Spurr — Stratigraphio Position of the Thomson Slates. 159 



Art. XXII. — The Stratigraphio Position of the Thomson* 

 Slates • by J. E. Spurr. f 



Location and Extent of the Series. 



The Thomson slates (St. Louis slates, Cloquet slates) are 

 schistose or slaty rocks which occupy an extensive area in 

 Eastern Minnesota. They afford continuous exposures along 

 the St. Louis River, not far from Duluth, near Thomson, 

 Carlton and Cloquet. Here they were first noted and 

 described. They are in this region well cleaved, but possess 

 evident and little-altered sedimentary characters. Further 

 west they are found along and beyond the Mississippi River, 

 in the vicinity of Little Falls. Here they have become schis- 

 tose ; they are often micaceous, staurolitic, hornblendic, or 

 garnetiferous. Frequent outcrops have been found, demon- 

 strating the continuity of the series between the St. Louis and 

 the Mississippi. In a north and south line, the belt has already 

 been shown, by the explorations of C. W. Hall, Merriam, 

 Upham, and others, to extend, roughly speaking, from near the 

 lower part of Mille Lacs to near the northern boundaries of 

 Aitkin and Carlton counties. The writer has been able to 

 trace these rocks as far north as T. 51-19, where an exposure 

 is found which will be referred to later. This prolongs the 

 known extent toward the north by 10 or 15 miles. The 

 dimensions of the belt are thus about 100 miles east and west 

 by 50 north and south, making an area of not far from 5000 

 square miles. 



For the lithologicalvariations from the simple cleaved slates, 

 found within this area, we may quote from Irving :;{: 



" They include, among the slates, fine-grained gray wacke- 

 slates, clay-slates, sericitic quartz-slates, true quartzites, mica- 

 slates (often hornblendic), staurolitic mica- slates (often garnet- 

 iferous), and hornblende schists ; and among the eruptives, 

 diabases, gabbros, and diorites, the latter presumably altered 

 forms of diabase or gabbro." 



Relations of the Thomson Series to the surrounding Hocks. 



On three sides, south, east, and west, the general boundaries 

 of the Thomson rocks are known. To the south and west lie 



*To this series the name St. Louis has been applied by Irving, Sweet, Tan 

 Hise, and others. Inasmuch as this name, however, is already well established 

 for a formation of quite different age, it has been thought best, in accordance 

 with a suggestion of Professor H. S. Williams, to designate the Minnesota series 

 in some other way. The term Thomson, therefore, which has already been used 

 by Professor N". H. Winchell, has been adopted. 



f Published by permission of the State Geologist of Minnesota. 



% Fifth Annual Report IT. S. Geol. Survey, p. 197. 



Am. Jour. Sci.— Third Series, Vol. XLVIII, No. 284.— August, 1894. 

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