284 THE VERMILION IRON-BEARING DISTRICT. 



of basic rocks, which are thus younger than the sediments. Splendid 

 examples of such dikes can be seen on Stuntz Island and elswhere. Some 

 of these dikes are older than, and some of them are essentially contem- 

 poraneous with, the Keweenawan rocks. 



The above facts concerning the relations of the Ogishke conglomerate 

 and the Knife Lake slate to the adjacent formations are so conclusive that 

 no doubt can remain as to the relative age of these sediments. They lie 

 immediately on the Archean Ely greenstone and the Soudan formation, 

 and are younger than these and than the granites of Vermilion Lake which 

 penetrate these two latter formations. They are older than the Griants 

 Range granite and than certain basic dikes that cut them. Since the sedi- 

 ments lie immediately upon the Archean and are overlain by another series 

 of clastic rocks, as has been found from the study of the contemporaneous 

 rocks in the Knife Lake ai'ea, they are here placed at the base of the 

 Algonkian, and are correlated with the Lower Huronian series of the other 

 iron-bearing districts of Lake Superior. 



OGISHKE CONGLOMERATE. 



This conglomerate was first so called because it is well developed on 

 and near Ogishke Muncie Lake, and the use of the term has been continued 

 on account of its appropriateness and because it was used in the early 

 literature of the Vermilion district. 



Attention is again called to the statement already made, that the con- 

 glomerate in some places differs somewhat petrographically from that of 

 the typical area, and that this variant phase has occasionally been called by 

 the local name of Stuntz conglomerate. (See p. 278.) 



PETROGRAPHIC CHARACTERS. 



Macroscopic characters. — The conglomerates in this western area all 

 possess a strong family resemblance. On the weathered surface the different 

 beds are white or grayish in color. This light color is due to pebbles of 

 rhyolite-porphyry, microgranite, granite, and granite-porphyry, which, as a 

 rule, have very light-colored weathered surfaces and are the main constit- 

 uents of the conglomerates. In a few places there is a good deal of jasper 

 present in angular fragments of various sizes. Greenstone fragments are 



I 



