18 THE GEOLOGY OF MINNESOTA. 



[Nature of the Archean igneous rucks. 



round which, to some extent, the later strata tended to be warped within narrow 

 limits. 



This later folding was accompanied by metamorphism. The Upper Keewatin 

 is converted to a micaceous rock about Snowbank and Moose lakes, and the Lower 

 Keewatin (?) to a muscovadyte about the southern confines of Disappointment lake. 

 These rocks are so similar to those produced by metamorphism of the Lower Keewatin 

 that on petrographic grounds they cannot be distinguished. Still, it is plain that 

 the rocks of the Upper Keewatin, in all known instances of such metamorphism, 

 retain more of their clastic characters than do those of the Lower Keewatin. 



[NOTE. There is an element of uncertainty in the assignment of rocks about Moose lake which contain 

 the jaspilyte already noted to the Upper Keewatin : and this uncertainty is to be extended to the mica schists 

 about Snowbank lake and the muscovadyte about Disappointment lake. The difference in metamorphism and 

 the conglomeratic composition are features that might still inhere in the Lower Keewatin, and the general 

 geographic position of these lakes rather favors the Lower Keewatin age of the surrounding schists and the 

 jaspilytes. Their strongest Upper Keewatin character consists in the occurrence of pebbles of older jaspilyte and 

 of granite in this conglomerate. This implies that some older terrane, probably the Lower Keewatin, had been 

 formed, folded and cut by granite prior to the creation of these strata, for we know of no granite nor jaspilyte 

 older than the Lower Keewatin.] 



THE IGNEOUS ROCKS OF THE ARCHEAN. 



Nature of the Archean igneous rocks. The igneous rocks of the Archean consist 

 of two grand divisions, viz.: the massive greenstones and the granites, of which the 

 greenstones are the older. With this grand distinction recognized, it is still true 

 that there are a few intermediate rocks, but these are so small in amount that they 

 are not worthy of class designation, and are so doubtful as to genesis that it is best 

 not to include them in a fundamental classification. They will, however, be treated 

 in a separate paragraph. There are also numerous dikes which are both granitic 

 and diabasic. The former are included in the Archean, but of the diabasic dikes a 

 considerable number are of later, probably Keweenawan, age. 



The Archean (jrci-n Atones show by their state of alteration that they are very old. 

 Their original crystalline characters are much obscured, and sometimes quite oblit- 

 erated. They are intimately associated with fragmental rocks that consist almost 

 wholly of the same minerals as the greenstones, and these fragmental rocks have 

 undergone such pressure and consolidation that in some instances they are very similar 

 to the crushed massive greenstones, and the whole mass has frequently been classed 

 as a unit under the designation Kawishiwin, a name derived from the Kawishiwi 

 river, on which these rocks are well exposed. The petrographic characters of these 

 rocks are given in another chapter, and their outward characters in the special 

 chapters descriptive of the geological plates in volume iv. 



That these rocks are older than the oldest known granite is shown by the 

 apophyses from the granite which penetrate them. Such are to be seen wherever 

 the two rocks approach each other, but may be specially mentioned at the northwest 



