536 THE MARQUETTE IRON-BEARING DISTRICT. 



CONTACTS OF THE GOODRICH QUARTZITE WITH THE LOWER MARQUETTE 

 SERIES AND WITH THE ARCHEAN. 



Direct contacts of the quartzite ^yllicll forms the base of the Upper 

 Marquette series with tlie uuderlying rocks are very numerous, and the evi- 

 dence in detail, as well as the more general facts, leaves no room for doubt 

 that this quartzite was laid down on a deeply eroded surface and that the 

 relations are those denoting a most profound lime-break. 



The detailed facts, which may be observed on the exceptionally fine 

 exposures about Republic Mountain, are these: (1) There is a slight but 

 very persistent discordance in stratification — more evident at any single 

 locality in dip than in strike — between the Goodrich quartzite and the under- 

 lying foi'mation of the lower series. This difference in dip is on the aver- 

 age not far from 15°. (2) The basal conglomerate 'o{ the upper series is 

 crowded with fragments of the iron-bearing member upon which it lies. 

 These fragments are often of large size and imperfectly rounded, and evi- 

 dently have not moved far. The included fragments at the immediate 

 contact are almost wholly from the subjacent formation, and from their 

 often irregular shapes and great preponderance might frequently be mis- 

 taken for the products of brecciation, if it were not for the sparse presence 

 in the conglomerate cement of quartz and feldspar derived from the more 

 distant granites. (3) The structural details of the contacts prove uncon- 

 formity. The layers of the underlying iron formation are often for short 

 distances traversed at large angles by the contact surface. Extending back 

 into the mass of the iron formation cracks are occasionally found into 

 which the fine material of the conglomerate cement has sifted. Finally, 

 in the conglomerates in the lower member of the upper series pebbles of all 

 the underlying rocks are seen, from the Archean to the top of the Lower 

 Marquette series. 



The more general facts of the relation of the Upper Marquette series 

 to the underlying rocks may be summed up in the statement that within 

 the narrow limits of the Republic area the upper series rest, in one locality 

 or another, on each of the older formations (Atlas Sheets IV, VII, X, and 

 XI). The maximum thickness of the Lower Marquette series is found at 



