ESTIMATES AND CAUSES OF CRUSTAL SHORTENING $7 



Calculating on the basis of a heterogeneous spheroid at the 

 beginning, i. e., upon the minimum change of pressures of 2^ 

 per cent., and assuming Laplace's laws of the relations of pres- 

 sures and densities, that "The variation in pressure in the 

 interior of the earth is proportional to the variation in the square 

 of the density" (see p. 75), Professor Slichter finds the surface 

 would be two-thirds of I per cent, greater than at present, or 

 1,700,000 square miles larger. 



Moreover, when the surface and volume were greater than 

 the present amounts, the effectiveness of gravity in producing 

 pressures would be less than assumed, because of the greater 

 size of the spheroid, so that the estimated enlargement of the 

 surface is short of the truth. However, it does not appear prac- 

 ticable to make a quantitative estimate of the value of this ele- 

 ment. 



Another estimate of the amount of surficial lessening as a 

 result of increased pressure may be made by a different line of 

 reasoning, as follows : The most probable conjecture which can 

 be made as to the average density which the material of the 

 earth would have if it could all be placed under conditions of 

 ordinary pressure and temperature is that obtained by Farring- 

 ton as the average specific gravity of meteoric falls, 3.69. 1 The 

 material of the crust probably does not represent the average 

 composition of the earth, for differentiation must have occurred 

 to some extent, upon any hypothesis as to the origin of the 

 earth. All inferences as to the composition of the interior of 

 the earth are based upon a considerable number of hypotheses, 

 none of which are verifiable. However, the meteoric falls, not 

 the finds, give us the density of the material which is now being 

 added to the earth. This is probably a better guide as to the 

 average composition of the earth than the average* of meteoric 

 finds, as suggested to me by Professor Chamberlin, because the 

 stony falls of the past have probably largely disintegrated. Of 

 course it is not certain that the material added at present to the 



1 The average specific gravity of meteorites, by O. C. Farrington : Journ. 

 Geol., Vol. V, 1897, pp. 126-130. 



