GEOLOGICAL SCALE AND STRUCTURE. 11 



sion is somewhat obscure. The entire interval in such instances may 

 purest carbonate of lime in the state. Under cover to the northward it is 

 found more magnesian in composition, being indistinguishable from the 

 Niagara that overlies it. It also becomes shaly and changeable in char- 

 acter at many points, but in this respect it is growing more like the 

 formation as found in western New York. As it becomes shaly the 

 thickness of the series is much increased. 



It is everywhere uneven in its bedding in its outcrops, being in 

 striking contrast in this respect to the formations below it and also above 

 it. The beds are markedly lenticular in shape, and extend but a few ieet 

 in any direction. They seldom rise to two feet in thickness. 



The uneven bedding, the crystalline and crinoidal characters, the 

 high colors, and particularly the red bands, taken in connection with the 

 chemical composition, combine to make the Clinton limestone an exceed- 

 ingly well-marked stratum throughout southwestern Ohio, and from the 

 hints yielded by the drill in northwestern Ohio, it seems to have much 

 the same character there. It becomes more shaly and much thicker to 

 the eastward. Throughout the northern central and central portions of 

 the state it carries bands of red shale universally. 



The fossils of the formation by which it is most definitely character- 

 ized will be found described in a subsequent chapter of this report. 



The Clinton limestone is directly followed at a number of points in 

 the territory occupied by it in outcrop, by a bed of very fine-grained ; 

 bluish-white clay containing many fossils distributed through it, the 

 fossils being cr3 T stalline and apparently composed of pure carbonate of 

 lime. Some of them are characteristic of the formation elsewhere, while 

 others are known onty in this bed. A similar bed of white clay is 

 reported at the same horizon by the drillers in northern Ohio, and the 

 drillings show the presence of fossils of the same characters. This clay 

 seam can be appropriately designated as the Clinton clay, but it merges 

 into and is indistinguishable from the lowest element in the overlying 

 group. It has been named by Mr. Foerste the Beaverto.wn marl. The 

 Clinton, in its outcrops, is entirely confined to the southwestern part of 

 the state. 



t>. The Niagara Group. 



The Clinton limestone is followed in ascending order by the Niagara 

 group, a series of shales and limestones that has considerable thickness 

 in its outcrops, and that occupies not less than thirteen thousand square 

 miles of territory in Ohio as a surface rock. 



The lowest member is the Niagara shale, a mass of light colored 

 clays with many thin c^careous bands. It has a thickness of one hun- 

 dred feet in Adams county, but it is reduced rapidly as it is followed 

 northward, and in Clark and Montgomery counties it is not more than 

 ten to fifteen feet thick. Still further to the northward, as appears from 

 the records of recent drillings, the shale sometimes disappears entirely ; 



