68 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



RELATIONS TO SURROUNDING BEDS. 



The foot wall of the ore is a black slate, described as being rich in 

 hematite and bearing large crystals of iron pyrite. No crosscuts have 

 been driven for any distance into the foot wall, so that it is impossible to 

 sav what thickness of the hematitic black slate there may be before the 

 greenish pyritiferous slate begins. In places a gray "soapstone" takes the 

 place of the black slate as the foot wall. 



The dump obtained by sinking the shaft in the material overlying the 

 ore shows large masses of conglomerate, the pebbles of which are rounded 

 and predominantly of volcanic rocks, with pebbles of chert and slate from 

 the iron formation and slates below. These fragments are well rounded. 

 The microscope also shows quartz grains with secondary enlargements, so 

 that there can be no doubt that the rock is a true conglomerate. Similar 

 conglomerates, except that the sedimentary fragments are wanting, have 

 been noticed farther north along the west side of the river. Just west of 

 the bridge at Mansfield, near the mine, there is also a small exposure of con- 

 glomerate, which shows an alternation of coarse and fine sediments, with a 

 strike nearly north and south, and a dip of 80° W. To the west, above this 

 conglomerate, and not more than 15 to 20 feet distant, are found the lavas 

 of the Hemlock volcanics. According to the mine captain, the succession 

 west from the ore body in the hanging wall is 20 to 25 feet of paint rock, 

 or, as it is usually called, red slate, then conglomerate, then greenstone. 

 It is difficult to diagnose the paint rock, as no specimens are to be had, but 

 it is highly probable that it is a ferruginous and extremely altered lava 

 sheet. Similar rocks are commonly found thus altered in association with 

 the ores in the Penokee-Gogebic and Marquette districts. Lending weight 

 to this conclusion is the fact that in some places an amygdaloidal green- 

 stone has been exposed in test pits immediately above the iron-bearing 

 formation. 



COMPOSITION OF ORE. 



The Mansfield mine up to the present time has raised only Bessemer 

 ore, and is the only mine in the Crystal Falls district which has supplied 

 any considerable quantity of ore of this character. An average of a num- 

 ber of analyses gives the following composition for the Bessemer ore: 



