Tlic Pittsburg Coal Bed. — White, 53 
attained by the spread of Coal Measures, we shall probably 
never know to a certainty, but that there is no inherent im- 
probability in the hypothesis, will appear from the fact that 
the oldest member of the Carboniferous period, the very 
hard and erosion-resisting sandstones of the Pocono, with 
its included coal-beds, extends to the North Mountain re- 
gion at several points along that great ridge. Of course' if 
the Coal Measures ever covered an area as wide as this 
lowest member of the Carboniferous, the probabilities are 
that the area of the Pittsburg bed which has escaped erosion 
is only a fragment of its former extent. But however this 
may be, its entire area of workable coal remaining in the 
states of Pennsylvania, Ohio, West Virginia, and Maryland, 
does not probably exceed 6,000 or 7,000 square miles. 
Structure. Dr. J. J. Stevenson, of the University of New 
York, was the first geologist to make a detailed study of the 
Pittsburg coal bed, and to describe the peculiar structure 
which so distinctly characterizes it, that the coal seam may 
be thereby identified with great certainty over a wide area. 
In Report K, Second Geological Survey of Pennsylvania, 
he shows that a series of thin parting slates and clays sub- 
divide the bed into several definite members, which may be 
grouped as follows: 
"Roof" coals. 
"Over"-clay. 
"Breast" coal. 
Parting. 
*'Bearing-in" coal. 
Parting. 
''Brick" coal. 
Parting. 
"Bottom" coal. 
"The "roof" coals are a number of thin layers of coal 
(two to twelve inches each) separated by shales or clays of 
varying thickness. Some of the layers are good coal, while 
others contain much dirt and other impurities. Their num- 
ber ranges from one to eight, or even more, and their com- 
Ijined thickness seldom exceeds three and one-half to four 
feet, while the separating slates and clays may be only half as 
