26 L. Lesquereux on the Coal Formations of North America. 
had been expanded without alteration through the whole extent 
of a coal basin, nothing would be easier than to fix at once the 
geological horizon of each bed of coal after the close study of a 
single section. The shales above the coal give by their fossils 
the only reliable data; but in many places they (the shales) are 
entirely wanting and are replaced by sandstone or limestone. 
In the western coal-fields of Kentucky, the first coal below the 
Mahoning sandstone, or the fourth coal above the conglomerates 
(the same as the Pomeroy coal of Ohio or the upper Freeport 
coal of Pennsylvania) whose shales sometimes reach in the 
a thickness of 10 feet, is immediately covered by the sandstone. 
There is scarcely a vein of coal worked to any great extent, that 
does not show a great diversity in the thickness, density and 
color of its roof shales. Hence the necessity of roofing differ- 
ently the tunnel of a mine in different places according to the 
nature of the shales. The bottom clay is almost always present; 
but its thickness, color and density are also variable. The lime- 
stone of the coal is the most irregular of all the formations. It 
is mostly local, sometimes only in boulders, and its numerous 
variations in thickness, composition and even fossils, cannot be 
accounted for by any satisfactory general rule. There is not in 
the United States a single bed of coal that is unvariably covered 
with limestone. The sandstone is generally extended with more 
regularity; but it has also its diversities of thickness and local 
disappearance. The only bed of sandstone which appears to be 
continuous in the whole extent of the coal-fields above the con- 
- glomerates, is the Mahoning sandstone. Though its thickness 
is also somewhat variable, it is found topping the 4th coal (coal 
E of Lesley’s Manual) from the anthracite basin of 
Pennsylvania to the western extremity of the coal-fields of Lli- 
nois and Western Kentucky. The Anvil-rock sandstone, top- 
ping the 12th coal of Western Kentucky, though ean 
great thickness, has not as yet been identified in the Hast. For 
the coal itself, the assertion of its continuity could be admitted 
as nearly true. ; 
hough a coal bed cannot be called a continuous sheet in 118 
(the. omeroy coal) is seen to have the same extent with scarcely 
ange in its thickn The Pittsburg coal which from its 
ore 
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