16 THE GEOLOGY OF MINNESOTA. 



[Upper Keewatin. 



region of Rainy lake and the north shores of Vermilion lake. The granites of more 

 southern localities, such as that at Ortonville, at St. Cloud and at Sauk Rapids and 

 in the eastern part of Morrison county, are believed to belong to the same category, 

 because of their association with large areas of highly altered and wholly recrystal- 

 lized schists that bear the characters of altered northern Lower Keewatin schists. 

 This of course is a character which is very unreliable for such an inference, but it is 

 the only guide to the age of these southern granites which can be invoked. 



Marble is also one of the constituents of the Lower Keewatin. It is seen at 

 Ogishke Muncie lake, extending for about four miles conformably with the general 

 structure. From this fact, and from the occurrence of microscopic debris of quartz, 

 feldspar and green hornblende in this marble, it is believed to date from the origin 

 of the rock, and was an oceanic chemical precipitate. A similar marble occurs in 

 the staurolite mica schists at Pike rapids, on the Mississippi. 



('jtj)cr Kci'ir<itht. The rocks which accumulated in the troughs after the folding 

 of the Lower Keewatin consist very largely of conglomerates, but they also include 

 graywackes, quartzytes, argillytes and jaspilytes. Jaspilyte, of the type seen in the 

 Lower Keewatin, is the matrix of the conglomerate on the north side of Moose lake. 

 Tn the same conglomerate are pebbles of a jaspilyte of an earlier date, and of granite 

 and flint. In favorable locations, as at Ogishke Muncie lake, where this conglom- 

 erate lies on the Lower Keewatin, it also contains many boulder masses of diabase 

 and of green and slaty rock similar to some seen in the Lower Keewatin. About 

 Vermilion lake it embraces the Stuntz conglomerate, which is composed largely of 

 pebbles of quartz-porphyry, whose source is probably from dikes of that rock cutting 

 the Lower Keewatin in the near vicinity. At Tower this conglomerate undergoes 

 sudden local variations. It is in some places composed largely of fine green schist, 

 which gradually acquires fine clastic grains and pebbles of jaspilyte (figure 2, plate 

 WW, vol. iv). In the near vicinity this schist is seen to surround larger and larger 

 masses of jaspilyte which attain the dimensions of ten or twenty feet across; and it 

 is very difficult, in such conditions, to distinguish this schist from the green schist 

 underlying from which, as a debris, it has been obtained; and this difficulty is 

 increased by the common folding, pressure and shearing to which they have both 

 been subjected. Beautiful illustrations of this schist containing pebbles and large 

 masses of jaspilyte are to be seen in the "South ridge" at a short distance from the 

 city of Tower, while the original schist, containing the jaspilyte in its native places, 

 can be seen conspicuously in the same ridge, and most prominently in the " North 

 ridge." There is no known general order of succession in the composition of the 

 Upper Keewatin. It can only be said that it is likely to be conglomeratic at the 

 bottom; still that is not always the case, for at Saganaga lake the bottom of the 



