April, 1917] Silurian Fossils from Ohio 189 



North of Osgood, both the lower and the upper Osgood 

 clays tend to become more indurated. This is true especially 

 of the lower Osgood clay, which becomes a more or less impure 

 limestone northward. The upper Osgood clay, on the con- 

 trary, although frequently reduced to a thickness of less than 

 two feet, usually may be recognized as the very characteristic 

 soapstone layer, immediately beneath the base of the Laurel 

 limestone, in Decatur and Franklin Counties. It is this upper 

 Osgood clay or soapstone horizon which is most readily identi- 

 fied, lithologically, in the Niagaran sections near Lewisburg, 

 Ludlow Falls, and Covington, Ohio, as forming a part of the 

 Osgood section in that area. Whatever rock occurs between 

 this clay horizon and the Euphemia dolomite is referred to the 

 Laurel limestone, and, in a similar manner, whatever rock 

 occurs between this clay horizon and the top of the Brassfield 

 Hmestone is included, along with the clay itself, in the Osgood 

 formation. Since there is little change in the thickness of the 

 Laurel limestone and of the Osgood clay in proceeding from 

 Lewisburg eastward to Ludlow Falls and Covington, it is prob- 

 able that these members of the Niagaran section extend farther 

 eastward, but no equivalent to the Laurel limestone is present 

 in the sections at Springfield, Yellow Springs, or Cedarville, and 

 the clayey section at those localities evidently is the northward 

 extension of the Crab Orchard clays of southern Ohio and 

 eastern Kentucky, and may not be the exact equivalent of the 

 Osgood clay of Indiana. Between Ludlow Falls and Covington, 

 on the west, and Springfield and Yellow Springs, on the east, no 

 clear exposures of the Niagaran strata immediately below the 

 Euphemia horizon are known. This evidently was the reason 

 why Prosser did not follow his Niagaran sections farther 

 eastward (Journal of Geology, 1916, pp. 334-365). 



In the more southern counties of Ohio, chiefly in Highland 

 and Adams Counties, the Niagaran section, in descending 

 order, is as follows: 



Cedarville dolomite, with sandy layers near top. 



West Union formation/ Vilify member 



[Bisher member 

 Crab Orchard clay shale 

 Dayton limestone 

 Brassfield limestone. 



