110 Phi/sical Theories of the Earth — Rsade 
section of the earth's surface, mean a radial shortening of about 
14 miles. The time taken in tlie elevation of the Appalachian 
chain, occuring as it did within the limits already mentioned, 
can only have been a fraction of the geological history of the 
globe; we are thus placed in the dilemma of invoking a sort of 
geological Frankenstein, for the total contraction of the globe 
since the dawn of geological time, on this estimate, is too tre- 
mendous to be admitted by any physicist. According to Sir 
W. Thomson, U. Darwin and other eminent authorities, the 
cooling of our earth cannot now have extended deeper than 
about 400 miles, so tlmt the total radial contraction, on the 
most favorable assumptions, cannot have been more than from 
10 to 15 miles since the tirst sedimentary beds were laid down. 
If therefore the earth's contraction is to be considered the cause 
of this estimated shortening of the Ap[)alachians, we await the 
discovery of some contracting agent, other than loss of heat. 
Having surmounted, if weare able, all these preliminary dif- 
ficulties, Ave are met with another of a different class. All 
mountain chains so far as known are composed of enormous 
thicknesses of sedimentary rocks; a fact first pointed out by an 
American geologist. Why should the earth in contracting 
select these particular areas for the compression of its crust and 
the piling up of its surplus material? If sediments are thrown 
down only on weak places in the crust, as some are fain to be- 
lieve, the compression would be continuous during the time of 
sedimentation. The exact opposite is the case. It has been 
attempted, in explanation of this awkward fact, to show that 
the rocks below have been weakened by sinking down into 
zones of higher temperature. This explanation is, it appears 
to me, altogether to pretty and complete; an individual case 
might l>e admitted but to assume that these several events 
always take place together, without exception, is rather a draft, 
on one's scientific faith. But to what extent would the crust 
really be weakened? The sinking rocks though increasing in 
temperature would be as strong as those replaced at the same 
temperature; so that the supposed weakness would only arise 
from the replacement of the original surface rocks by the new 
sediments. These sediments while sinking are undergoing con- 
solidation by pressure, chemical reactions and increasing tem- 
perature; so that, for ail we know to the contrary, they 
