110. & F Peokham-—The Origin. of Bitumons. 
If these distillates are re-distilled, the second distillate may be 
divided into several different materials, beginning with marsh- 
gas and ending with very dense oils heavily charged with 
paraffine. It is impossible to conduct this primary or secon- 
~ dary distillation without producing marsh-gas, but the amount 
of this gas and the density of the fluid produced will depend on 
the temperature at which the distillation is carried on and the 
rapidity of the process. The use of superheated steam is 
found to increase the quantity of distillate and to prevent over- 
heating, and the formation of other hydrocarbons than those 
belonging to the paraffine series. 
The section compiled by Mr. J. F. Carll, for Report III of 
the Reports of the Second Geological Survey of Pennsylvania, 
shows the Devonian shales above the Corniferous limestone 
and below the Bradford third oil-sand 1600 feet in thickness. 
This shale outcrops along lake Erie, between Buffalo, New 
York, and Cieveland, Ohio, where it is for the most part the 
surface rock in the neighborhood of Erie and southward to 
Union City, Pennsylvania. No one can examine this shale 
without noticing the immense quantity of fucoidal remains 
that it contains. N. S. Shaler estimates the Devonian black 
shale of Kentucky to cover 18,000 square miles at an average 
depth of 100 feet, and to yield on distillation fifteen per cent of 
‘fluid distillate. It is not necessary to follow him in his cal- 
culations of the enormous bulk of this distillate as represented 
in barrels; the important point in this connection, is that it is 
a very persistent formation, being revealed by borings over a 
very wide area, and doubtless extending eastward beyond the 
boundaries of Kentucky, beneath the coal-measures which con- 
tain the petroleum 
If, however, the Devonian black shales are inadequate both 
ville limestone and other Silurian rocks that underlie that sg 97 
( i ilu- 
rian limestone in the basin of middle Tennessee is about 1000 
feet thick. Including the Upper Silurian limestones, the 
whole thickness of the limestones, in which are found occa- 
sionally little pockets or geodes and cavities of petroleum, is 
not far from 1200 feet. The most of the petroleum has been 
found in the upper part (the Nashville) of the Lower Silurian, 
as, for example, the larger cavities near or on the upper Cum- 
berland River, in the neighborhood of the Kentucky line, both 
within Kentucky and Tennessee.” These limestones underlie 
the whole petroleum region of southeastern Kentucky and 
middle Tennessee. | 
The objection urged by Professor Andrews, that the coals in 
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