6 
BULLETIN OF THE LABORATORIES 
also quite distinct from the Dayton limestone beneath. But at Center- 
ville, and numerous other places throughout the state, there is quite a 
gradual change from one into the other. Whatever may have been 
the cause of the increase of the carbonate of magnesium, it seems to 
have originated outside of the special area here described. It began 
to operate in the Clinton Group. At Todd’s Fork the change was 
rather gradual, but at Huffman’s quarry, after dolomitisation elsewhere 
had became quite advanced, it seems to have come suddenly, remained 
long enough to deposit the thin Beavertown marl, and then to have 
retreated again, giving way to the formation of quite a pure limestone 
again. Meanwhile, on the outskirts of this area the dolomitic charac- 
ter was still maintained, and a second advance of the causes leading 
to dolomitisation took place, gradual at the outskirts, quite sudden at 
Huffman’s quarry. After this second advance of the cause which may 
have given rise to dolomitisation, it became permanent, the silicious 
elements which had accompanied its earlier advance became less, and 
the comparative amount of dolomite increased until in the Springfield 
and Guelph, or Cedarville strata, the rock had become a typical dol- 
omite. 
It is evident, therefore, that the problems as to the cause of the 
rapid succession of the limestones and shale or clays of the Lower Si- 
lurian is a different one from that of the cause of the insertion of 
the Beavertown marl and the Niagara shales in the Upper Siluri- 
an Group of Ohio. While those of the Upper Silurian were part 
of a general process of dolomitisation, more or less gradually infecting 
the sediments of the series, those of the Lower Silurian evidently are 
the result of a much more rapid and much more frequent alteration. 
Both of these processes remain still to be explained. Much has been 
written; it may be equally valuable to examine the question without 
any special theory in view. The chemical relations of the Clinton 
Group the rocks above and below can at least be well established. 
It remains to note a few points about the relative amount of iron 
present in the Clinton Group. In the two analyses from Huffman’s 
quarry noted “ light red ” and “dark red” it will be noticed how 
small a variation of the amount of iron present is sufficient to make a 
marked difference in appearance to the eye. The Camp Corwin speci- 
men is from the uppermost layers of the Clinton Group, at the locali- 
ty mentioned and is a picked specimen, the usual amount present in 
