566 R. C. ALLEN 



slate, chert, conglomerate, and breccia resembling lithologically the 

 succession in the upper or Princeton series, but the regular suc- 

 cession of formations shown on both limbs of the fold terminates 

 abruptly at the line indicated as a fault on the map. Another 

 cross-fault probably trends diagonally northeast through section 28, 

 producing a horizontal displacement of not less than 700 or 800 ft. 

 in the N.E. \ of section 32 and from 150 to 200 ft. in the N. ^ of 

 the N.W. \ of section 28. The offset in the latter locality may be 

 explained by folding, but the sharpness of the break in the former 

 locality strongly suggests faulting. In any case, the extension and 

 direction of the fault as indicated on the map is to a considerable 

 degree hypothetical. 



Knowledge of the structure of the northeast two-fifths of the 

 Gwinn syncKnorium pertains chiefly to the northeast Kmb. The 

 most conspicuous structural feature of this limb is the broad cross- 

 anticline responsible for the extraordinary surface exposure of the 

 iron formation in the vicinity of the Austin and Stephenson mines, 

 giving rise to two prominent synclines, the northern one carrying 

 the Princeton No. 2 ore body and the southern one the Austin- 

 Stephenson deposit (see cross-section I-II). Northward from 

 Princeton No. 2 mine the east limb is overturned and dips at an 

 angle of about 80° to the northeast, about parallel to a faulted 

 contact with black slate extending from somewhere north of the 

 Old Swanzy pit in the S.W. \ of the N.E. \ of section 18 southeast 

 for a distance of probably more than a mile. Where observed in 

 the Swanzy pit and in the Princeton No. i pit in the S.E. \ of 

 section 18, the dip of the fault plane is northeast about 75° or 80°. 

 Both the'iron formation and the adjacent slate are intensely sheared 

 along the zone of faulting. The belt of slates adjacent to the fault 

 on the northeast may be stratigraphically either above or below the 

 iron formation so far as the writer has proof. The upper and the 

 lower slate members of the Gwinn series are lithologically very 

 similar. Drill holes and the mine workings show that the iron 

 formation in this vicinity lies directly on the basal arkose member 

 of the Gwinn series with here and there a few feet of black slate 

 lying between them. This makes it very probable that the slate 

 belt northeast of the fault belongs to the upper slate member of 



