324 R. M. Deeley—Isostasy. 
of less density than those beneath lowlands and seas, and it became 
clear that the elevated regions were sustained by the buoyancy of the 
lighter rocks beneath them. The theory that the earth’s crust was 
in isostatic equilibrium thus arose. 
Careful measurements have shown that a perfect condition of 
isostatic equilibrium does not exist. Beneath many mountain 
ranges the deep-seated rocks are often sufficiently ight in character to 
support mountains of greater elevation than exist above them, whilst 
in other cases the rocks are not sufficiently dense to support the 
weights above. In many cases these departures from perfect isostasy 
are very marked and present considerable theoretical difficulties. 
Barrell, for instance, concludes that ‘‘Isostasy ....is nearly 
perfect, or is very imperfect, or even non-existent, according to the 
size and relief of the area considered ”’. 
A very interesting suggestion of Barrell’s is that the more or less 
rigid rocks of the lithosphere are separated from the rigid materials of 
the nucleus by a region where the rocks are very soft and allow the 
lithosphere above to float on it. It is taken to extend from a depth 
of about 200 km. to 400 km. or more. 
Barrell suggests that the reason why the condition of isostatic 
equilibrium is not more perfect than it is, is that the rocks of the 
lithosphere are so immensely strong that they are able to act as a 
girder and support the local excess weights in this way. In support 
of this he calculates that at a depth of 25 km. the rocks are five times 
as strong as they are at the surface. It seems very doubtful, 
however, whether the crust could support in this way some of the 
immense departures from isostatic equilibrium that are known. 
Perhaps the difficulty may be explained in another way. In 
a mountainous area the action of denuding agents keeps reducing the 
height of the mountains, and the material thus removed is spread over 
the neighbouring sea-floor. For a condition of isostatic equilibrium 
to persist such mountains should rise and the sea-floor be depressed. . 
For such a change of level to take place there must be a flow of rock 
in the region of the asthenosphere from below the sea towards the 
mountains. Now if the rocks of the asthenosphere were liquid, but 
very viscous, such a flow could readily take place until isostatic 
equilibrium was reached. On the other hand, if the rocks were 
plastic, and very soft, such a flow would not be able to occur until 
the stresses reached some particular magnitude depending upon the 
plasticity of the rock. In the case of a “liquid the rate of shear is 
always exactly proportional to the stress producing it; but in the 
case of a plastic body such flow cannot take place until the stress 
reaches some particular magnitude depending upon the degree of 
plasticity of the rock. It thus arises that although the asthenosphere 
may be of very considerable thickness, the resistance to flow would 
be very great if the movement had to effect a portion of it, especially 
if a thousand kilometres or more long. From this it will be seen 
that perfect isostatic equilibrium is only possible when the astheno- 
sphere is in a liquid condition. 
It is probable that in places where the relief is considerable, and 
the condition of isostatic equilibrium approaches perfection, the 
