166 GEOLOGY OF OHIO. 
The other theory to which I have referred is, that the coal seams lie 
in a series of basins of hmited extent, and that the identification of any 
one seam—except, perhaps, the Pittsburgh—throughout an area of sey- 
eral counties, is a stretch of the imagination. After somewhat extended 
observations in the Alleghany and Illinois coal fields, and careful com- 
parison of the reports made by others, I am led to believe that, as is so 
frequently the case with strongly opposed theories, the truth lies between 
the two. 
In tracing these different beds of coal from town to town and from county 
to county, they are seen to exhibit marked changes in their thickness, 
character, and relations to each other; and a section formed by the coal 
strata in one district is never quite the same as that furnished by an- 
other. Some of the seams are extremely local, occupying an area of 
perpaps not more than a few hundred acres, while others, like the Pitts- 
burgh and Nelsonville seams, underlie many thousand square miles. 
Whoever will take the trouble to examine the sections of the coal strata 
of western Pennsylvania, given by Prof. Rogers in the second volume of the 
Geology of Pennsylvania, and compare them with those now published, 
beginning at the east and passing to the west and south, will be forced 
to conclude either as I have claimed, that a skeleton or frame-work runs 
through the entire series, and that some of the strata are continuous 
over the greater part of the breadth of the north end of the Alleghany 
coal field, or that the sections taken at different points present a remark- 
able and incomprehensible series of coincidences. 
The classification of our coal strata has grown entirely out of our ex- 
perience. On first entering one of the valleys which traverse the coal 
area, the number, order, and characters of the coal seams, with their re- 
lations to each other and the associated strata, were learned as an inde- 
pendent lesson in local geology. In passing to another valley another 
series of outcrops was studied, and the differences and coincidences 
were compared. The system of sections now published is simply the 
record of observations made in the manner I have described. The 
classification of our coal seams, reported in the preceding pages, has been 
tested in various ways, and by different geologists, who have had much 
experience in this kind of work, and its general accuracy may be con- 
sidered as demonstrated. | | 
But all this shows simply the structure of the northern end of the Al- 
leghany coal basin. How far the central and southern portions of this 
ereat trough—750 miles in length—correspond with the northern end 
remains to be accurately determined by further investigation. The 
facts reported by Prof. Safford, and my own observations in Kentucky 
