51, 

 range 19 west, and the Mesabi graywackes is complete* Touching the 

 staurolitic schists along the Mississippi river, usually regarded as the 

 Thomson series changed by becoming crystalline, Spurr adds that they 

 correspond exactly to, the green schists and crystalline schists of the- 

 Mesabi district. 



Turning to dynamic characteristics, Spurr says that one of the greatest 

 differences between the least altered Keewatin near Virginia and the 

 Mesabi (Animikie) slates is the steeply dipping cleavage in the former. 

 This cleavage is for northeastern Minnesota a distinctly pre-Animikie 

 character. It is seen in many localities within the Keewatin between 

 Saganaga lake and lake Vermilion, and is a stronglj^- imprinted character 

 at Virginia, Stony Brook (lying 40 miles south), Cloquet, Carlton, and 

 Thomson, and eastward into Duluth. 



On the foregoing lithologic and structural grounds Spurr correlates 

 the rocks around Carlton and Thomson with the Lower Huronian rather 

 than with the Upper. With this correlation every geologist who, within 

 the knowledge of the writer, has subsequently worked in this region has 

 come into general accord. 



THE PRESENT VIEW 



The studies of recent years, as set forth in the foregoing summary of 

 petrographic characters, have led the writer to the conviction that the 

 basal rocks of the district described all belong to a single unit of geologic 

 time. This unit or period was terminated by a series of volcanic dis- 

 turbances resulting in extensive accumulations of granite in the Missis- 

 sippi River region, a large number of granitic dikes in the district 

 crossed by the Ram and Snake rivers, and a complete metamorphism 

 of the vast series of silicious elastics along the Kettle River valley. The 

 petrographic characters of the sedimentaries have been thereby so 

 changed that no positive recognition of their clastic character is to be 

 seen until Mahtowa and Carlton are approached as one traverses the 

 state from the Mississippi river toward Duluth. 



It has been shown in the foregoing discussion that the rocks exhibit 

 for some miles, in a succession of stages which can be followed step by 

 step from Thomson southward, the graded alteration of coarse and fine 

 graywackes into sharply crystalline hornblende and hornblende-biotite 

 schists to the west of Sturgeon lake. 



*Loc. cit., p. 165. 



