990 THE GEOLOGY OP MINNESOTA. 



[The oldest known rocks. 



While, therefore, in the region northwest of lake Superior we have in the 

 Archean an enormous amount of acid igneous rock, as in Canada, it is found to be 

 of at least two epochs, and in each case to be the cause of, or at least cotemporary 

 with, a profound and extensive metamorphism of the earlier acid clastic rocks. It 

 is not chronologically a part of those elastics, but as a rock mass it is later. 



It is to be noticed that there are three epochs of greenstone, viz.: (1) The 

 igneous basal mass which is believed to be the representative of the first crust of the 

 earth. (2) The agglomeratic and fragmental greenstone which preceded and accom- 

 panied the first quartz-porphyry. This contains the jaspilyte iron ores at Town. 

 Volcanic action added largely to this greenstone. (3) The greenstone of the Upper 

 Keewatin. This is usually distinctly clastic, and is also in some part of volcanic 

 ejecta. 



The writer has elsewhere shown* that, as a mass, these greenstones are pre- 

 dominantly of fragmental structure and of volcanic origin, but also embrace truly 

 massive rocks. The term Kawishiwin, used by the Minnesota Survey, embraces the 

 first two above, being those of the Lower Keewatin. 



3. The nature of the greensand that produced the iron ores of the Mesabi Iron 

 range. It has already been stated that the original greensand of the Mesabi Iron 

 range, the source of the hematite now extensively wrought in northern Minnesota, 

 was probably not derived from organic agencies. It was the suggestion of Dr. J. E. 

 Wolff, adopted by Mr. Spurr, that this substance is glauconite and hence might have 

 been the product of foraminiferal organisms (Bulletin x) and that suggestion has 

 generally been accepted, at least by the officers of the Minnesota Survey. It was 

 through the careful inductive research of Mr. Spurr that the ultimate source of the 

 ore was found in this greensand. 



In the course of the microscopical work embodied in this volume several 

 discoveries have been made which, at first disregarded or otherwise explained, have 

 led to the abandonment of the idea that foraminiferal organisms were responsible 

 for the greensand. When these new facts are adjusted with others that were known 

 before, but which, appearing anomalous, were disregarded or otherwise explained, 

 the greensand appears to have resulted from a volcanic sand, and the taconyte rock 

 itself, both the granular and the massive, from igneous forces. 



This conclusion is as much a surprise to the writer as it can be to any one, since 

 he has formerly considered the origination of the hematite of the Mesabi range 

 definitively traced to an organic origin. 



Attention may first be directed to some facts, in part mentioned by Mr. Spurr, 

 which unite with others since noted, to compel to the new view. 



* The origin of the Archean Greenstones, Twenty-third Annual Report, Minnesota Survey, pp. 4-35, 1894. 



