of the Earth 9 s Crust in Cooling. 589 



VII. On the causes of Isostasy. 



From a detailed consideration of the evidence for and 

 against the theory of isostasy, Professor Joseph Barrell * 

 has come to the conclusion that while the uppermost layers 

 of the crust of the earth are probably very strong, the 

 strength gradually diminishes with depth, so that at a 

 depth of about 400 kilometres, in the middle of the layer 

 of weakness, called the asthenosphere, the strength is only 

 about 2V °f wna t ^ is at the surface. In virtue of this 

 weakness a large area of excessive mass per unit area tends 

 to sink, while lighter areas rise. The smaller the dense 

 area is, the worse is the compensation, in consequence of 

 the fact that the maximum shearing stress occurs at a 

 smaller depth, and less stress is transmitted to the astheno- 

 sphere. Barrell considers that the adjustment takes place 

 by progressive local melting under strain, with subsequent 

 recrystallization when the tangential stresses have been 

 removed. Section VI. of the present paper may give an 

 additional important cause of the maintenance of isostasy. 



A further problem of extreme difficulty is that of deciding 

 how the continents and ocean basins ever came to be formed. 

 If there were no isostatic compensation, they might be 

 attributed to crumpling by compression, of a type not very 

 different from that which produces mountain ranges. It 

 appears, however, that continental areas, though varying 

 enormously in height and shape, are fairly permanent in 

 position, and that their height is really due to their being 

 made of lighter materials than the ocean bed. The question 

 is to decide how the lighter materials succeeded in being 

 collected to certain points, leaving the denser rocks exposed 

 in other parts. The most satisfactory explanation of any 

 widespread inequality in the shape of the crust of the earth 

 is the theory of Gravitational Instability, due to Jeans. 

 This theory shows that if the earth be regarded as com- 

 pressible, and any arbitrary displacements be given to its 

 constituent particles, the alteration in the potential energy 

 is of three parts : — 



(1) The elasticity of the materials causes it to be 



increased. 



(2) The normal displacements of matter at the boundary 



cause it to be increased. 



(3) The changes of density within the earth lead to 



changes in the gravitational potential which in 

 general cause the potential energy to be diminished. 



Journal of Geology/ vol. xxii. (1914) pp. 28, 145, 209, 289, 441 

 355, 729 ; vol. xxiii. (1915) pp. 27, " 

 present writer, ' Observatory,' April 1916 



537, 655, 729 ; vol. xxiii. (1915) pp. 27, 425, 499 ; or a review by the 



>ril I "" 



