166 GEOLOGY OF OHIO. 



The other theory to which I have referred is, that the coal seams lie 

 in a series of basins of limited extent, and that the identification of any- 

 one seam — except, perhaps, the Pittsburgh — throughout an area of sev- 

 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. Kogers 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, wil] 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 

 s ties of outcrops was studied, and the differences and coincidences 

 wjre compared. The system of sections now published is simply the 

 jecord 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 

 great trough— 750 miles in length— correspond with the northern end 

 letnains to be accurately determined by further investigation. The 

 facts reported by Prof. Safford, and my own observations in Kentucky 



