APPLICATION OP PRINCIPLE OF TRANSGRESSIVE OVERLAP 583 



This fauna is clearly of early Trenton (Black Eiver) or Chazy age — 

 a fact which makes the underlying sandstone more nearly equivalent to 

 the Saint Peter of Minnesota (transgressional portion; see beyond) than 

 to any part of the basal sandstone series of the Upper Mississippi region. 



On Sulphur island higher strata rest upon the Huronian quartzites 

 without the intervention of the sandstones.* This, as suggested by 

 Eominger, very likely represents a submerged reef or mound of the Bara- 

 boo type of Wisconsin; but this does not seem to be the case in the En- 

 campement d'Ours section, where a great thickness of strata, comparable 

 to the basal Superior sandstone, succeeds the Huronian. There can be 

 little question that at this portion of the shore the Middle Ordovicic 

 strata overlapped the Cambric and rested with a basal sandstone on the 

 pre-Cambric. The reference of this bed to the Cambric is clearly 

 erroneous. 



On the island of Lacloche (Cloche island) a similar reddish, greenish, 

 and whitish sandstone, from 20 to 30 feet thick, rests on the pre-Cambric 

 crystallines. It passes upward into arenaceous dolomites and limestones 

 with an abundance of fossils, which first appear in the upper layers of 

 the sandstone and which clearly establish the age of the formation as 

 Black Eiver. The probable identity of this sandstone and that on Saint 

 Joseph and Encampement islands with the Saint Marys sandstone of 

 Sault Sainte Marie was early pointed out by Logan, who considered it 

 improbable that these sandstones are the equivalent of the Potsdam of 

 New York. 



From a number of localities a siliceous dolomite varying up to 100 

 feet in thickness has been recorded as lying above the Superior sandstone ; 

 this formation, named the Hermansville limestone by van Hise and 

 Bayley,f is generally regarded as of Beekmantown age, though the evi- 

 dence for this is by no means conclusive. In the Iron Mountain region 

 Upper Cambric fossils are recorded from the basal sandstone, but this 

 does not prove that the basal sandstone of Marquette and the pictured 

 rocks is of the same age. In fact, from their position with reference 

 to the transgression of the Cambric sea, these more northern sandstones 

 must be regarded as of later age than that of the Menominee district. 

 If the Hermansville limestone (Auxtrains formation would be a better 

 name, from the more typical exposure on that stream) proves eventually 

 to be Beekmantown rather than Chazy (that is, Upper Stones Eiver or 

 Lowville), the late Cambric or early Ordovicic age of part of the Superior 

 sandstone must be conceded. In that case, however, the basal sandstone 

 of Sault Sainte Marie and eastward is of much later age, belonging to 



* Rominger : Loc. cit. 

 t Menominee folio. 



LIV — Bull. Geol. Soc. Am., Vol. 17, 1905 



