370 C. W. HALL — KEEWATIN OF EASTERN AND CENTRAL MINNESOTA 



graywackes at Thomson, examination led him and J. W. Dawson to the 

 conclusion that the}^ must be evidence of keratose sponges.* That de- 

 cision assio;ned the rocks containing them to an early Paleozoic terrane. 

 The geologists of the northwest for more than 20 years have held to 

 the Huronian age of these rocks, using the term Huronian in its broad 

 sense. This assi<2;nnient has had as the strongest argument urged in its 

 favor a general litliologic resemblance, reinforced by geographic situa- 

 tion. Spurr writes one paragraph in the history of correlation so well 

 that he may be quoted in part : 



"In the Third Annual Report of the United States Geological Survey, Irving t 

 first hinted at the correlation of the 'Saint Louis slates' with the Aniniikie of 

 northeastern Minnesota, as observed at that time around Giviiflint lake and 

 Thunder bay. He pointed out the general lithological resemblance between the 

 two series and noted the difference in that the ' Saint Louis slatps ' are cleaved. 

 In the same report, however, he J suggested the correlation of the uncleaved 

 Animikie slates with the folded schists lying further north, and his descriptions 

 and accompanying diagram clearly show that he included among these schists the 

 larger part, if not the whole, of what we now know as Keewatin (Lower Huro- 

 nian). In the Fifth Annual Report^ he first confidently assigned to the Thomson 

 (Saint Louis) series a place equivalent to that of the Animikie. ... In the 

 Seventh Annual Report || he again refers to the Saint Louis slates as Animikie, 

 and here first hints as to what horizon of the Huronian they were believed by 

 him to belong — that is, the same as that of the upper slates of the Animikie series 

 as represented by the Mesabi range." T[ 



In the pa[)er referred to (page 163) Spurr noted in the Virginia area 

 of the Mesabi range a transition from the holocrystalline mica and horn- 

 blende schists, so ])ronounced in their habit in contact with the granites, 

 to easily recognizable sedimentary and only slightly altered silicious and 

 clay slates and graywackes at the most southward lying points where 

 exposed, which are also most distant from the granite contact. 



These Keewatin rocks, Spurr adds,'^^* " possess a strongly marked re- 

 gional cleavage or schistosity not far from vertical," trending north 70 

 degrees east. This east-northeast to west-southwest direction, with gen- 

 erally southward dip, it may here be emphasized, is a very common 

 attitude of the Keewatin schists and slates from the gold-mining district 

 north of Rainy lake to the southernmost exposures now known on the 

 Snake river of east-central Minnesota, an air-line breadth of 200 miles. 



* Transactions Royal Soc. Canada, vol. i, ser. iv, p. 250. 



t Roland Duer Irving : Copper-bearing rocks of lake Superior. Monograph v, U. S. Geol. Survey, 

 1883, p. 102. 

 X Op. cit., p. 170. 



g Archean Formations of the Northwestern States, p. I'JG. 

 il Classifieation of Cambrian Formations, p. 422. 

 1[ Amer. Jour. Sci., vol. 148, pp. IGO, IGl. 

 ** Ibid., p. 1G3. 



