THE STRENGTH OF THE EARTH'S CRUST 219 



probably better interpreted by o . 0030 as a divisor, since as a class 

 they must be assumed as due to excesses or deficiencies of mass 

 which are both near and large. This does not mean, however, that 

 the larger masses are not assumed as scattered uniformly, according 

 to the laws of chance, through the crust. It is seen, then, that 

 Hayford and Bowie have favored those interpretations which gave 

 a large anomaly per unit mass and have ascribed the total anomaly 

 as on the average to be interpreted on this basis, obtaining there- 

 by a smaller figure as the mean departure in feet from the level 

 for perfect compensation. They have not discussed, furthermore, 

 in the text the influence of deeper-seated variations of density, 

 which might give considerable residuals, nor the possibility that 

 departures from the mean density in opposite directions might 

 balance each other so as to give equal pressures at the bottom of the 

 zone of compensation. The latter case will not seem improbably 

 to the geologist. The great batholiths of the Archean appear to 

 make a universal floor in the crust. They range in composition, 

 from granites to gabbros and have come to rest at various levels. 

 Light and heavy masses may well be irregularly distributed in 

 the same vertical cylinder. If at the time of origin the whole 

 were too heavy, a tendency would have arisen for the column to 

 sink until equilibrium was attained. If the whole, on the con- 

 trary, were too light, the column would have tended to rise until 

 a heavier base balanced the lighter mass above. Thus, if irregular 

 distribution of density arose as the result of vertical igneous 

 intrusion, the whole region would tend to seek that level where 

 the irregularities would balance. 



In order to gain quantitative ideas as to this possibility of 

 partly explaining the anomalies, the writer has made calculations 

 on the following assumptions. A station is situated upon the axis 

 of a vertical cylinder extending from the station to a depth of 

 114km. The radius is taken successively at 58.8, 166.7, ^-nd 

 1,190 km. Let such a cyHnder be divided into five equal cylinders 

 by horizontal planes. Let each of the five be equivalent in mass 

 to a cylinder of the same radius but only 100 ft. in depth and of 

 density 2.67; in other words, the unit mass as used by Hayford 

 and Bowie. What will be the attraction in dynes per gram pro- 



