326 ELEMENTARY GEOLOGY 
does not contract, because it is already cold, is slowly 
folded as the hot core shrinks from loss of heat. It 
is very much like the wrinkling of the surface of an 
apple, which upon drying, loses vapor from the pulp, 
which hence contracts, causing the less watery skin 
to pucker (Fig. 195). 
This theory appears to explain most of the phe- 
nomena of mountain folding. 
Granted that early in the 
history of the earth there were 
lines of weakness, due to one 
cause or another, these would 
again and again serve as places 
of folding as the outer rocks 
found it necessary to conform 
to the shrinking interior. At 
one time the area might be 
lowered, at another raised ; for 
Fra. 195. although the general level of 
Wrinkled surface of an apple 
cased Ue diol ECE the crust 1s always in process 
of lowering, locally, now and 
then, parts would be raised and folded higher than 
others. 
This would account for the frequent evidences of 
subsidence, and for the occasional elevation along 
these and other lines, which would occur whenever, 
in the constant shrinkage, it became necessary for 
