EFFECT OF THE GLACIAL SHEET 171 



tinents, to be in a state of very unstable equilibrium, easily 

 swayed up and down by forces which operate in the outer 

 part of the earth or upon its surface. Thus when the glacial 

 sheet was laid upon the northern lands the effect seems to 

 have been to bear down the surface of the earth on which it 

 was imposed, to a depth in a way proportionate to the thick- 

 ness of the ice. When the glaciers melted away the de- 

 pressed areas quickly recovered their position, and, as we 

 have noted above, rose even higher than they are at the 

 present time. The accumulations of sediment washed from 

 the land and deposited on the sea-floor appear also to bear 

 down the surfaces on which they rest, in substantially the 

 same way in which they are affected by the burden of a 

 glacial envelope. On the other hand the removal of weight 

 from the land by the action of rain-water, by which vast 

 quantities of material are taken away, appears to favor the 

 constant uprising of the land, so that, though endlessly 

 down-worn by the action of the elements, the continents 

 remain ever undestroyed. 



As the central parts of the earth are constantly shrinking 

 from the loss of heat, while the portion next the surface 

 remains unshrunken for the reason that it has lono- since 

 parted with the greater part of its caloric, the outer envelope 

 of rocks has constantly to wrinkle into the broad upcurves of 

 the continents and downcurves of the sea-floor in order to fit 

 itself to the diminished internal mass. The action may be in 

 a measure paralleled by that which takes place in an apple. 

 the skin of which wrinkles as the juice dries away from the 

 inner parts. The folding of the outer envelope is due, as we 

 readily perceive, to the fact that the interior of the sphere 

 shrinks much, while the outer part thereof does not contract 



