PLATE LXXXVI. 



THE VERMILION LAKE PLATE, 1899. N. H. WINCHELL. 



At Tower and along the south shore of Vermilion lake the bottom member of 

 the Upper Keewatin, known as the Stuntz conglomerate, is well exposed. This con- 

 glomerate assumes very diverse characters. In much of the "south ridge" it con- 

 tains many large jaspilyte masses, twenty, forty or fifty feet in diameter, and these 

 are separated by scant green beds of schistose rock which present the aspect of dikes, 

 but are, instead, the green sediment that was washed into the openings between 

 these large masses. At other points near adjacent this green schist embraces many 

 siliceous grains which are mainly composed of fine, detrital jaspilyte, and at still 

 others it becomes a well banded graywacke. These fragmental jaspilyte masses 

 were derived from the original jaspilyte of the Lower Keewatin, which is found at 

 the west end of the "south ridge" and in the "north ridge." In many places this 

 conglomerate is also composed largely of quartz-porphyry derived from older quartz- 

 porphyry of the Lower Keewatin. This is its character at Stuntz island and along 

 the lake shore west from that island. The largest of the rounded pieces are some- 

 times six to ten inches in diameter, but the size runs down to the smallest dimen- 

 sions, thus forming a grit and graywacke. The native Lower Keewatin quartz- 

 porphyry from which these pebbles are derived is found in the northern part of 

 Stuntz island and at the corner where, in T. 62-15, the sections numbered 13, 14, 

 23 and 24 come together. There is almost an insensible gradation downward, in the 

 conglomerate on Stuntz island, from coarse to fine. Hence, when the contact occurs 

 between the Upper Keewatin and the Lower, it is very difficult to determine the pre- 

 cise place of contact. The debris from the original quartz-porphyry, when hardened, 

 is much like the rock from which it was derived. This fact is repeated where the 

 conglomerate lies on greenstone, and where it lies on granite. It suggests that 

 between the Upper Keewatin and the Lower there occurred a long period of land 

 surface, during which the Lower Keewatin was decayed to great depth, and that on 

 submergence at the opening of the Upper Keewatin the decayed materials were not 

 entirely removed and assorted by transporting and sedimentary action. This phe- 

 nomenon is also well exhibited at the bottom of the Cretaceous in the Minnesota 

 valley and in Mower county, where the kaolin is evidently in part residual and in 

 part sedimentary. 



Anyone who studies the jaspilyte should bear in mind that there is a sedimen- 

 tary jaspilyte, belonging in the Lower Keewatin, as well as one. which was originally 

 a basic rhyolyte. The former is interstratified with green schists and varies like a 



