404 A. C. LANE 



(i per cent or 2 per cent, as though wind-blown) in the dolomites, and 

 occasional beds of sandstone are characteristic all over the state. 



It is exposed only in the Upper Peninsula. It forms the shore of 

 Lake Michigan and Lake Huron in a continuous ridge which rises 

 to the north almost at the dip of the beds, which is about 50 feet to 

 the mile, from the lake level 580 feet A.T. in somewhat less than 10 

 miles to an elevation of about 800 feet A.T. Here and there it out- 

 crops and very often the soil over it is thin. 



While as a whole it is dolomitic, there are horizons, notably that 

 of the Fiborn and Rex quarries, which run nearly pure calcium 

 carbonate. They probably occur in the lower part beneath the 

 Guelph. 



The absence of sediment and the absence of iron and the fact that 

 it is succeeded by a salt series suggests that the climate was not a 

 very rainy one. 



21. Salina (or Lower Monroe). — The term Monroe was intro- 

 duced by me in 1893' to 1895, and as at first used without definition 

 did not include all the beds down to the Niagara. In my later and 

 more formal definition (Vol. V) I made it practically include all the 

 Silurian above the Niagara, having found it impracticable to separate 

 the Salina from the beds above. The difficulty still remains. The 

 last salt bed is not always at the same horizon. 



According to Grabau, there should be a marked hiatus and 

 disconformity between the Niagara and the next overlying beds in 

 Monroe County. The thickening as we go north, which is rapid 

 and very great, would then be practically by addition at the bottom 

 of beds formed during this disconformity.^ Now, comparing wells 

 at Britton, Milan, Romulus, and Wyandotte we do find an increasing 

 thickness, and five feet of rock salt at Milan seems to be almost 

 directly above the Niagara, whereas in Wyandotte there are 275 feet 

 of dolomites below the rock salt and above the white dolomite. The 

 rock salt at Milan is 717 to 722 feet below the base of the Sylvania, 

 and may be continuous with a bed of 790 to 900 feet below it at 

 Wyandotte, 1,080-1,190 or 1,235 feet. Again, as we go north the salt 

 beds seem to occur higher up. Gypsum (anhydrite) certainly occurs 



1 Report for 1891-92, 66. 



2 Bull. G.S.A., XIX, 554. Compare also Schuchert, op. cit., PI. 68. 



