412 THE CBYSTAL FALLS IRON-BEARING DISTRICT. 



schists have also been found in test pits on both sides of the western exten- 

 sion of the Groveland syndine in sec. 36, T. 42 N., R. 30 W. In sec. 31, 

 T. 42 N., R. 29 W. (the Groveland section), they have been penetrated in 

 10 drill holes, besides numerous test pits, giving altogether a cross section 

 more than half a mile in length from north to south. In the northern half of 

 sections 32 and 33 numerous test pits have exposed the Mansfield formation, 

 proving that it borders on both sides the narrow syncline, the interior of 

 which for a mile and a half is occupied by the magnetic Grroveland jasper. 

 Tln-ough sees. 34, 35, a,nd 36, T. 42 N., R. 29 W., and sec. 31, T. 42 N., 

 R. 28 W., the mica-schists have not been discovered, probably both because 

 they are but feebly represented and because but few test pits have been 

 sunk through the Cambrian blanket. In sees. 32 and 33, T. 42 N., R. 28 

 W., the mica-schists have been found in scattered test pits and borings on 

 both sides of the interior jasper of the Felch Mountain syncline, and also 

 on the south side of section 33. 



The thickness of the Mansfield formation is so small — not more than 

 200 feet — that it produces no very noticeable effects on the general topog- 

 raphy, in spite of the ease with which it weathers. In the western portion 

 of the district, through sees. 34 and 35, T. 42 N., R. 30 W., with the dolo- 

 mite it underlies a broad low-lying plain, which is bounded on the south 

 by a ridge of the Sturgeon quartzite backed by the Archean plateau. On 

 the north, a broad ridge, through which diagonally pass the Archean gran- 

 ites and gneisses, the quartzite, and the dolomite, defines this valley as far 

 east as the middle of section 35 ; in the northern and central portions of this 

 section it spreads out into a swampy lowland, diversified by glacial sand 

 plains, expressive of the gradual widening of the trough and of the gen- 

 erally horizontal attitude of the soft rocks of the interior. The most defi- 

 nite topographical feature directly due to the Mansfield schists is the narrow 

 steep-sided valley which runs east from this lowland for nearly 2 miles, 

 on the south side of the Groveland syncline. The ancient stream valley 

 filled with the Cambrian sandstone, already mentioned, follows along this 

 narrow belt. 



PETROGRAPHICAL CHARACTERS. 



The hand specimens from the various test pits, the drill cores, and the 

 few small outcrops indicate that the Mansfield formation is quite uniform 

 in character throughout the Felch Mountain area. The great majority of 



