194 C K. LEITH 



succession is the same as in the Northwest, and that the greenstones 

 should be considered the bottom rock of the geological scale. 



Winched 1 attempts to explain the origin of the Archean igneous 

 rocks. From field evidence and petrographic discriminations and 

 associations, it is believed that the alkaline magma from which the 

 igneous rocks were derived is the result of aqueo-igneous fusion of the 

 fragmentals of the Archean itself ; that when deeply buried, under 

 heat and pressure, the Archean elastics were rendered plastic, penetrat- 

 ing openings in the adjacent and superjacent strata; and that when 

 the plastic mass was not moved from its place it was simply recrystal- 

 lized in situ. 



The clastic rocks must have been derived from the basal greenstone, 

 which is considered representative of the original crust of the earth. 

 The presence in such elastics of sufficient potassa and silica to yield 

 upon fusion the granitic magmas is explained on the hypothesis that 

 they must have come from the waters depositing the fragmentals, and 

 primarily from the atmosphere, in its condition normal to the Archean 

 age, just following the congealing of the first crust. 



While numerous instances of such transition from clastic to igneous 

 rock have been noted in Minnesota, there has been a careful study of 

 but one. That was the case of the granite and porphyry which intrude 

 the elastics at Kekequabic Lake. 



Winched 2 presents some additional poin.s on the geology of north- 

 eastern Minnesota. 



The Laurentian includes, in Minnesota, an acid crystalline schist 

 of sedimentary origin, and a massive igneous rock, although the igne- 

 ous rock is younger than the crystalline schist portion, and should 

 have a different designation. The conclusions reached are that: (i) 

 the sedimentary Laurentian is a crystalline condition of sedimentary 

 strata, which are conformably a portion of the sedimentary schists ; (2) 

 the igneous Laurentian is the result of a more intense metamorphism, car- 

 ried even to fusion of some strata. These conclusionsresult particularly 

 from the study of a section from Tower northward, through Vermilion 



1 The origin of the Archean igneous rocks, by N. H. Winchell : Proc. Am. Assoc. 

 Adv. Sci., Vol. XLVII, 1898, pp. 303, 304 (Abstract). Also Am. Geol., Vol. XXII, 

 1898, pp. 299-310. 



2 Some new features in the geology of northeastern Minnesota, by N. H. WlN- 

 CHELL : Am. Geol., Vol. XX, 1897, pp. 41-51. 



