28 THE GEOLOGY OF MINNESOTA. 



[Origin of the Archean granites. 



It has been presumed by Hunt that by a long period of lixiviation by circu- 

 lating waters the elements of the granite, as well as of all the other later crystalline 

 rocks, were extracted from the early doleritic crust and brought to the surface and 

 deposited in such form that they constituted, on consolidation, the rocks under 

 consideration. By an extended series of chemical and mineralogical examinations 

 he hnds that all the minerals of the granite, etc., are derivable from the early ferro- 

 maguesian crust, by the long-continued forces which he advocates. Recent researches 

 by Michel Levy also indicate that, starting with the ferro-magnesian scoria or outer 

 magma of the earth, represented by some such rock as basalt, there may have been 

 a profound transformation effected in that basic magma, in deep-seated reservoirs, 

 through the action of mineralizing waters circulating through the surrounding rock 

 walls. The end products of this transformation are the alkaline magmas which 

 produce the granites, etc., while intermediate stages in the transformation give rise 

 to igneous magmas of intermediate characters. Michel Levy also allows for extensive 

 other alteration of the igneous magmas by endomorphous metamorphism. /'. <.. by the 

 fusion and incorporation of material from the walls of the reservoirs and from the 

 minor openings in the clastic rocks through which the molten masses may pass in 

 their way to the surface. 



Against such supposed change from a ferro-magnesian magma to an alkaline one 

 there rises, however, an important obstacle, which appears to be insurmountable. It 

 consists in the fundamental profound chemical difference that exists between these 

 two magmas. It may be admitted, for the moment, that if a process of concentration 

 of certain of the elements of the ferro-magnesian magma be long enough continued, 

 accompanied by a slow extraction of other elements, there would be produced 

 two magmas of contrasted characters. These two rocks, the end products of the 

 supposed differentiation by whatever process, whether at the surface or in deep reser- 

 voirs, would contain all the elements of the original magma, and no others. If other 

 elements are found they must be credited to some foreign source. If it be found 

 again that the actual proportions existing between the elements in the derived 

 rock be those that they could not possibly be supplied by the original magma, such 

 excess of some of the elements must also be attributed to some foreign source. 

 Allowing for these considerations, it appears at once that, while there is no potassium 

 in the ferro-magnesian magma, it is the dominating base in the alkaline magma. It 

 must hence have been derived from an extraneous source. It also appears that the 

 amount of silicium present in the derived rock in proportion to that which could be 

 supplied cotemporaueously with the needed quantity of the other elements, such as 

 lime and alumina, is far too great. It is not necessary to enter into extensive or 

 minute comparisons. The single fact that potash could not be supplied by the ferro- 



