232 The American Geologist. April, i890 
now to reproduce the hydrography of Pennsylvania as then 
existing with any degree of certainty. The Appalachian Rev- 
olution, as Prof. LeConte has happily termed the great change 
next to be described,- was at hand and its iconoclastic zeal 
entirely destroyed nearly all traces of pre-existing conditions 
When it was over, Pennsylvania started on a new career of 
existence. 
The Revolution was on this wise : The Earth-force which 
in earlier days had shown signs of its intensity again man- 
ifested itself. The vast system of low lands, marshes, pools 
and rivers that we have just described and the massive 
palaeozoic sediments on which they rested, many thousands of 
feet in thickness, began slowly to yield to its energy. This 
force, whatever its nature, was exerted from the southeast and 
in a horizontal direction. But it was irresistible. Before it 
the solid and massive palaeozoic deposits from three to seven 
miles in thickness, yielded as a spread table cloth before a 
pressing hand and rose in gigantic waves or sank in huge fur- 
rows at right angles to the direction of pressure. Stiff as 
these beds were their stiffness availed them nothing ; heavy 
as they were their weight was as that of a feather before the 
incalculable Earth-force. Their stiffness and their weight 
were as vanishing quantities when compared with the thrust 
that came to bear upon them. Arch behind arch slowly arose, 
trough after trough as slowly subsided until the whole central 
part of the state, previously flat and monotonous, became one 
complicated system of ridges and furrows where horizontality 
was almost unknown. The greatest intensity was felt in the 
southeast where consequently the arches are steepest. Grad- 
ually their abruptness grows less to the westward and their 
steep slopes flatten down until they disappear. So severe was 
the pressure at the place of onset that not a few of the anti- 
clinals are actually overthrown, their tops having been pushed 
beyond their bases so that both sides slope in the same direc- 
tion. Thus in the Cumberland valley the successive waves 
are so overthrown and pinched together that there is only one 
continuous dip varying from 50 to 80 degrees across the whole 
limestone outcrop between the South and the Blue mountains. 
The intense compression to which this part of the continent 
was subjected is sufficiently proved by the distorted condition 
of the strata. But another consequence is equally inevitable 
