54 THE GEOLOGY OF MINNESOTA. 



[Age of tlie Puekwunge conglomerate. 



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disappear in some directions sooner than in others. There can be no doubt that 

 under favorable conditions the surface eruptives flowed for many miles, and that in 

 other conditions they were shut out, and a continuous series of fragmental strata 

 was the result. The fine, red conglomerates and the red sandstones and shales were 

 extended to great distances toward the southwest, and have been penetrated by 

 deep wells at Minneapolis, Belle Flaine, Mankato, Hastings, Red Wing and other 

 points, where they have been found in some instances, so thick that they constituted 

 an effectual barrier against the further sinking of the wells. 



Age of the Puekwunge conglomerate and sandstone. As in the eastern part of the 

 1'nited States, the term Potsdam has been applied, in the Northwest, to both of these 

 sandstones, /. e., to the quartzyte overlying the Puekwunge conglomerate, which has 

 also had the names Sioux quartzyte, Baraboo quartzyte, Barron County quartzyte, 

 and to the sandstones lying above the trap rocks, containing an Upper Cambian 

 fauna which, in Minnesota, has been divided between Hinckley and St. Croix. From 

 this circumstance considerable confusion of nomenclature has resulted. There is no 

 doubt, however, that in the midst of what in some places is a continuous series of 

 similar sandstone strata, the traps of the Manitou were interposed, thus affording an 

 important character for dividing the sandstone into two parts. It also seems prob- 

 able that even where no actual eruption occurred the cotemporary sandstone deposits 

 were considerably affected in composition, and that, in others, the lower strata were 

 hardened and cemented into quartzitic strata by the chemical action attending 

 volcanic ejection in some adjoining portion of the ocean. If it be inquired what 

 portion of this series is represented at Potsdam, N. Y., where the name was first 

 applied to any part of this sandstone, there need be little hesitation in answering- 

 to the earlier. Hence it is more in keeping with the practice of the best authority 

 in geological nomenclature to restrict the term Potsdam to the quartzitic sandstone 

 which, about lake Superior, underlies the traps of the Manitou eruptives and overlies 

 the Puekwunge conglomerate, /. e., to the sandstone seen at a mile west of Grand 

 Portage village, and in Grand Portage island, to the quartzyte seen near New Ulm 

 and in Cottonwood county, to the quartzyte in Pipestone county, and to the equiva- 

 lents of this in the various deep wells. In some of the deep wells in the southern 

 part of the state an enormous thickness of red shale and sandstone has been encoun- 

 tered, and this is supposed to be due to cotemporary volcanic action in such contiguity 

 that the detrital and volcanic debris was colored by the ferruginous oxidation which 

 rapidly pervades superficial volcanic ejections; but, owing to the occurrence of 

 similar red shale in these lower quartzytes and sandstones, there must have been an 

 earlier source for such sediment. There is here an indication of eruptive action in 

 the central part of the state during the prevalence of what has been noted as the 



