Origin of Anthracite. — Keyes. 413 
bituminous coal was formed; along the edges, anthracite. 
According to the hypothesis advanced, the coal marshes must 
have skirted or fringed stationary or slowly rising shores, and 
the principal changes in the character of the plant masses 
are regarded as taking place before these were covered by sed- 
iments to any great extent. In consequence the vegetal 
deposits must have passed through practically no changes 
since the time of their original burial. 
In regard to the formation of coal marshes in general, 
nearly all evidence available goes to show that the plant re- 
mains accumulated along slowly sinking shores where the 
subsidence was interrupted somewhat from time to time. The 
swamps would thus be constantly advancing inland and not 
seaward, which last mightbe inferred from what has just been 
stated; they would gradually creep up the shores, instead of 
outward with the growing deltas. Closely following the peri- 
pheral swamp deposits, the beds formed in the more open sea 
would extend farther and farther landward, as the sinking 
took place, slowly covering the areas formerly occupied by 
coastal marshes. This appears to be clearly the case in the 
Central and Western Interior coal fields, w r here the strati- 
graphical arrangement is much more simple than in the more 
• astern districts. But in the fields of the Mississippi basin, in 
Iowa and Missouri especially, there are coal beds which ap- 
pear to have originated under very different physical condi- 
tions from those under which most of the coal seams of the 
region were formed. A notable example of this kind is the 
Mystic vein, which is the most extensive individual bed at 
present known in the entire field. Its extent and continuity 
have been definitely made out for upwards of fifty miles in 
one direction and over forty in another. The vegetal accu- 
mulations were made under conditions manifestly maritime. 
Evenly bedded limestones a few feet in thickness occur a short 
distance above and below the coal ; and other but thicker 
limestones exist seventeen and fifty feet above. Throughout 
the entire extent of the Mystic area there is a remarkable uni- 
formity in the thickness of the coal and its associate Strata. 
Still more remarkable is the character of the coal itself. Al- 
though a bituminous variety, it has the subconchoidal frac- 
ture, the bright luster, and the line uniform texture of 
