404 
ELEMENTS OF GEOLOGY. 
minous; but as we travel south-eastward, where they no long¬ 
er remain level and unbroken, the same seams become pro¬ 
gressively debitumenized in proportion as the rocks become 
more bent and distorted. At first, on the Ohio River, the 
proportion of hydrogen, oxygen, and other volatile matters 
ranges from forty to fifty per cent. Eastward of this line, 
on the Monongahela, it still approaches forty per cent., where 
the strata begin to experience some gentle fiexures. On en¬ 
tering the Alleghany Mountains, where the distinct anticlinal 
axes begin to show themselves, but before the dislocations 
are considerable, the volatile matter is generally in the pro¬ 
portion of eighteen or twenty per cent. At length, when we 
arrive at some insulated coal-fields associated with the bold¬ 
est flexures of the Appalachian chain, where the strata have 
been actually turned over, as near Pottsville, we find the 
coal to contain only from six per cent, of volatile matter, thus 
becoming a genuine anthracite. 
Clay-ironstone. —Bands and nodules of clay-ironstone are 
common in coal-measures, and are formed, says Sir H. De la 
Beche, of carbonate of iron mingled mechanically with earthy 
matter, like that constituting the shales. Mr. Hunt, of the 
Museum of Practical Geology, instituted a series of experi¬ 
ments to illustrate the production of this substance, and found 
that decomposing vegetable matter, such as would be dis¬ 
tributed through all coal strata, prevented the further oxida^ 
tion of the proto-salts of iron, and converted the peroxide 
into protoxide by taking a portion of its oxygen to form car¬ 
bonic acid. Such carbonic acid, meeting with the protoxide 
of iron in solution, would unite with it and form a carbon¬ 
ate of iron; and this mingling with fine mud, when the ex¬ 
cess of carbonic acid was removed, might form beds or nod¬ 
ules of argillaceous ironstone.'^ 
Intercalated Marine Beds in Coal. —Both in the coal-fields 
of Europe and America the association of fresh, brackish- 
water, and marine strata with coal-seams of terrestrial origin 
is frequently recognized. Thus, for example, a deposit near 
Shrewsbury, probably formed in brackish water, has been de¬ 
scribed by Sir R. Murchison as the youngest member of the 
coal-measures of that district, at the point where they are in 
contact with the overlying Permian group. It consists of 
shales and sandstones about 150 feet thick, with coal and 
traces of plants ; including a bed of limestone varying from 
two to nine feet in thickness, which is cellular, and resembles 
some lacustrine limestones of France and Germany. It has 
been traced for 30 miles in a straight line, and can be recog- 
* Memoirs of Geol. Survey, pp. 51, 255, etc. 
