578 GEOLOGY, 



mobile element, should have gathered toward the poles, and the less 

 mobile solid earth should have protruded about the equator, but the 

 distribution of land and water, present and past, gives no clear evidence 

 of this. The equatorial belt contains a less percentage of land than 

 the area north of it and more than that south of it. It varies but 

 slightly from the average for the whole globe. 



While the doctrine of tidal retardation is theoretically sound, and 

 while the relations of the moon to the earth have probably been appre- 

 ciably affected by tidal action, geological evidence indicates that it 

 has not been sufficiently effective in producing crustal deformations 

 to be clearly detected by its own distinctive results. This may be 

 due (1) to the fact that there are compensating agencies that tend to 

 acceleration of rotation, and (2) to the probable fact that the central 

 rigidity of the earth is too high to give a very effective body-tide. Hence 

 the process of retardation may have been too slow to have been geologi- 

 cally appreciable in the known period. The recent estimates of the 

 effective rigidity of the earth are greater than former ones, and they 

 may need to be modified yet further in the same direction. 



Distribution of rigidity. — An important consideration in this con- 

 nection is the distribution of interior rigidity. It is certain that the 

 rigidity of the outermost part, taken as a mass, is somewhat less than 

 that of rock of an average surface type, for it is fissured, and there is 

 no reason to suppose that the rigidity of the rock next below the fissure 

 zone rises at once to the rigidity of steel, and hence if the average rigidity 

 of the whole earth is equal to that of steel, a portion of the interior 

 must have a rigidity much higher than steel. There is probably some 

 law of increase from surface to center, and there are theoretical grounds 

 for thinking that it is in some way connected with the laws of pressure, 

 density, compressibility, and temperature. All of these factors probably 

 affect rigidity, but in different ways. The modulus of rigidity of steel 

 is about 770 X 10^ grms. per sq. cm. Milne and Gray^ found that of 

 granite to be 128 X 10^ The ratio of the rigidity of steel to that of 

 rock is, therefore, about 6:1. If it be assumed that the rigidity 

 increases in depth directly as the density, the rigidity will nowhere 

 reach that of steel, being only about two-thirds as much at the center. 



1 Quar. Jour. Geol. Soc, Vol. 39, 1883, p. 140. Everett (Units and Physical Con- 

 stants) gives 837X10« for steel, but as the modulus for granite seems low, we have 

 taken the lower estimate for steel to avoid exaggerating the ratio between them. 



