b THE GEOLOGY OF MINNESOTA. 



[Origin of the fragmental rocks. 



by its lacking the ferro-magnesian minerals.* One of the most remarkable phases 

 of the coarser forms of the Archean fragmentals is to be seen at Zeta lake, which lies 

 between Kekequabic and Ogishke Muncie lakes. The shores of this lake are almost 

 wholly composed of a conglomerate in which appear conspicuous feldspar crystals, 

 evidently also of fragmental origin, giving the rock the aspect of a porphyry. A 

 similar composition is presented in the hills made of this conglomerate eastward 

 from Moose lake (No. 2170), but here, in addition to crystals of feldspar, this conglom- 

 erate contains also fragmental crystals of hornblende (No. 2171). In numerous 

 instances it has been observed that beds plainly of detrital origin alternate with 

 those which contain such crystals of feldspar, indicating the proximity of volcanic 

 vents and occasional explosive extrusion, accompanied by rapid oceanic assortment 

 and stratification. 



Origin of the fragmented rocks of the Archean. If we be allowed to infer the 

 origin of these fragmental rocks from their nature, as given above, we are reduced 

 to two methods of origin, viz.: volcanic ejection and ordinary erosion. Several 

 geologists have already alluded to the probable volcanic source of much of the 

 material which makes up the Keewatin of Canada and of Minnesota. We find that 

 the Keewatin is separable into two parts, and that the volcanic materials are most 

 abundant in the earlier or Lower Keewatin, while in the later Keewatin not only is 

 there much less of distinctly volcanic debris,, but also more of evident erosion and 

 sedimentation. 



The oldest detrital rocks are abundantly mingled with volcanic tuff of a basic 

 nature. Indeed the fragmental greenstones of the Keewatin are so intimately 

 associated with the massive greenstones that they cannot always be distinguished 

 from them, whether in the field or in the microscopic thin section. Starting from 

 the plainly igneous rocks, the characters gradually change by the loss of one feature 

 after another and the acquirement of slightly different features, until finally the 

 whole petrographic nature of the rock at one end of the series is so altered that it 

 is not warrantable to class the rocks at both ends of the series in the same category. 

 At one end of the series the rock considered is plainly an igneous one, and at the 

 other it is plainly a fragmental one, and it is only by the most minute and pains- 

 taking comparisons that some of the steps in the series can be assigned to this or 

 the other end of the scale. Without stopping at this place to specify these minute 

 differences, or to indicate which characters are distinctly igneous and which are 

 clastic, it is intended only to call attention to the significance of such an indefinite 

 and gradual transition. 



* The rock over which the short portage passes, from Saganaga lake to Oak lake, is a part of this conglomerate, but so 

 closely resembles the original granite of Saganaga lake that several geologists have reported it as a part of the Saganaga granite. 



