6 BULLETIN OF THE LABORATORI^^S 



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 inlecting 

 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 to 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 (iroup. 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 



