G('nexi'< of I roil- O rex. — JvniJxill. 357 
■wherever oolitic iron-(jres iire developed within the Clinton series, 
they are found to graduate into non-ferriferous limestones, more 
or less crinoidal. and usually in circumstances only moderate!}' 
favorable to weathering action. An equally significant fact is the 
iibsence of valuable iron-ores where the Clinton limestone, as in 
southern Ohio, is massive and unaccompanied by a considerable 
thickness of overlying shales. Wherever, on the other hand, the 
limestone occurs in numerous thin beds, and so alternates with 
more or less ferruginous shales: or again, wherever overtopped 
by shales, it seldom fails, especially in steep dips, to graduate 
unequally into oolitic hematite by replacement. Even in Ohio, 
where the Clinton grouj) is represented by a single but compara- 
tively thick limestone memlter under gentle dips, the upper por- 
tion of the limestone is sometimes replaced by hematite,* though 
■of no economic importance. Imperfect replacement likewise 
occurs where the limestone becomes shaly and expands in thick- 
ness. 
Non-ferriferous Clinton limestones, more or less magnesian, 
into which their associated iron-ores graduate, may ])e assumed 
to have been deposited in clear and moderately deep continental 
seas. That these seas were ramified by all ))ut insulated laud- 
surfaces is indicated by the :il)iindance of intercalated siliceous 
sediments from sub-aerial rock-decay. Tt is sometimes held that 
these limestones, and at least the oolitic hematites developed upon 
the .same horizons and passing into each cither, were necessarily 
deposited together. Vet direct ferric precipitation from ex- 
tremely instal)le natural solutions of ferrous salts cannot well be 
believed to have taken place so far from inland sources as where 
conditions existed favoral)le to the accuniuhdion of non-siliceous 
and expansive linu'stones. 
Again, notwithstanding the fact that the Clinton iron-ores merge 
into pureniaiinc limestones, liaA'ethev,on theother hand. sometimes 
been iissumed to afford i)roof of wide-spread marshes. A theory of 
this kind, however, is likewise opposed by the necessity of at- 
tinbuting expansive limestones of tlic Clinton type to mid-sea. 
and inferentially deeji-sea. deposition. And the ol)jection still 
stands that ferric hydrate in suspension, no moic than ferrous 
salts in solution, can have materially contril)ute(l to niarini' non- 
siliceous limestones. The conclusion therefore seems justified 
*Geol. Surv. of Ohio, vi, 1:*. 
