STRUCTURAL GEOLOGY. 27 



Origin of the Archean granites.] 



It is a remarkable fact that, although several geologists have assumed a primor- 

 dial ferro-magnesian rock as the earliest condition of solidified matter on the cooling 

 globe, not one has been able to indicate a spot where it can be seen, but have assumed 

 that the oldest known rock is a granitic or alkaline one. Button originally pointed 

 out that this granite is younger than the strata that lie upon it, and Lawson has 

 shown* conclusively that it breaks through the mica schists and the fragmental 

 rocks of the Keewatin. As already fully stated, this is also the case in Minnesota, 

 and it follows that the oldest rock at the surface cannot be these eruptive granites, 

 whatever may have been their origin. 



Those who have followed this chapter from the beginning will have learned 

 that this granite is also more recent than that great greenstone series which plays 

 so important a part in the geology of the Lake Superior region. They will also have 

 noted that between the date of origin of the massive greenstones and the granitic intru- 

 sion there was formed an important series of detrital rocks, and that these detrital 

 rocks, very largely composed of volcanic ash and to some extent of oceanic precipitates, 

 such as silica, oxide of iron and lime, have been metamorphosed into the mica schists 

 and gneisses which are cut by these granites. In other words, between the origina- 

 tion of the greenstones and the intrusion of the granite there was an immense lapse of 

 time, which was characterized by a widespread, if not by a universal, scene of vul- 

 canian activity.' These volcanoes were, as indicated by the generally stratiform 

 condition of these fragmentals, closely surrounded by the primeval ocean into which 

 their ejectamenta fell,+ and its erosion products must have mingled with the volcanic 

 to constitute the cotemporary strata. Such strata would necessarily sometimes be 

 acid and alkaline, and sometimes irony and magnesian, and would have embraced 

 any oceanic chemical precipitates. 



Origin of flip. Airhean r/rf mites. Recent careful studies of the Archean granites 

 have demonstrated several important facts respecting their nature and structural 

 relations, showing their intimate connection with the gneisses, and thence with the 

 mica schists. Before, however, mentioning these bonds of affinity, it will be best to 

 refer briefly to some ideas that have lately been promulgated concerning their origin. 



Relying on the result reached theoretically by numerous petrographers and upon 

 the observed fact that, in Minnesota, the "greenstones" are the oldest of the Archeau 

 rocks, we are at once brought to the consideration of the question of the possible 

 derivation of the granites from the greenstones by some process of alteration or 

 differentiation. 



* Geology of the Lake of the Woods. G< ni:,<ii<->,i Survey <.' ',//,,/,/, 1886. 



fThe Kawishiwin agglomerate at Ely, Minn. Aiin-ri-nn (,' w<i ; //v. vol. ix, pp. .'i.W-368, 18ffi. In this paper, and in others 

 written in the course of the prosecution of the geological survey, the greenstones were erroneously assumed to have been later 

 than the granites and gneisses, following the order established by the Canadian Geological Survey. 



