PREFACE. vil 
elements, haye received no notice whatever. The Upper Coal Measures, 400 feet in 
thickness, which almost rival the Lower Measures in importance, are also passed 
without any methodical discussion, aside from a single short chapter devoted toa 
single coal seam. Even the lowermost portion of the Lower Coal Measures I have 
not found time to study in a systematic way, but, so far as this section is concerned, 
the facts of the present economic development in the only important fields are 
clearly and fully given. I can only say that I have done what I could with the time 
and means at my disposal. Being unable to cover the field, I was obliged to choose 
what portion I should occupy. I selected that portion of the Lower Coal Measures 
included between the Mercer horizon and the Mahoning sandstone as clearly the 
most important mineral-producing belt of the State. But even in this part of the 
scale, there are some dark corners still left, especially in Southern Ohio. Its main 
elements, however, are set in so plain an order that he who runs may read. 
The same explanations must cover, in the main, the omission of several other 
subjects that certainly deserve a place in a volume treating of the economic geology 
of the State. Among these must be named lime-production, a large and growing 
interest in Ohio, and which has been treated only incidentally here; cement-manu- 
facture, which, though at present developed to but small extent, is likely soon to 
attain much greater importance; the production of gypsum, which is, however, con- 
fined to one locality ; and, finally, the production of salt, bromine, petroleum and tn- 
flammable gas, substances which are in almost all cases associated in their appear- 
ance in Ohio. The manufacture of salt has long been carried on in the State ina 
very large way. 
A chapter has been prepared on this last-named group of substances, with 
special reference to the deep borings for natural gas in which so much interest is 
now taken and so much money expended in various sections of the State, but it is 
found impossible to include it here, for the reason that the volume has already grown 
beyond proper limits in size. 
This chapter, embodying as it does the results of all the more important ex- 
plorations for oil and especially the facts accumulated in the recent very extensive 
and expensive drillings for natural gas, is believed to be filled with timely and prac- 
tical information which can be made to do good service in directing, restricting or 
discouraging the expenditure of money in this novel and enticing quest. The chap- 
ter will be presented to the Legislature at its next session, with the request that it be 
published as a supplement to the volume now issued. 
The chemical work herein reported will be recognized by intelligent readers as 
one of the most important features of the present volume. The analyses are based 
on a different system of sampling irom any heretofore used in the geological work of 
the State, and, representing average instead of exceptional values, they bring to light 
the real characteristics of our several coal seams, and can be safely followed as prac- 
tical guides in the fields which they occupy. It is coming to be clearly recognized 
that the highest chemical skill is valueless in this line of investigation, unless the 
samples on which it is exercised are taken by some system which will make it cer- 
tain that they really and adequately represent the seams from which they are de- 
rived. 
Without the promise of Professor Lord’s codperation in this department, I 
should not have ventured upon the preparation of the volume, and without the 
