ANNUAL REPORT FOR 1875. 70 



region. Similar indications were found on Pelican river, and to the 

 northwest of Pelican Lake. 



The " Greenstone group " found between Jenny and Grandfather 

 Bull Falls on the Wisconsin river, the quartzite near Wausau, and 

 the silicious rock of Marshal] hill, evidently Huronian, give additional 

 points for determining the general direction of the Menomonee range. 

 The southwestern termination of it, where it disappears under the 

 Potsdam, would seem to be at Black River Falls. Between this 

 range and the Penokee mountain range there is probably a nearly con- 

 tinuous area of Laurentian. And between the Menomonee Huronian 

 range and the irregular line of the northern edge of the Potsdam 

 there is a continuous Laurentian belt. 



The geographical boundaries of these formations can only be deter- 

 mined by more detailed explorations. 



It is a matter of great practical importance to trace the outlines of 

 the Huronian and Laurentian formations in the Arcliasau region of 

 the state, for, judging from results obtained from the adjacent region 

 of the Upper Peninsula of Michigan where exactly the same forma- 

 tions exist, the Laurentian rocks contain no useful minerals, while 

 the Huronian areas promise an abundance of iron, if not more pre- 

 cious metals. 



Hamilton ojg fam^l^t Hi ^aBBBBB&J —There is a region of "Wiscon- 

 sin, on the shore of Lake Michigan, beginning in the city of Milwau- 

 kee, extending northward to the vicinity of the county line of Ozaukee, 

 running inland half a dozen miles, shaped something like a segment 

 of a circle, which has been placed by geologists in the Upper Helder- 

 berg epoch, and has been called by one scientist in connection with 

 the present survey of the state, the Hamilton formation. 



The undersigned, soon after he began studying the geology of "Wis- 

 consin, was led to doubt whether the region in question belongs to the 

 Devonian age, as it must, if it is either Upper Helderberg or Hamil- 

 ton. The proper place for a full discussion of the subject will be in 

 the final report, after the geological survey of the state is completed. 

 Yet it may not be out of place to give here, briefly as may be consist- 

 ent with clearness, the reasons which have forced the present head of 

 the survey to conclude, against his predecessors, against his associates, 

 that the limited area under consideration is Lower Helderberg, and 

 consequently belongs to the Upper Silurian and not to the Devonian 



age. 



In North American Geology, following the nomenclature of Prof. 

 Dana and the New York geologists, the proper place of the Lower 



