THE GEOLOGICAL SECTION OF MICHIGAN 411 



But the emergence this time was not a mere recession of the sea- 

 level. There was an Appalachian warping and gentle folding extend- 

 ing clear to Michigan, for as Grabau has pointed out, not only is there 

 a disconformity of the Sylvania and overlying beds, but both together 

 were folded and eroded before the Upper Helderberg was laid down, 

 both around Alpena and to the mouth. One result of this was to 

 permit an incursion from somewhere (judging by the thickening of 

 the limestones from north of Alpena) of the first Devonian-looking 

 fossils known, as described by Grabau. He tells me the same things 

 come from the Saskatchewan. 



It seems clear, comparing the records of the various wells, that 

 to the northwest the Sylvania is replaced by a series of limestones. 



Take the large group of wells at Marine City reported in Vol. V. 

 In all of them at about 1,000 to 1,100 feet down, and about 500 to 600 

 feet above the first salt, 300 feet or so above a well-marked gypsum 

 bed (which may be really the most fitting place at which to draw the 

 top of the Salina) we find from 60 to over 100 feet of sandstone, often 

 calcareous and passing into a sandy limestone or arenaceous dolomite, 

 like the top of the Lower Monroe Raisin River beds. The same 

 horizon is plain in New Baltimore 940-1,275; St. Clair 1,050-1,270, 

 and Port Huron. There can be hardly a doubt that this corresponds 

 to the Goderich Group III of Hunt, and so presumably to the Sylvania 

 and part of the Upper Monroe. As usual passing from the outcrop the 

 unconformities seem less. In its limestone facies it is impossible 

 so surely to assign a thickness, but it seems to be about 170 feet. 

 How much of this should be attributed to a thickened base of the 

 Upper Monroe is a matter for further research. The fairly uniform 

 thickness for Middle and Upper Monroe from Lake St. Clair to 

 Alpena suggests no appreciable disconformity in this region. So far 

 as one can judge the Upper and Middle Monroe are absent on the 

 west side of the state. Even around Mackinaw and at Cheboygan 

 there are no indications of them known to me. They are not shown 

 on any of Schuchert's maps. I am not so sure they should not be 

 placed on Plate 72. 



24. Upper Monroe. Detroit River Series (275 ft.). — This series 

 seems to have been deposited in a long, narrow trough at the very 

 end of the Silurian at a time when most of the continent was out of 



