THE LOWER HURONIAN. 307 



in both places there is a very striking difference in the Hthologic character 

 of the two rocks. The iron-bearing formation is made wp essentially of 

 beds of magnetite alternating with very quartzose bands, whereas the 

 cong'lonierate is the normal greenstone conglomerate, although verj- much 

 altered. There is no transition lietween the two, and the relationships 

 apjoear to be those of two unconformable series of rocks. The evidence of 

 this unconformity is, however, not absolutely conclusive in the Vermilion 

 district. The opinion of the majority of those who were studying these 

 rocks for the Survey was in favor of this unconformable relationship, and 

 in this respect was in thorough agreement with the conclusions reached and 

 already published by the Minnesota survey. However, in view of the fact 

 that some of the rocks were intensely plicated, it was recognized that it was 

 possible for them to change both their strike and dip within a very short 

 distance, even within the distance which separated them from the Animikie 

 and in which there were no exposures. Such a change might possibly 

 bring them into perfect conformity with the Animikie. In view of this 

 possibility we could not all agree to accept the unconformable relationship 

 as proved. In 1900, shortly after work was begun in the Mesabi, Mr. C. K. 

 Leith had the good fortune to find a place where the relationship between 

 these two series is unmistakably shown." At this point, north of Biwabik, 

 the vertical Lower Huronian beds were found overlain by the low south- 

 ward-di^Dpiug rocks of the UjDper Huronian series, with a thin basal 

 conglomerate at the bottom. The correctness of the opinions previously 

 held were thus demonstrated beyond all doubt. 



EELATIONS TO THE KEWEENAW AN. 



Relations to the Keiveenmvan gabhro. — In Ts. 63 and 64 N., Rs. 8 and 9 

 W., the Ogishke conglomerates and Knife Lake slates are found in many 

 places almost in jiixtaposition with the Duluth gabbro mass of Minnesota. 

 In no cases were actual contacts observed between them, as invariably a 

 topographic depression, occupied either merely by lower ground or, as in 

 most cases, by water, intervened. The Keweenawau gabbro has been long 

 recognized as one of the youngest rocks occurring in Minnesota. Where 

 the conglomerates and gabbro are in contact the gabbro has been found to 

 metamorphose the conglomerates and slates very extensively, and hence 



aMon. U. S. Geol. Survey Vol. XLIII, 1903, p. 181. 



