PREFACE. ix 



In several of the maps, and notably in Nos. 7 and 8, there has been a slight de- 

 parture from the general plan in the following particular, viz., the Kittanning coal 

 is indicated in areas where it has descended below drainage, but where one or both 

 of the seams are still known to exist. It would perhaps have been better to have 

 maintained the same system throughout the entire field; but persons of intelligence 

 enough to take interest in the maps will scarcely be misled by the features named. 

 All of the maps are drawn to one scale, viz., two miles to one inch, except maps 9 

 and 10. Among the advantages to be derived from them, the two named below are 

 especially prominent. 



1 . The continuity of the various seams,and their identity with welt known 

 seams of adjo.ning states is fully and finally demonstrated. Since the publication 

 of Volume V, iu 1S84, there has been, in fact, no adequate ground for doubt in re- 

 gard to this subject. The connections of the main seams were fully recognized and 

 they were continuously traced in a general way at that time ; so that no open minded 

 student of our geology has since that day, been in doubt as to the true order, but 

 the present maps, with their unbroken lines of outcrop, make the whole subject so 

 plain that even the wayfaring man can henceforth have no excuse for mistaking or 

 confusing their places in the system. The propriety of replacing the local and nu- 

 merical designations of our coal seams with the names first given to the same seams 

 in Pennsylvania, is fully established by these maps. The laws of scientific nomen- 

 clature do not allow us to multiply names unnecessarily for one and the same object. 

 A fossil must always be known by the first name given to it, in connection with ade- 

 quate description and publication. So, also, an identifiable stratum must retain the 

 designation under which it was first made known to the scientific world. This com- 

 mon law requires us to adopt the Pennsylvania designations of our coal seams, be- 

 cause the latter were first described in Pennsylvania. The Pittsburg and the Sharon 

 seams, especially the former, have always been known in Ohio by their Pennsyl- 

 vania names, but from lack of knowledge of the true equivalents of the remaining 

 seams, local names and worse than this, numerical designations that have not even 

 the merit that one and the same number is applied to one and the same seam in all 

 instances, have been fastened upon them, and thus the recognition of their identity 

 has been seriously obstructed. In conclusion, it can truly be said that these ques- 

 tions are permanently settled for both the practical man and the scientific student 

 by our maps. 



A few questions of subordinate interest remain as to the proper places in the 

 scale of certain seams in particular fields, but the large questions have passed 

 from debatable ground into demonstrated certainties. 



2. The second and by far the more important service that these maps are able 

 to render is found in the fact that they, for the first time, make it possible to deter- 

 mine the areas of otir several coal fields. The areas above drainage can be directly 

 measured, by cutting them out of the maps, with proper care, weighing the several 

 areas in a chemical balance, and comparing their weight with that of some standard 

 area ; or, better still, they can be determined by the use of the planimeter. Prof. C. 

 N. Brown has at my instance applied this instrument to the maps under considera- 

 tion, and the results which were thus attained will forthwith be stated. 



Our coal seams cannot, however, be supposed to terminate abruptly with their 

 disappearance below drainage. Some extension of them undercover is believed in 

 by everyone ; but very different judgments will be formed by different observers as 

 to the limits within which they can be reasonably expected. Theoretical views will 

 color our opinions on such a question. An estimate made by one who holds to the 

 essentially marginal character of our coal deposits will differ widely from an esti- 

 mate made by another who counts it entirely possible that all the seams of the Ohio 

 scale could be cut by a single shaft at the center of the field. Any measurement of 

 the acreage of our coal resources must, therefore, in this respect, be a matter of 



