320 REID— THE DISTRIBUTION OF LAND 



logical speculations, such as a land connection between Africa and 

 Brazil in middle geologic times, and a similar connection across the 

 North Atlantic in Tertiary times. 



Some attempts have been made to explain the existence of oceans 

 and continents. Lowthian Green advanced the tetrahedral hypoth- 

 esis in 1875.'* The corners and edges of the tetrahedron are sup- 

 posed to be land areas, and the faces water areas. The advocates 

 of this hypothesis differ materially in locating the corners and the 

 edges ; and the dynamical arguments in favor of the tetrahedral 

 form are entirely unsound. 



In 1878 George Darwin^ suggested that under the tidal action 

 of the moon north-south wrinkles might develop, which would later, 

 under the same forces, have their equatorial portions pulled towards 

 the west. The general form of the continents conform but slightly 

 to this plan, and the geologic structure is largely at variance with it. 



The idea of an earth cooling and contracting goes back to the 

 time of Leibnitz. Dana^ suggested that the portions of the earth's 

 crust which solidified first would blanket the rock under them and 

 keep it warm ; these regions would become continents ; whereas, 

 violent convection in the still liquid regions would bring more heat 

 to the surface there and dissipate it, thus cooling these parts, and 

 causing them to contract more and become the ocean beds. 



Pratt's studies of the deflection of the vertical in India led him 

 to the conception now denoted by the name of isostasy ; he con- 

 sidered that the difference of density under the continents and 

 oceans was due to unequal contraction, but he did not assign any 

 cause of this inequality.'^ 



Faye,^ accepting Pratt's conclusions, ascribed the greater density 

 under the ocean to the lower temperature there. Taking the tem- 

 perature of the sea bottom at a depth of 4000 m. at 1° C., the mean 

 surface temperature of the land (at sea level) at 16°, and the tem- 



4 " Vestiges of a Molten Globe," Honolulu, 1875. 



5 Problems Connected with the Tides of a Viscous Spheroid," Proc. R. 

 S., 1878, XXVIII., 194-199, and Phil. Trans. R. S., 1879, CLVIL, 539-593- 



^ " On the Volcanoes of the Moon," Anicr. Jour. Set., 1846, II., 335-355. 

 ■^ " Figure of the Earth," 4th ed., 1871, pp. 201, 202. 



s " Sur les variations seulaires de la figure mathematique de la Terra," 

 C.R., 1880, XC, 1 189-91. 



