PRE-CAMBRIAN FORMATIONS OF GWINN IRON DISTRICT 567 



the Gwinn series. North of the middle of section 18, details of the 

 structure are unknown but the distribution of formations indicated 

 by the few exposures and drill holes suggests deformation by both 

 folding and faulting of a complex character. 



ARCHEAN SYSTEM 



The Archean system comprises both acid and basic plutonic 

 rocks, granite greatly predominating. These rocks inclose the 

 synclinorium on the west, north, and east sides, encircling the north 

 and northwest sides in bold hills and protruding through the drift, 

 in low knobs on the east side from New Swanzy northward. Numer- 

 ous drill holes reach the system after penetrating the overlying 

 sedimentaries within the borders of the synclinorium. 



ALGONKIAN SYSTEM 



The Algonkian system is represented by two unconformable 

 series of Huronian sedimentary rocks, the Princeton (upper) and 

 the Gwinn (lower) series. Both series are intruded by basic dikes, 

 probably of Keweenawan age. The basal conglomerate of the 

 Gwinn series contains pebbles and bowlders of quartzite, quartz 

 slate, and siHceous, cherty, sKghtly dolomitic slate derived from a 

 third sedimentary series unconformably below the Gwinn series but 

 not present so far as known in the Gwinn syncKnorium. 



MIDDLE HURONIAN 

 GWINN SERIES 



There are four members of the Gwinn series, viz., from the 

 base upward, (i) conglomerate and arkose, (2) black slate and 

 gray slate, (3) iron formation, and (4) black slate, gray slate, 

 and graywacke. 



I. Conglomerate-arkose. — -The basal member of the Gwinn series 

 is mainly arkose and arkose conglomerate. It lies on an uneven 

 surface of Archean granite and is reported to occur in isolated 

 patches over a considerable area outside of the Gwinn synclinorium. 

 Within the fold its thickness varies from practically nothing to 

 above 60 ft. The dominant phase of the member is arkose or 

 decomposed granite. It is evident that the arkose has its origin 



