414 • A. C. LANE 



It is next to impossible to trace these largely paleontological 

 subdivisions in the wells, especially the difference between Amherst- 

 burg and Anderdon. But the tendency to a dolomite top, with 

 sulphur reduced from gypsum and anhydrite, and limestone lower 

 can be plainly followed. 



Above the undoubted Lucas either as an extension of the base of 

 the Dundee or as a still higher member of the Detroit River series 

 (which it would be interesting to compare with the New York Helder- 

 berg) was an intercalation of limestone in the Lucas which did not 

 reach as far as New Baltimore. By the time we get to Alpena, 

 limestone occurs at various horizons. There is clearly a tendency 

 to replacement of dolomite by limestone toward the north in the 

 direction probably of the open sea. The Michigan Monroe seems 

 to have been, like the Black Sea and Caspian, turned northside south. 



At the close of the Monroe the state was so elevated that slight 

 folds which occurred at the same time could be planed off, and the 

 underlying formation in numerous places from Mackinaw to Monroe 

 County, made into a dolomite conglomerate, calcirudite. So far 

 as we know it remained above water during the opening stages of 

 the Devonian Helderbergian. There is distinct reason to believe 

 that this uplift was not a mere rise and fall of the sea strand produced 

 perhaps by disturbances thousands of miles away like the earlier 

 changes in Niagara time, but a tilting by which the west was more 

 elevated than the east and a certain amount of folding took place. 

 The Limestone Mountain fold on Keweenaw Point may have taken 

 place this early. The Sylvania uplift seems to have been decidedly 

 most at the south, opening up and depressing the land at the north. 

 By the close of the Amherstburg the effect of a new uplift made itself 

 felt in cutting off the northward connections and the conditions for 

 formation of dolomite and anhydrite were re-established and with 

 them the Silurian fauna. The same disturbance that cut Michigan 

 off once more may have opened up New York to the Helderbergian, 

 so that while the Coeymans and Port Ewen beds were forming in 

 New York, 300 or 400 feet in all, Michigan was mainly out of water, 

 and not until the Schoharie (Hall in Foster and Whitney, II, 225), 

 did deposition that has been recognized by its fossils begin in 

 Michigan. 



