THE CARBONIFEROUS SYSTEM. 139 



Coal No. 4. 



Throughout the greater part of the belt of outcrop of the Lower Coal 

 Measures in Ohio, at a distance varying from 20 to 90 feet above Coal 

 No. 3, another coal, another limestone, and another ore bed are found, 

 which are no less marked and constant features in the series than those 

 just described. The resemblance between the two groups is so close that 

 it is not always easy to discriminate between them, and this has led to 

 some errors in our earlier reports. 



The great variation in the interval which separates them has also led to 

 some difference of opinion, and has been considered by those who hold to 

 the theory of the strict parallelism of coal seams as proof of a want of con- 

 tinuity and identity in one or both groups. These strata have, however, 

 been traced with great care through many counties by several members 

 of the Corps who have had much experience as coal geologists, and they 

 are all agreed in regard to the relations of these limestone groups to 

 each other and to the associated strata. They also coincide with me in 

 the opinion that the kind of parallelism here shown is an argument 

 against the theory which has been referred to, rather than against the 

 continuity and identity of each group throughout the region it traverses. 



Where the interval between the limestones is considerable, two and 

 sometimes three coal seams are found between them. With the excep- 

 tion of Coal No. 4, these are generally quite local in their extent, and 

 rarely attain workable thickness. One of these may be seen in the 

 northern part of Tuscarawas county, between Dover and Mineral Point. 

 It occupies about the middle of the space between the limestones, has a 

 maximum thickness of about three feet, and is of inferior quality. In 

 the Eeport of Progress for 1870 this seam is described and enumerated as 

 Coal No. 4, but subsequent observation proved that it was so local and 

 unimportant that it was regarded as unworthy to be numbered as one of 

 our series of lower coals. It has, therefore, been designated in our later 

 reports as Coal 3a. 



In the valley of the Killbuck and that of the Tuscarawas the lime- 

 stones over Coals No. 3 and No. 4 may be traced almost continuously for 

 nearly 100 miles, where their relations to each other and the associated 

 rocks are so apparent that no one can mistake them. Here the distance 

 between the limestones is found to vary from twenty to ninety feet, af- 

 fording an excellent illustration of the local subsidences which took 

 place during the formation of our Coal Measures. 



Like the lower limestone seam, Coal No. 4 is exceedingly variable, 

 as regards both its quality and thickness. It is also prone to divide 

 into two or more benches, which are generally separated by fire-clay, 



