190 GEOLOGY. 



formation is (3) the Virginia slate, which includes, besides slate, shale, gray- 

 wacke, and some limestone. Its thickness has not been determined. Like the 

 Biwabik formation, it is locally metamorphosed by igneous contact. All the 

 beds dip south and southeast at angles of 5° to 20°. The dipping beds are 

 gently cross folded, the axes of the folds pitching south. The unconformity 

 of the series with older rocks is shown by (1) basal conglomerate, (2) discord- 

 ance of dip, (3) differences in amount of deformation and metamorphism, (4) dis- 

 tribution, and (5) relations to intrusive rocks. 



In the Vermilion region, 1 the Animikean series consists of two main conform- 

 able members, the Gunflint formation below, and the Rove slate formation 

 above. The former ( = ,the Biwabik of the Mesabi region) is mainly of chert 

 and granular quartzose bands with more or less carbonate, bands of jaspar, mag- 

 netic chert, and quartz with crystals of actinolite, griinerite, etc., together with 

 more or less iron carbonate, magnetite, hematite, and limonite (not commercially 

 ^luable). The formation has been estimated to be something like 800 feet in 

 thickness. The Rove slate formation (probably = the Virginia formation of the 

 Mesabi region) has, in addition to the slate, some graywacke and some quartzite, 

 and is locally metamorphosed by igneous content. The system also contains 

 some igneous rock. Its thickness is undetermined. The structure of the system 

 is rather simple, being largely monocinal. 



Iron ore in the Lake Buperior region. 



Production. — The region about Lake Superior yields more iron ore 

 than any other area of equal size in the world. The productiveness 

 of the region has nearly trebled in ten years. In 1902 ; the aggregate 

 production of ore from that part of this region which lies within the 

 United States was 26,977,404 long tons. 2 This is about 76 per cent, 

 of all that was produced in the United States that year; of this, the 

 Mesabi region produced nearly one-half. In 1900, the last year for which 

 statistics for other countries are available, the iron ore produced in the 

 Lake Superior region of the United States was about 23 per cent, of 

 the total production for that year. 



Geologic positions. — The ores of this region are partly in the Archean, 

 as in the Vermilion region of Minnesota, partly in the Huronian, as 

 in the Marquette region of Michigan, but most largely in the Animikean, 

 as in the Menominee and Gogebic regions of Michigan and Wisconsin, 

 in the Mesabi region of Minnesota, and some of those in the Marquette 

 region. The following table gives the production in tons for these several 

 regions for the ten years preceding 1902 : 



1 Clements, Mono. XLV, U. S. Geol. Surv. 



2 Mineral Resources of the United States for 1902. 



