THE OIL- AND GAS-PRODUCING ROCKS OF OHIO 823 



in the upper members, and wells are known that have had a 

 large initial production ; but it is a general rule that such wells 

 rapidly decrease, falling far short of their early promise. 



THE PRINCIPAL DIVISIONS OF THE GEOLOGICAL SCALE IN OHIO, AND THE 

 OIL- AND GAS-BEARING MEMBERS. 



Goose Run sand 

 Mitchell sand 

 Coal measures -{ First Cow Run sand 



Macksburg 500-foot sand 

 Second Cow Run sand 



Maxton sand 



Carboniferous 



r 



Lower 

 Carboniferous 



Logan group \ 



Berea grit 



Devonian 



Silurian 



Ordovician 



Mountain limestone 



f Keener sand 



Big Injun ; „. T . , 



° . ' < Big Iniun sand 

 series ° J 



[_ [_ Squaw sand 



Ohio shales 



Lower Helderberg sand 

 Clinton sand 

 Trenton limestone 



The Tre?ito?i limestofie. — Quite the reverse, however, with the 

 first member, known as the " Trenton limestone." Not only is 

 this the most important source of oil in Ohio, but probably it 

 is not excelled by any single formation in the world. It 

 forms the floor, so to speak, of the entire state, being found 

 wherever the drill penetrates to a sufficient depth. It is dis- 

 puted whether or not the formation reaches the surface any- 

 where, but Dr. Orton considered the exposure near Point Pleas- 

 ant in Clermont county as forming the top of the Trenton. 1 



The composition of the Trenton has been made the subject 

 of careful investigation by Orton, who found that the oil- and 



1 Geological Survey of Ohio, Vol. VII, p. 4. 



