ESTIMATES AND CAUSES OF CRUSTAL SHORTENING 59 



added pressure produces important deformation. In another 

 place (pp. 8-9) in this number of the Journal, Powell suggests 

 that the modulus of compressibility varies under different con- 

 ditions, and that so slight a change of pressure as that due to 

 unloading and loading by denudation, has caused important 

 expansion and compression. If this be so, so important a change 

 of pressure as results from change of the rotation period of the 

 earth might have produced a more important effect upon its 

 volume than would be obtained by supposing the modulus of 

 compression to remain the same under all pressures. 



It is not supposed that the numerical results given (pp. 56- 

 58) for surficial lessening of the earth due to increased pressure, 

 following upon lessened speed of rotation, approach exactness. 

 However, it is to be noted that the numbers obtained by two dif- 

 ferent methods are concordant, and moreover, that all of the 

 hypotheses used in obtaining these numbers have been so made 

 as to obtain minimum results rather than maximum, and they are 

 therefore probably much too small. It therefore appears highly 

 probable that crustal shortening resulting from increased pres- 

 sure as the speed of rotation of the earth has lessened, is one 

 of the chief causes for earth contraction. 



Change in physical condition. — Another cause of the earth's 

 contraction is the change in the physical condition of the mat- 

 ter of the earth's interior. In so far as liquid material has 

 changed to a solid amorphous material, this has produced con- 

 traction. Further, if liquid or solid amorphous material has 

 changed to a crystalline condition, this has resulted in more 

 important contraction. 1 This contraction is supposed to be due 

 to the closer arrangement of the molecules. According to 

 Delesse, 2 in passing from the crystalline to glassy state, granite 

 decreases in density 9 to 1 1 per cent., syenite 8 to 9 per cent., 

 diorite 6 to 8 per cent., dolerite 5 to 7 per cent., and trachyte 3 

 to 5 per cent. Barus has shown in the case of diabase, an 



1 So far as I am aware, Lyell was the first to suggest that deformation might 

 result from a change from a liquid to a crystalline condition. (Principles of geology, 

 by Charles Lyell : 10th ed., London, 1867, Vol I, pp. 134-135 ; Vol. II, p. 236.) 



2 See Manual of geology, by James D. Dana : 4th ed., 1895, p. 265. 



