PRE-CAMBRIAN FORMATIONS OF GWINN IRON DISTRICT 571 



4. The upper slate. — -The upper slate member is from 30 to 100 

 ft. thick. It is unconformably overlain by the basal conglomerate 

 of the Princeton series. Its relation to the underlying iron-bearing 

 member is largely gradational. It comprises an interbedded series 

 of black slate, gray slate, and dark graywacke-quartzite. The 

 black graphitic phase is more commonly directly above the iron 

 formation than the gray slate, and the graywacke-quartzite phase 

 seems to be in upper and middle horizons. 



PRINCETON SERIES 



The Princeton series consists of an interbedded series of slates, 

 ferruginous slates, and cherts, quartzites, ferruginous quartzites, 

 and graywacke with a basal conglomerate. The series is 400-500 

 ft. thick. Probably the entire thickness is not represented in the 

 Gwinn fold. It is rarely seen in outcrops but it has been penetrated 

 by numerous drill holes and some open pits. For the purpose of 

 this article the interesting member is the basal conglomerate. 



The basal conglomerate varies from 30 to 50 ft. to more than 

 100 ft. in thickness. Nearly all of the many drill holes which cross 

 its horizon show its presence but here and there it is represented by 

 a coarse graywacke. So far as known, the only exposures are in 

 the S.E. \ of the N.E. \ of section 18, T. 45, R. 25, where a number 

 of exposures occur on a low brush-covered ridge. Adjacent to them 

 on the east the upper slate member of the Gwinn series is exposed 

 in pits. The strike of the conglomerate is N. 70° W. and the dip 

 80° N. 



The matrix of the conglomerate is coarse, dark graywacke- 

 quartzite, the pebbles are chert and siliceous black slate, quartz, and 

 arkose, derived from the underlying Gwinn series, and quartzite. 

 The matrix carries a good deal of disseminated ferruginous material 

 and some very small fragments of iron ore. There are also a good 

 many small irregular cavities in the rock which are lined with 

 hematite and limonite produced by weathering-out of iron-bearing 

 fragments of some kind. The largest chert fragments are two to 

 three inches long and one-half to an inch wide. All of them show 

 wear by attrition, the smaller ones being generally lens shaped. 



So far as can be ascertained, the Princeton and Gwinn series are 

 structurally almost accordant. The strike of the conglomerate 



