Pennsylvania Geology Final Report Volume 1 1981

14 GEOLOGICAL SUKVET OF PENNSYLVANIA.
the human remains was found to be about seven thousand
years. But in both these cases, which are exceptionally
favorable for estimating the time of the occupation of the
mounds by man, no knowledge is obtained respecting the
far greater lapse of time previous to their iirst occupation
during which the two rivers mentioned had been doing the
same work in the same Avay, work the beginning of which
goes back into the last geological age.
A bridge was built by the Romans from one vertical wall
to the other of a deep and narrow ravine in the center of
France. For unknown ages a river of Auvergne had been
working its channel down through a lava bed, undermining
and throwing down one after another of its basaltic
columns, grinding them up into black mud and delivering
the mud to the Loire to be deposited in the Bay of Biscay.
The Roman bridge is broken, but its arches still cling to
the walls of the chasm, showing that this lias not been sensibly
widened in two thousand years. A flagrant proof of
the extreme slowness with which the erosion of the surface
of the earth has ever been going on and we ; may turn from
the basaltic columns in Auvergne to the great canon of
Colorado or any of the gaps in the mountains of middle
Pennsylvania with a sentiment of profoundest conviction
for their vast antiquity. The process of destruction is
evident ;
it goes on before our eyes daily and annually; but
unless we have a sound conviction of its infinite sloio-
ness we shall fall into the popular superstition which prattles
about convulsions of nature which never occurred, and fails
to realize the true character of the events which the geologist
has to describe.
The lesson of geological antiquity is taught with equal
clearness by the series of volcanic eruptions which mark
the whole history of the earth from the beginning to the
present day; and although evidence of the exercises of the
eruptive forces on an exceptionally grand scale at certain
times is not to be mistaken, corresponding to the greater
and the more widespread earthquakes which have some-
times varied the importance of calamities in human history,
they cannot be considered in any other light than as excep-