324 REID— DISTRIBUTION OF LAND AND WATER. 



where, combined with a squeezing out of some material from under 

 the seas. It is not clear how this expansion could have been main- 

 tained as the crust cooled. 



Chamberlin thinks that the earth is divided into six segments, 

 three in the northern hemisphere and three in the southern ; that 

 the edges of these segments would be squeezed up leaving the de- 

 pressed faces as the incipient ocean basins ; that a preponderance 

 of heavier planetesimal matter was deposited under areas of high 

 barometer; that there were three such areas in the southern hemi- 

 sphere, partly on account of the three segments, but also on account 

 of "the peculiar spacial requirements of a hemisphere."^- (It may 

 be remarked that this requirement of a hemisphere is not recog- 

 nized by mathematicians, geophysicists, or meteorologists, and there 

 are only two areas of high pressure in the northern hemisphere, 

 which also change with the seasons.) Chamberlin also thinks that 

 the heavier materials in the crust were carried by erosion to the 

 oceans leaving lighter materials on the continents; and the accumu- 

 lation of heavier material perpetuated and accentuated the ocean 

 basins. The hypotheses on which this explanation is based are far 

 too numerous to make it at all acceptable. 



In 1873 Dana looked upon the greater ,nsity under the ocean 

 as due to the character of the mineral ingredients there, but could 

 not account for their distribution.^^ Iddings has pointed out that 

 the rocks collected from the Pacific islands, have, on account of 

 their composition, a higher density than the rocks of the continents, 

 and, so far as our knowledge goes, fit in with the general principle 

 of isostasy.^* 



We may say, in closing, that the existence of a water hemisphere 

 and a land hemisphere is due to the non-coincidence of the center 

 of mass and the center of figure of the earth; that this is due to a 

 difference of density in the two hemispheres, probably confined to a 

 hundred miles or so of the surface ; and that this, in turn, is due, 

 not to unequal contraction or anything of that kind, but to a differ- 

 ence in the composition of the rock in the two areas, 



12 "The Origin of the Earth," Chap. VIIL, Chicago, 1916. 



13 Amer. Jour. Set., 1873, VI., 168-169. 



14 "The Problem of Volcanism," pp. 123-125. 



