84 GEOLOGICAL SURVEY OF WISCONSIN. 



and the Hamilton, there are the epochs of the Lower Helderberg and 

 the Oriskany in the Upper Silurian age and the Cauda-galli, Schoharie, 

 Corniferous, and Marcellus epochs in the Lower Devonian age. 



During all that long period, therefore, while the ancient sea was 

 depositing, under different conditions, these half dozen formations, 

 the little semi-circular patch of Wisconsin, under discussion, must 

 have been dry land, with the Salina for surface rock, and then must 

 have become submerged long enough for the deposition of the De- 

 vonian Hamilton, again to be emerged at the close of that epoch, and 

 remain dry land to the present hour. And this submergence must 

 have been just far enough, according to such a hypothesis, to leave at 

 every point on its margin a semi-circular rim of Salina. It would be 

 impossible to harmonize a supposition of this kind with the known 

 facts of rock-structure of surrounding regions, or to make it con- 

 sistent with the recognized laws of dynamical geology. It is much 

 more rational to suppose that the land was slowly rising; that, con- 

 sequently, the Silurian sea was slowly receding; that the last edge to 

 emerge was the region in question; that during its own epoch the 

 Salina was deposited; that, emergence still slowly going on, the 

 "Water-lime of the Lower Helderberg, being next in order of time, 

 was laid down over a narrower area, after which the ocean disappeared 

 from our shores, leaving the territory where Wisconsin now is, hence- 

 forth a part of the abiding continent. 



The paleontological indications for determining the age of the for- 

 mation are far less satisfactory than the stratigraphical. There is no 

 doubt that these rocks are of the same epoch as those mapped by Mr. 

 Rominger in the Michigan survey at the Straits of Mackinac as 

 Helderberg. He says (p. 28, Paleozoic rocks), speaking of the pale- 

 ontological evidence: "Lcperditia alta and Spirifer modestus are 

 known as Lower Helderberg species. On such a meagre representa- 

 tion of such a rich fauna, I would hesitate to base conclusions regard- 

 ing the age of the formation." Yet, as already stated above, he 

 concludes from stratigraphy that the rocks are Lower Helderberg. 

 The undersigned, while investigating the lithological and stratigraph- 

 ical characters and relations of the formation, has left mainly to 

 others an examination of its fossils. Orthis plicata, Avicula rugosa 

 and Tentaculites, designated by Mr. Vanuxem as characteristic of the 

 Water-lime group, have been found by the chief geologist at Humbolt 

 Falls. Leperditia alta, a still more important characteristic fossil, 

 has also been found. 



The most satisfactory paleontological evidence of this formation is 

 JEurypterus remipes, which has not been discovered in the region. 



