124 GEOLOGY OF OHIO. 



chiefly employed as household fuels, for which they are specially adapted, 

 and, in small portions, for enriching the gas from inferior varieties. 



The marked differences which are observable in the varieties of coal 

 which I have enumerated are due, as I conceive, mainly to the circum- 

 stances in which they were deposited. By Prof. Lesquereux they are 

 ascribed to differences in the character of the vegetation from which they 

 were formed ; but this can be accepted as only a very partial explana- 

 tion. Nearly all of our coal seams exhibit considerable variation of 

 quality at different localities and in different parts of the same bed. Our 

 cubical coals show changes in the relative quantities of volatile matter 

 and fixed carbon which they contain, and in their tendency to cement in 

 the fire ; they also sometimes merge into cannel, in part or entirely, in 

 passing from one township or county into another. But these differ- 

 ences, striking as they are, are not accompanied by any appreciable 

 change in the vegetation, so far as we can judge by examination of the 

 coal itself, or from the impressions of plants contained in the roof-stones 

 or fire-clays. It is possible that the open burning character which the 

 Briar Hill seam so generally exhibits may be in some degree due to the 

 kind of the vegetation from which it "was formed ; but this is a mere 

 conjecture, which derives no support from the plant remains found with 

 it. As has been already stated, the open burning coals have a distinctly 

 laminated structure which is recognizable at a glance. This is so char- 

 acteristic that it may be always accepted as proof that a coal which pos- 

 sesses it is not cementing, whatever its chemical composition may be. 

 The distinction between coking and open burning coals is evidently not 

 dependent upon the relative proportions of volatile matter and fixed car- 

 bon, since the semi-bituminous coals of Pennsylvania and Maryland, 

 which contain only from 17 to 20 per cent, of volatile matter, are emi- 

 nently coking, while the typical furnace coals, such as the Briar Hill 

 and Brazil, contain nearly twice as much volatile matter, and yet do not 

 coke. All the cubical coals are more or less laminated — i. e., exhibit 

 alternations of bright and dull lines. In the cementing coals the pitchy 

 layers are broad, and the lines of separation between them are thin and 

 broken ; hence these coals exhibit on their cleavage planes smooth sur- 

 faces of a black, pitchy appearance, by which an experienced eye can at 

 once identify them. The cause of the lamination of our coals is as yet 

 unknown, but I have supposed it possible that it was dependent upon 

 an annual contribution of vegetable debris, or a periodical variation in 

 the quantity of water in the coal marsh. This is an interesting subject, 

 but one which will be really understood only when it shall have received 

 more attention than has yet been given to it. 



