640 GEOLOGY OF OHIO. 
pact, light stone, while the Springfield division produces a less compact, 
more easily worked stone, and the top beds are almost universally con- 
verted into quicklime. 
The Helderberg or Water-lime rocks, which cover a large area, are 
almost without exception bituminous dolomites, but in character vary 
from dark to light and from compact to open or vesicular. The Cornif- 
erous limestones are most extensively quarried in and about Sandusky, 
and furnish one of the finest materials obtained in the state, while all of 
the overlying formations are almost devoid of building-stone quarries. 
As regards composition, the stones from these various formations vary 
from almost typical limestones to almost typical dolomites, and there seem 
to be no rules which will enable one to decide upon the quality or dura- 
bility of the stone from its composition. Experience also demonstrates 
that the composition, as regards the proportion of lime and magnesia, 
does not determine the value of the stone as material for the prodution 
of quicklime, and it would therefore appear that the value of the stone is 
more largely dependent upon its accessory constituents and its micro- 
scopic structure. 
There is a progressive increase in the amount of magnesia from the 
Lower Silurian limestones to the Corniferous. The Cincinnati lime- 
stones of the Lower Silurian contain from 1 to 5 per cent. of magnesian 
carbonate, while the Clinton limestones of the Upper Silurian contain 
on an average about 12 per cent. The Dayton limestone of the Niagara 
period contains about the same amount, while the upper divisions of the 
Niagara and the Helderberg formations are made up mainly of nearly 
typical dolomites. As regards composition the next following Corni- 
ferous limestones are very variable. At Bellefontaine the stone is a dolo- 
mite, and at Columbus it is as good a limestone, containing on an average 
93 to 95 per cent. of carbonate of lime, and the Hocking Valley furnaces 
are largely using it for a flux. 
In structure there is less diversity in the Ohio limestones than in 
those of some of the other states, since the odlitic and concretionary 
forms do not appear; but all other types are found, and therefore the 
greatest diversity exists in the ease with which stones may be worked. 
There are the open, porous varieties, and the varieties which once were 
open and porous, but which have been again partially consolidated by 
the filling of the pores; others in which the pores have been entirely 
filled ; and other varieties in which large crystals have developed them- 
