478 
Van Rise — Earth Movements. 
pressure increased to the present amount, and important con 
raction may have resulted from the increased pressure. 
The surficial contraction due to change of oblateness and 
that due to increased pressure, may together be of equal and 
possibly of greater importance than that due to secular cooling. 
Important contraction has doubtless also resulted from a 
change in the physical condition of a part of the earth’s interior. 
In the change from liquid rock to a solid amorphous condition, 
a contraction occurs 1 . Further, more important contraction re¬ 
sults from a change from the glassy to a crystalline condition. 
In the direct change from a liquid to a crystalline condition 
the contraction equals the sum of the contractions of the two 
stages mentioned. In the case of one rock, the amount of this 
contraction, as shown by Barus, is as much as 13 per cent. 
Contraction to some small extent may also have resulted by 
a change from a less complex to a more complex molecular 
structure. If physical changes of these kinds have been ex¬ 
tensive during geological time, and this can hardly be doubted, 
this is an important cause for contraction. However, some of 
these physical changes may have been a part of the conse¬ 
quences of increased pressure and secular cooling, in which case 
these causes are not wholly independent. 
Finally, the water and air now upon the surface of the earth, 
and possibly also gas and water which have been lost to the 
earth 2 , may have been originally occluded deep within its inter¬ 
ior. The escape of this water and gas from the interior would 
result in contraction. 
The cumulative effects of these various causes for surficial 
contraction are possibly sufficient to explain all the observed phe¬ 
nomena of mountain-building, and if they are not, still other causes 
for contraction may be discovered in the future. Thus the objec¬ 
tions to the contract]onal theory of mountain-making have far 
less weight than they had when only a single cause, loss of heat 
due to secular cooling, was assigned for contraction. 
1 Manual of geology, by J. D. Dana: 4th ed., 1895, p. 265. 
2 A group of hypotheses bearing on climatic changes, by T. C. Cham¬ 
berlin: Journ. of Geol., Vol. V, 1897, pp. 653-683. 
