1885. ] of the Appalachian Mountains. 263 
as a snow-plough in front of the mighty engine, and in their turn 
communicated the movement and the crumpling to the north- 
western country beyond them. 
It is possible that considerable correction might be made in 
these figures if more accurate details of structure were attainable. 
This correction might be in either direction. That it would not 
all tend to diminish the result seems clear from the following 
considerations : 
(1) No account is taken of the condensation of the strata 
during compression. 
(2) The line of section does not pass through the most dis- 
torted district. 
(3) All effects of compression in and beyond the South mount- 
ain and west of the Bald Eagle range are disregarded. 
(4) The summits of the arches and the bottoms of the troughs ~ 
are assumed to be, but are not quite flat. 
(5) No account is taken of numerous intermediate minor folds 
or of several faults, at some of which the older strata to the 
south-east have been shoved uphill over the edge of the newer 
north-western beds at an angle which observation does not yet 
enable us to determine. 
On the other hand the height of the creases in the Cumber- 
berland valley may be somewhat less than that shown in the dia- 
gram and assumed in the calculation. All these minor considera- 
tions cannot affect the main point at issue. The figures above 
given indicate a compression of the superficial layer of the crust 
in Pennsylvania during the process of crumpling by which eighty- 
eight miles of the surface have disappeared, sixty-five miles at 
present being all that remain from a former breadth of 153 miles. 
Even if only one-half of these figures be taken into account, the 
problem remains equally difficult. How can such a shortening 
of the surface be accounted for? 
We are at no loss for a force competent to produce it. The 
tangential pressure developed by a contracting nucleus under a 
solid crust is amply sufficient to deform and corrugate that crust. 
But to realize the effect which this tangential pressure has pro- 
duced is less easy. ag ee 
A shortening of the circumference by eighty-eight miles is 
equivalent to a shortening of the diameter of the earth by about 
twenty-seven miles. Are we prepared to admit that the globe 
