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THE CARBONIFEROUS SYSTEM. 173 
be done by art, since the sides of the fault are often beautifully polished, 
and present the glazed and striated surfaces which are technically called 
slickensides. Occasionally the slickensided faces are not in apposition, 
but a “clay seam” of greater or less thickness is interposed between 
them. 
The most considerable fault which has come to my knowledge in Ohio 
is one mentioned by Prof. Stevenson. This occurs in Coal No. 8, at Nefi’s 
Siding, in Belmont county. Here the down-throw is about three feet. 
The peculiar polished surface so often exhibited by the sides of the 
fissure in a fault which cuts through a soft and friable coal has excited 
considerable wonder and speculation among miners. The lamine of the 
coal are generally curved downward on one side and upward on the other, 
and they are blended together as though by the action of heat. The 
whole aspect of the slickensided surface is such as would naturally lead 
one to suppose that the coal had been fused along the line of fracture, 
and yet it is quite evident that it has been affected by no greater degree 
of heat than that excited by friction; and it is not even certain that 
heat has had any agency in producing slickensides in coal or other rocks. 
That a soft coal fractured without heat should not have been crushed and 
pulverized, appears at first sight somewhat remarkable; but it should 
be remembered that it has been held, as it were,/in an immense vise, and 
that the masses on the opposite side of the fracture have been pressed 
together with incalculable force. This has not only held the particles 
in close juxtaposition, but has condensed and compacted them. By the 
action of a powerful hydraulic press, many substances reduced to a fine 
powder can be rendered as dense and hard as stone or ivory. Such pres- 
sure, combined with motion, has, as I conceive, produced the polished 
surfaces called slickensides. 
Horsebacks—This name is somewhat vaguely applied to obstacles en- 
countered in mining. In coal mines it is generally used to designate a 
mass of rock which rises from the bottom, or (much more frequently) 
comes down from the roof and cuts out the coal. The “horsebacks,” 
which consist of swells of the bottom, usually represent knolls or ridges 
in the coal marsh, on which little or no peat accumulated. In some 
cases, also, the pressure of superincumbent rock seems to have partially 
squeezed out the softer carbonaceous sheet over a ridge or swell of resist- 
ant material underlying it. Such cases are, however, extremely rare. 
Nearly all the “horsebacks” met with in our coal mines have been pro- 
duced by currents of water, which have more or less completely cut away 
the coal, and have deposited in its place sand, afterward hardened into 
sandstone. When, however, the current which produced the excavation 
