372 C. ^V. HALL — KEEWATIN OF EASTERN AND CENTRAL MINNP:SOTA 



The observations of Spurr and his interpretation of them are accepted 

 by the writer, inasmuch as the writer's own studies lead to the conchi- 

 sion that the rocks from Cloquet to the exposures west of Sturgeon lake 

 belong to the same series, which series, in the nomenclature of this paper, 

 is designated Keewatin. 



The schists, still followed for miles southwestward from Sturgeon lake, 

 present no further variation than an intenser metamorphism would bring 

 about, namely, the occurrence of a coarser texture, the introduction of 

 quartz veins and lenses, and the presence of garnets, both minerals giv- 

 ing proof of great alteration, and the latter particularly an index of 

 contact metamorphism, as in the Crystal Falls iron-bearing region,* and 

 " due to the reactions between solutions passing between the intruded 

 and intruding rocks and carr3dng dissolved salts from the one into the 

 other.'' t In many localities, literature shows, garnets are a frequent 

 contact mineral, and their presence and distribution are a guide in 

 structural problems. 



It is recognized that southwest of Sturgeon lake step-by-step deter- 

 mination of the rock- and time-continuity, so clearl}'- traced from Stony 

 brook to that point, can not be followed, owing to the covering of glacial 

 drift, which leaves only occasional exposures in view. The rock rela- 

 tionship of these isolated exposures must for the present be a matter of 

 opinion rather than actual demonstration. The o[)inion of the writer is 

 that westward from Sturgeon lake to the Mississippi river and beyond 

 the rocks are a continuation of the same Keewatin schists as occur in 

 the Saint Louis and Kettle River valleys, broken, displaced, folded, and 

 altered by crustal movements and the intrusion of the dikes, bosses, 

 and laccolites of hornblende and hornblende-biotite granites. These 

 granites have graduall}^ replaced the schists, until in Benton, Sherburne, 

 and Stearns counties not an exi)osure of the schists has yet been reported. 

 Profile II, plate 30, shows this displacement. 



This opinion is hesitatingly put forth, although it has been held by 

 the writer for several years. Other discoveries are probable, and other 

 rock formations may be brought to light. Archean knobs may be found 

 protruding through the complex here assigned to the Keewatin. The 

 local reasons for this assignment have been sufficiently dwelt upon, if 

 not witli convincing clearness or array of proof Within any geologic 

 region there are certain rock associations and relationships that, once 

 recognized, are usually reliable. The observations in central and eastern 

 Minnesota, on which the age relationships are herein based, are rein- 



*Clements, Smj'th, Bailey, and Van Hise : The Crystal Falls iron-be. ning district of Michigan, 

 etcetera. Monograph xxxvi, U. S. Geol. Survey, 1899, p. 415. 



fVan Hise, Bagley, and Smytii : Marquette iron-bearing district of Michigan. Monograph 

 xxviii, U. S. Geol. Survey, 1897, p. 514. 



