VI PREFACE. 
Lower Coal Measures. In so far, therefore, as the title suggests a distinction be- 
tween general and economic geology, it is, to a certain extent, misleading. Any 
properly written account of the geology of a Coal Measure county would be in the 
main a chapter of economic geology, and a considerable part of the materials 
furnished in the three volumes of the geology of Ohio, already issued, would by 
a true classification fall under the same head. : 
The order of arrangement of the Coal Measures, involving the persistency and 
extent of the leading elements, I have counted strictly within the province of 
economic geology, and I have accordingly devoted a large space in the present 
volume to the interpretation of the order of the Lower Coal Measures. Coal seams 
and beds of iron-ore and fire-clay have characters of their own. In entering an 
undeveloped field, it is not enough to know that the openings made show coal, ore 
or clay of good volume and quality, but in order to warrant any safe forecast as to 
the persistency of these elements, we need to know the horizons to which they are 
severally to be referred. A good showing at one horizon would stand in a very 
different light from an equally good showing in another, asa basis for the invest- 
ment of capital. 
The best single service that the present volume can claim to have rendered is in 
the determination of the leading horizons throughout the entire coal field of the 
State. Several important changes have been made in the reading of the record. 
The Leetonia coal has been shown to be the Lower Kittanning seam or the first 
above the Ferriferous limestone. The Canfield cannel seam has been shown to 
belong to the Ferriferous limestone horizon. The Hammondsville Strip Vein has 
been shown to be the Middle Kittanning seam, and the Clay Vein coal of the Ohio 
Valley, the Lower Kittanning seam, or the first and second seams, respectively, above 
the Ferriferous limestone. The Steubenville Shaft coal has been referred to the 
Lower Freeport horizon; the Osnaburg coal of Stark county, and the Pike Run or 
Dennison coal of Tuscarawas county, which are the same seam, are shown to be 
Middle Kittanning and not Upper Freeport in age. The Carbondale coal of Athens 
county, has been so clearly proved to be the Nelsonville seam that the question of 
its age cannot longer be regarded as an open one. The Waterloo goal of Lawrence 
county has been shown to be the Upper Freeport seam and not the Middle Kittan- 
ning. The clay seam of East Liverpool and the Upper Ohio Valley has been 
proved to be the Kittanning clay and not the Lower Mercer. The Blackband ore 
of Stark and Tuscarawas counties has been proved to occupy the horizon of the 
Upper Freeport coal. The Putnam Hill limestone has been shcwn to be a com- 
panion seam to the Ferriferous limestone, and not this limestone itself, underlying 
it and almost alternating with it in its appearance, in the girdle of the old coal gulf. 
As a matter of course, a great number of other changes will follow those already 
named. It would be too much to expect that all of these changes should be at once 
adopted. The leading ones have already recommended themselves to those who 
are practically engaged in the development of the various fields, and the new order 
is proving itself a safe and certain guide in all recent explorations. In regard to its 
final acceptance I entertain no doubt. 
It is a matter of regret to me that I have been obliged to leave entire sections 
even of the Ohio coal field without any adequate notice in the present volume. The 
Barren Measures, 400 to 500 feet in thickness, and containing a number of valuable 
