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—+. 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 isan interesting subject, 
but one which will be really understood only when it shall have received 
more attention than has yet been given to it. 
