104 KEMPS ORE DEPOSITS. 



ores available. Class (d) embraces limonites closely related to 

 those of Example 2, where they are referred to. Class (e) has a 

 flaggy structure and its ores are related to Class (c), but are less 

 distinctly banded and are mere local varieties of ferruginous schists. 



2.02.20. The ore bodies have been in earlier years generally 

 regarded as true beds of greater or less extent and often of great 

 irregularity. They approximate a lenticular shape in the simplest 

 development, as is better shown in the other less disturbed districts. 

 In the Marquette region this is at times obscured by the excessive 

 disturbances. They often follow the foldings of the walls, partic- 

 ularly in synclinal troughs. Later developments have brought 

 out the fact that the ore bodies are associated with some underly- 

 ing rock that is relatively impervious. The favorite one is the so- 

 called soaprock, an altered igneous intrusion that is chiefly in 

 dikes. Beds of jasper seem to play the same role. Van Hise, in 

 his paper of February, 1892, notes four varieties (1) Deposits on 

 the contact of a quartzite conglomerate (the base of the Upper 

 Marquette) and the ore-bearing formation ; (2) deposits resting 

 upon soaprock, which grades into massive diorite ; (3) deposits 

 resting upon dikes of soaprock, which follow along or cut across 

 the ore-bearing formations ; (4) deposits interbedded in the jasper 

 or chert. In Figure 18 a generalized section taken from Van Hise 

 exhibits these different varieties. On the east the soft hematites 

 (limonites) are first met ; then in going west the red and specular 

 hematites ; and then the magnetic character increases, until at the 

 western end of the district the magnetites are most abundant. 

 South of the Marquette region and between it and the Menominee 

 is found the Felch Mountain area. It consists of three small basins 

 now cut off by erosion from the main exposures, with which it was 

 doubtless at one time connected. 



2.02.21. The origin of these ore bodies has been a subject of 

 much controversy. Detailed descriptions of the various hypoth- 

 eses will be found in Wads worth's monograph. 1 Only the im- 

 portant attempts at explanation are instanced here. The early 

 survey of Foster and Whitney (1851) attributed an eruptive origin? 

 and the same difficult thesis has since been attempted by Wads- 

 worth (1880), who bases his argument chiefly on the analogy of 

 the banded jaspers to laminated felsites, and to the fact that they 

 and the ore curve around masses of inclosing schist or break across 



1 M. E. Wads worth, " Notes on the Iron and Copper Districts of Lake 

 Superior," Ball. Mus. Comp. ZooL, Vol. VII., No. 1, July, 1880. 



