PLATE LXX. 



POKEGAMA LAKE PLATE OF THE MESABI IRON RANGE, 1899. U. S. GRANT. 

 The rock at Pokegama falls is a quartzyte, dipping southerly. It also appears 







on Prairie river, at the lower falls. Above Pokegama falls, about a quarter of a mile, 

 it forms a vertical bluff on the right bank, exposing in all about twenty-five feet of 

 the bedding. This rock has been uniformly supposed to lie below the iron ore horizon 

 of the Mesabi range. There is a coarse quartzose sandstone seen below the ore at 

 the Cincinnati mine near Biwabik, and also below the ore on the Gogebic Iron range 

 (Aurora mine), and the Pokegama rock has been parallelized with that horizon. That 

 is the view presented in most of the reports of the survey (except volume v, page 

 992), but there are facts that indicate that it is above rather than below the ore. 



1. In descending Prairie river, at the lower falls this rock appears to dip south- 

 erly and to run below the taconyte seen there, but the superposition is not seen. 

 There is a break in the continuity of observation north of the strike of 'the ore- 

 bearing rock sufficiently broad to allow space for a synclinal which might throw the 

 iron ore under the quartzyte. 



2. The rock becomes conglomerate in irregular patches. The pebbles then 

 contained in it are such that they cannot all have come from the Archean, viz., red 

 quartz-porphyry, diabase, pumice, very fine kaolinic debris, both white and red, 

 hornstone, flint, like that at Gunflint lake, micaceous slate, white and lavender 

 quartz, green schist and red shale, resembling catlinite. The rock is essentially a 

 quartz conglomerate. 



3. It is overlain by a fine, unctuous shale, which is red, but is streaked with 

 white by reason of sedimentation. 



4. Where the Diamond mine exploration was made, a few miles further east, 

 the ore, which is in the upper part of this quartzyte, is not taconitic. It is limonitic 

 rather than hematitic. No ore is known there of the character and quality of that 

 at the lower falls of Prairie river. 



This conglomerate, therefore, seems to contain Kevveenawan and Animikie as 

 well as Archeau debris. It may be of the age of the Puckwunge (Potsdam) sand- 

 stone and conglomerate. The overlying unctuous shale may be of the Keweenawan, 

 similar to that seen along the banks of the St. Louis river above Fond du Lac, or of 

 the Cretaceous, appearing more like the latter. If it should prove to be Cretaceous 

 that would suggest the same age for the quartzyte. The preponderance of evidence 

 at present is toward the Puckwunge (Potsdam) age of this quartzyte. 



