600 GEOLOGY OF OHIO. 



ending with that of Franklin county, comprises one less formation 

 than it. 



Marion county, to the northward, has a greater range, by a single for- 

 mation — the Niagara limestone occupying a small corner in the north- 

 western part of the county, while the Waverly sandstone appears upon 

 the eastern boundary. 



Pickaway county, to the south, and Delaware, to the north, exactly 

 agree with Franklin, in both the range and composition of their geolog- 

 ical columns. The following formations, named in descending order, are 

 found in Franklin county : 



4. Waverly group. 



3. Huron shale. 



2. Corniferous limestone. 



1. Lower Helberberg limestone, or waterlime. 



These formations represent three main divisions of geological time, as 

 is shown in the appended diagram : 



f Cleveland sbale. 

 Carboniferous Age — Waverly group -! Waverly sandstone. 



(_ Waverly shalt^s. 

 Devonian Age I Huron shales. 



( Corniferous limestone. 

 Upper Silurian Age — Waterlime group. 



These elements will now be briefly described, commencing with the 

 lowest : 



1. The Lower Helderberg limestone, or waterlime, is a late-found, but 

 important, member of the geological series of Ohio. Its presence in the 

 State was first recognized at the beginning of the present Geological Sur- 

 vey ; but as the structure of the State has been more accurately deter- 

 mined, it has been found to occupy a larger area than any other limestone. 

 It makes, however, but a very small contribution to the surface of Frank- 

 lin county, its outcrops being limited to a few points on Little Darby 

 and Big Darby creeks, on the extreme western border. The valleys of 

 both these streams now lie in this formation, for several miles above and 

 below Georgesville ; but, on account of the heavy drift deposits that occur 

 here, the rock is seen but at a few points. The best exposure is found 

 in the bed and bank of Big Darby, one mile below Georgesville, and in 

 the sections formed by small tributaries of the stream in the same 

 vicinity. It is also seen in the banks of the Little Darby, one mile 

 above Georgesville. The last nanied exposure is a continuation of the 

 outcrop below "West Jefferson, in Madison county. As both of the local- 

 ities named contain the same elements, the description of one will 

 answer for both. 



