122 GEOLOGY OF OHIO. 
general rule, been changed to our beds of bituminous coal—to anthracite 
where local causes have carried the distillation further. In formations 
more modern than the Carboniferous, the accumulations of bituminous 
vegetation are, as before stated, usually classed as lignites, though they 
have been formed in the same way as our coals. These contain more 
water and oxygen, and are less valuable as fuels than the true coals, but 
they shade into them imperceptibly, and locally nature has accelerated her 
processes, and by volcanic heat has distilled lignites to anthracite, as at 
the Placer Mountain, New Mexico, and on Queen Charlotte’s Island, 
where excellent anthracite has been produced from Cretaceous lignites, 
and at Los Bronces, in Sonora, Triassic coal is converted into anthracite 
by a similar cause. In China there are extensive deposits of Mesozoic 
coal, which have been converted into good anthracite throughout consid- 
erable districts. At the present time we see the formation of coal only 
in its initial periods, viz., the growth of vegetation and the accumula- 
tion of bituminized vegetable tissue in marshes where oxidation is pre- 
vented or retarded by water. By artificial processes we can, however, 
hasten the changes in vegetable matter, and by distillation produce lig- 
nite, bituminous coal, and anthracite. In eastern America all the coal 
strata, excepting the small Triassic basins of Virginia and North Caro- 
lina, are of Carboniferous age. In the valley of the Mississippi, where 
they have suffered no local metamorphosis, they are all of the bitumin- 
ous class. In the Alleghanies the same strata, having been somewhat 
affected by the causes which resulted in the upheaval of the mountains, 
have lost a portion of their volatile matter, and have become what are 
known as semi-bituminous coals. To this group belong the coals of 
Frostburgh, Broad Top, Blossburgh, etc. Still further east the Carbon- 
iferous strata are more metamorphosed, and all the coal of eastern Penn- 
sylvania is anthracite. In Rhode Island a coal basin of limited extent 
of the same age with those of Pennsylvania seems to have been still 
nearer the focus of metamorphic action, and here the coal is partially 
converted into graphite, forming the variety known as graphitic anthra- 
cite. 
All the coals within the Ohio coal field are classed as bituminous coals, 
but among them we find those which form several different varieties 
when classified by their physical structure, their chemical composition, 
and their uses in the arts. These are, first, the dry, open-burning, or 
furnace coals; second, cementing, or coking coals; third, cannel coals. 
Of these the first and second varieties are sometimes classed as cubical, or 
block coals, from their tendency to break into more or less cubical blocks. 
The first variety enumerated includes those that do not coke and ad- 
