Coal Strata of Pennsylvania. 27 
Derry in Westmoreland county, about fourteen miles — of 
Greensburg, (see this Journal, vol. xlix, p. 216.) The markings 
here are all upon the surface of a white coal grit or sandstone, 
forming a bare ledge of rock, no vegetation, scarcely even any 
mosses or lichens having grown on the pure quartzose stone. 
The ledge is about thirty five feet in its longest diameter, and 
about thirty two in breadth. It projects above the general level 
about three feet at one end, and slopes down gradually but irregu- 
larly till it is only an inch or two high at the lower extremity. 
There are several other similar but much smaller ledges of bare 
sandstone within a very short distance, on which there are no 
footprints. The large ledge is covered with distinct representa- 
tions, not only of the tracks of birds and dogs, but also of other 
animals. Those of birds in particular, are cut sharply and deep- 
ly, and there is one long series of steps in succession, as if a bird 
had walked from the lower to the upper end of the ledge, and 
then over thé end, so as. to leave on a steep slope inclined to the 
horizon at an angle of 22°, a clear and vivid imprint. I found it 
quite impossible to imagine the layers of sand when in a soft 
state or capable of receiving footprints, to have been ever so 
placed, as to admit of such an impression being made, and it is 
obvious that after the loose sand had consolidated and the ledge 
had acquired its present. form by denudation, no bird could have 
left the slightest trace of its passage over so hard arock. I may 
state indeed that I have never seen in Germany, England, or 
America, any impressions of the tracks of birds or quadrupeds in- 
dented on so coarse a sandstone as that of Derry, although casts 
in relief have frequently been taken in such a matrix, the sand 
having been poured into the hollows made in the soft mud on 
which the animals had trodden 
Another serious objection to the. supposed geological antiquity 
of the footprints, presented itself to me on my first view of this 
ledge near Derry. . The impressions are most of them extremely 
sharp, although the sandstone has not only been exposed for ages 
to the weather, but evidently acted upon by currents of water, 
_ which have shaped out channels and cavities of various depths, 
and some perfectly round, deep, and regularly formed pot-holes, 
: one of them eighteen inches deep and more than a foot in dis Os 
eter. Icounted on this single ledge no less than nineteen pot 
holes, between some of which, part of the principal series of bird 
travks passes. 
