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SCmNCK 



[N. S. Vol. VIII. No. 198. 



tids?); and corals allied to Archseocyathus 

 and Dictyocyathus. The thin limestones 

 ■which occur in the upper half of the terrane 

 are supposed to have originated chiefly 

 from foraminifera (Globigerina, etc.). 



20. The Oldest Knoton Rock. By Profes- 

 sor N. H. WiNCHELL, Minneapolis, Minn. 

 With a brief description of the other mem- 

 bers of the Archean series as made out by 

 the Geological Survey of Minnesota, this 

 paper more particularly described the so- 

 called greenstones of this State, which the 

 author considers the bottom of the Archean 

 scale and the representative of the original 

 crust of the earth formed from the molten 

 mass by the earliest consolidation. The 

 greenstones are divisible into two parts, 

 one igneous and the other clastic, the latter 

 succeeding the former with a confused and 

 apparently sometimes non-conformable su- 

 perposition, somewhat as surface eruptive 

 rocks might be superposed, in the presence 

 of oceanic action, upon a massive of the 

 same nature at the same place. The clas- 

 tic portions of the greenstones vary to more 

 silicious rocks, constituting great thickness- 

 es of graywackes, phyllites and conglomer- 

 ates ; and as such they have been converted 

 by widespread metamorphism into mica 

 schists and gneisses, the alteration com- 

 ing on by degrees, increasing in intensity 

 toward centers of granitic intrusion and 

 toward the great areas of granite and igne- 

 ous gneiss. 



Such granite and such metamorphic rocks, 

 as a whole, have been considered the base- 

 ment rock, the oldest known rocks of the 

 country. But, following up the long known 

 fact that the Laurentian granite and igneous 

 gneisses cut the schists and sedimentary 

 gneisses and hence are younger, they are 

 thus shown to be younger also than the 

 bottom greenstones. They occasionally 

 penetrate these greenstones and change 

 them to amphibolj'te and pyroxene gneiss. 

 These metamorphic schists and gneisses 



seem to be a representative of the sedimen- 

 tary portion of the Lower Laurentian of 

 Canada, while the igneous granite and 

 gneisses are as plainly a general parallel of 

 the igneous portion of that series. It fol- 

 lows, therefore, that the Canadian Lauren- 

 tian is, as a whole, of later date than the 

 greenstones, if the succession is the same as 

 in the Northwest, and that the greenstones 

 should be considered the bottom rock of the 

 geological scale. 



21. The Origin of the Archean Igneous 

 Rocks. By N. H. Winchell. The green- 

 stones, which were discussed in the preced- 

 ing paper, are supposed to represent the 

 primeval crust of the earth. The author 

 denies the possibility of the derivation of 

 the alkaline magma from these ferro-mag- 

 nesian rocks by any of the methods of 

 lixiviation or of differentiation which are 

 currently proposed by geologists who have 

 lately discussed the origin of the igneous 

 rocks. Accepting this ferro-magnesian 

 crust as the rock of the first magma of the 

 earth, the author shows that it could not 

 give rise to minerals consisting largely of a 

 potash base, such as orthoclase and micro- 

 cline, which constitute the greatest dis- 

 tinguishing element in the alkaline magma. 

 Neither could it give rise to the preponder- 

 ating percentage of silica which accompanies 

 the potash minerals. These characters, 

 therefore, must have some other source. 

 The potash is believed to have existed in 

 the ocean itself which immediately followed 

 the consolidation of the first crust. Such 

 an alkaline ocean, especially if heated, 

 would hold in solution much silica. Hence 

 followed the precipitation of alkaline sili- 

 cates, and of excess of silica ; hence, also, 

 the alkaline character of the schists and 

 gneisses when its sediments were formed 

 into rock and metamorphosed ; and hence,' 

 when fused, the alkaline magma. As to 

 the cause of this potassic ocean by which 

 the great stock of the world's potash was 



