260 > Pennsylvania before and after the Elevation  [March, 
thrown and probably at most exposures approaching or exceed- 
ing a dip of 45°. Over this distance I propose to assume an 
average dip of 40°, which to me seems fairly to represent the 
facts and to be rather below than above the truth. A simple cal- 
culation then proves that these twenty-nine miles of strata, if flat- 
tened out, would measure about thirty-eight miles. 
If from the accompanying section the distances representing 
these twenty-nine miles are taken out and measured by the scale, 
a result in close agreement with the above will be obtained. The 
two confirm each other. 
In regard to the Cumberland valley the problem is more diffi- 
cult. South of the Blue mountain the structure of the country 
is very different. Here the Medina sandstone is lost, having been 
entirely removed by erosion, and we are consequently compelled 
to adopt another stratum. The valley consists entirely of Cam- 
bro-Silurian limestone and slate dipping steeply, and the latter 
showing strongly developed cleavage. On the north-western 
sides of the arches the strata are usually inverted. It is evident 
at once that compression has here been much greater and the 
tangential compressing force much more intense than along the 
other portion of the line. Professor Rogers says in the Geologi- 
cal Survey of Pennsylvania (Vol. 1, p. 240) : 
“ The dip of the south-eastern leg of the arch is from 45° to 
60°, while that of the north-western inverted side is from 60° to 
80°.” “The formations repeat themselves in several narrow, 
short and parallel anticlinal and synclinal outcrops. They are 
- folded into a very uniform steep south-eastern dip in a series of 
compressed flexures, readily discernible on the Cumberland Val- 
ley Railroad. Along the south-eastern margin of the valley so 
general is the inversion of the folded limestone that this forma- 
tion appears everywhere to dip under the primal rocks of the 
South mountains, and had we not fully established their true 
position we might imagine the former to be the lower or older 
group ” (p. 1113). 
Adopting Professor Rogers’s figures, it is clear that the results 
obtainable from the Cumberland valley must far exceed in pro- 
portion those above given from the counties lying farther to the 
north-west. The dips above stated show that the inverted sides 
of the arches altogether underlie the others, and the measure- 
ments obtained for them must therefore be added to those found 
for the south-eastern or uninverted sides. The latter will be first 
