20 THE GEOLOGY OF MINNESOTA. 



[Granites of the Archean. 



greenstones are given in the special chapters of volume iv. These granites are of at 

 least two dates. As already stated, one series cuts the Lower Keewatin only, and 

 the other cuts the Upper Keewatin. In numerous instances have been seen two 

 series of granitic dikes, in the same rock, one series cutting the other. This occurs 

 north of Vermilion lake, and at West Seagull lake. It is, of course, not known that 

 these represent the two principal granitic epochs of the state, but it may be assumed. 



The granites are more variable than the greenstones. They vary in texture and 

 in composition; yet, as a whole, are better preserved than the greenstones. They 

 very rarely show the effects of dynamic action, while the greenstones are frequently 

 converted to schistose rocks by such action. The coarsest granites (aside from 

 pegmatitic veins) are those of Saganaga lake, in which the quartzes are sometimes 

 nearly half an inch in diameter, and are distributed evenly throughout the rock, and 

 of Ortonville, in which the feldspars, orthoclase and microcline, are porphyritic, and 

 sometimes are an inch and a half in larger dimension. A similar coarsely porphyritic 

 granite occurs in Stearns county. The granites sometimes become gneissic. This word 

 may betaken in two senses, viz.: the granite may become foliated by the occurrence of 

 divisional planes, along which are a few more mica scales than elsewhere, but which 

 do not affect the megascopic aspect, except on weathering. These divisional planes 

 are separated from two to four inches. This rock, when quarried, appears massive 

 and homogeneous. It is frequently seen at Bassimenan lake. Another form of 

 igneous gneiss is that in which the minerals and even the mass of the rock are elon- 

 gated uniformly in one direction, but in an interlocking mesh, making still a massive 

 and nearly homogeneous rock, but on quarrying, having a distinct rift in one direction. 

 These variations in the massive granites should not be confounded with the banding 

 of the true gneisses derived from the metamorphism of sedimentary beds, which 

 constitute probably seventy-five per cent of the gneissic rocks of the state. 



The chemical composition varies in such a way that the alkali present is soda, 

 as well as potash, making soda granites, or soda syenytes when the quartz is wanting. 

 Frequently, also, the Archean granites contain some of the soda-lime feldspars, and 

 they then pass into diorytes and to quartz diorytes. Rarely augite takes the place, 

 in part, of hornblende, and still more rarely fluorite is found. Commensurate with 

 these variations the rock takes different aspects, sometimes becoming quite dark 

 with hornblende and biotite, and sometimes nearly white, because of the absence or 

 scarcity of those minerals. 



Distribution of the Archean granites: Owing to the fact that the granites graduate 

 into gneisses, and that gneiss, which graduates into mica schist, may be of igneous 

 or of aqueous origin, it is a priori impossible to indicate precisely the limits of the 

 Archean granites. To this statement it should be added that no attempt has been 



