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due to lateral pressure owing to the contraction of the heated 
interior and consequent wrinkling of the crust to accommodate 
it to the diminished nucleus. 
Let ABCD be a layer of rock of unit of width, length J, 
and depth & And suppose the abutments at AC and BD to 
approach each other through the space le, where e is a small 
fraction. Then the layer of rock in question would take some 
new form, as one of those given in the figure, or any other 
whatsoever possible. 
Call AB “The datum level.” Let a, a, &c., be the areas 
formed by the upper curved line above AB, and 5, b, &c., the 
areas formed by the same line below AB. 
In like manner let a, @ be similar areas for the lower 
datum level CD. Then the space included between the curved 
lines must be equal to 
Ab Cd=kl (1 + e). 
It is also evidently equal to 
ABCD+a+a+&.+8+8+&e. 
—b—b—&.-—a—a—&e. 
or, denoting the sums of the quantities similarly situated by 
the symbol &, we get 
pl (1 +e) =k + 3 (a) —& (2) + 5 (6) —> (4). 
- kle= (@ —& (b+) +5 (8) - 5 ©) (1). 
