MID-SILURIC DELTA FANS OF NORTH AMERICA 475 



Niagaran strata known, although the Niagaran beds have a wide distribu- 

 tion. To imagine a connection across this region with a distant Arctic 

 Sea would require the subsequent complete removal of the deposits thus 

 formed. That is, of course, not impossible, but to say the least is highly 

 improbable. Such a supposition would also require the removal of all 

 deposits formed in this Arctic Salina Sea, unless, what is not impossible, 

 some of the beds classed as Niagaran in the north prove to be of later 

 age. There is, then, in this direction a possibility of a connection with 

 an Arctic Sea, but, as will be shown later, the characters of the deposits 

 within the Salina basin are such as to practically forbid such an assump- 

 tion. 



If we now examine the deposits of northern and western Michigan, 

 eastern Wisconsin, Illinois, Indiana, northwestern Ohio, and southeastern 

 Michigan, we find that practically everywhere the Upper Siluric or Mon- 

 roe strata rest on the Niagaran. This means that these regions were land 

 during Salina time and so freed from receiving deposits. The widespread 

 erosion of the Upper Magaran and the frequent absence of late Niagaran 

 beds may be attributed to such erosion before the Upper Siluric Sea cov- 

 ered this region again. From Ohio to the Appalachians the formations 

 are out of sight beneath younger deposits, but when they reappear in the 

 disturbed eastern region similar relationships exist. Thus at Cumber- 

 land, Maryland, the Salina is wholly wanting, the Lower Monroe beds 

 of Upper Siluric or post-Salina age resting with a disconformitv and 

 hiatus on the Niagaran, the highest beds represented being, according to 

 Prouty, 74 of Rochester age. These Lower Monroe beds of the Cumber- 

 land region have generally been referred to the Salina, but I have con- 

 vinced myself by personal observation that such a reference is erroneous. 

 They form one continuous stratigraphic series, with the beds recognized 

 by their fossils as Upper Monroe, and their thickness does not differ ma- 

 terially from that of the Lower Monroe of Michigan which overlies the 

 Salina. What few fossils have so far been obtained from these limestones 

 indicate a late Siluric age and bear out this correlation. 



Between the limestone of Upper Monroe age and the highly fossilifer- 

 ous Niagaran limestones lie .11 feet of sandstone, often ripple-marked, 

 and 30 feet of thin-bedded calcilutytes, mostly nodular, and alternating 

 with shaly layers. The sandstones probably represent the Kecl'er sand- 

 stones of the section farther east. They comprise, in descending order, a 

 3-foot bed of ripple-marked sandstones, iron stained and somewhat disin- 



74 Wm, F. Proilty : The Meso Silurian deposits of Maryland. Ahum-, .lour. Scl., vol. 

 xxvi, 1008, pp. 563-574. 



