264 Pennsylvania before and after the Elevation [March, 
actually contracted to that extent during the formation of the 
Appalachian earth-folds? If not, how shall the facts above stated 
be explained ? 
The suggestion has been made that the subsidence of the 
present bed of the Atlantic from a continental level in late Palæ- 
ozoic days may have supplied the necessary compressing force 
and have produced the shortening here pointed out. But the 
cause is totally inadequate. Supposing that the whole Atlantic 
area had subsided from the mean height of one mile above sea- 
level to five miles below it at’its middle or deepest point—a max- 
imum supposition—then a short calculation will prove that the 
new and flattened arc so produced, of 60° in extent, would meas- 
ure only about three miles less than the previously existing one. 
This amount, if wholly expended in crushing the thick Appa- 
lachian sediments, would evidently be far from sufficient for the 
solution of our problem. 
Without therefore denying the occurrence of such a subsi- 
dence, we can lay it out of the case as inadequate. 
Is it possible to believe that there was an accumulation of 
strain on the crust during preceding ages, which was relieved by 
the occurrence of this paroxysmal compression and corrugation. 
Such a supposition would meet with strong opponents in many 
quarters. 
Mathematicians tell us that the crust cannot endure the strain 
which would be caused by the shrinking away of the nucleus in 
consequence of contraction, but must close down at once upon 
the latter as it sinks. Some of them add that this would be the 
case were it many times as resistent as now. If so, any such . 
_ accumulation of strain is evidently impossible. 
_ But there is a possibility that the interior of the earth may be 
of such a nature as to allow of some considerable amount of 
shrinkage without leaving the crust completely unsupported. 
This partial support might enable contraction to proceed for some 
time before the closure of the crust upon the shrinking nucleus 
followed. A cellular structure about the place of junction be- 
> tween the cool and heated portion might render possible such a 
condition of things. Our ignorance of the earth’s interior is at 
present so dense that any supposition which does not clash with 
_ well established facts is admissible for the purpose of argument.’ 
= 1In this connection I may remark that since writing the above passage I find that 
