THE lEON BEAKING MEMBER. 



197 



for petrograjihic purposes, the samples would of course have been selected 

 in a different fashion. Nevertheless, as taken, they represent quite fairlj- 

 the more ferruginous portions of this phase of rocks, one (No. IV) repre- 

 senting a less ferruginous portion. Of tliese analyses, I represents a 

 thickness of 40 feet exposed in the SW. \ of Sec. 17, T. 44 N., R. 3 W., 

 Wisconsin; II, 41 inches in the NE. | of Sec. 15, T. 44 N., R. 3 W., Wis- 

 consin; III, 19 feet exposed on the west cliff at Penokee gap, NW. \ of 

 Sec. 14, T. 44 N., R.'3 W., Wisconsin; IV, 7 feet 6 inches immediately 

 overlying the rock represented by III ; V, 10 feet from the same cliff; VI, 

 50 feet exposed in the NE. \ of Sec. 14, T. 44 N., R. 3 W., Wisconsin; 

 VII, 58 feet in the SW. \ of Sec. 10, T. 44 N., R. 2 W., Wisconsin ; VIII, 

 25 feet in the SW. J of Sec. 1, T. 44 N., R. 2 W., Wisconsin. 



Analysea of magnetitic slates. 



It will be seen that several of these analyses show a proj^ortion of iron 

 sesquioxide beyond that which is required by the protoxide contents indi- 

 cated to make up the mineral magnetite. In fact the excess is even slight!}- 

 greater tlian is indicated at first sight, since a certain amount of the iron 

 protoxide must be assigned to the amphibolic mineral whicli is always 

 present. Disregarding the small correction to be made on this account and 

 considering all of the iron protoxide indicated by the analyses as contained 

 in magnetite, we find that the following respective proportions of magnetite 

 and hematite are indicated for the several samples. I, 5080 per cent and 

 1-37 per cent; II, 27-20" and 30-63; III, 5056 and 11-40; IV, 23-38 per 

 cent of magnetite and no hematite; V, 5038 })er cent of magnetite and no 



