224 



JOSEPH BARRELL 



density will be decreased to 0.053 that of water. The data may 

 then be tabulated as follows: 



TABLE XV 

 Computed Anomalies Due to Densities of Surface Formations 



Deficiencies or 

 Excesses of Density 



Unit mass . . . . 

 Cenozoic 



Pre-Cambrian 



Anomalies in Dynes 



per Gram Due to 

 Thickness of 5 ,000 Ft. 



0.0034 



— 0.012 



— 0.006 

 +0.005 

 +0.008 



These mean anomalies of the pre-Cambrian due to the greater 

 density of the outer 5,000 ft. of rock, when compared to the Cenozoic 

 anomalies, are, as shown by this tabulation, at a minimum o.oii 

 greater, at a maximum 0.020 greater, at a mean 0.0155 greater. 

 The difference of the means shown by geodetic measurement was 

 0.024. The specific gravities seem to have been taken as far 

 apart in limits as is allowable and the assumed mean thickness of 

 sediments as 5,000 ft. beneath the Cenozoic stations is a generous 

 figure; the mean thickness is more likely to be less, rather than 

 greater. The means for the geodetic anomalies as related to 

 geologic formations are perhaps subject to about the same degree 

 of error as the determinations of the anomalies from the specific 

 gravities and thickness. The result, although not of a high order 

 of accuracy, shows that although the range in specific gravities 

 accounts for a considerable part, perhaps one-half or two-thirds, 

 of the relation of anomalies to geologic formations, it can hardly 

 account for the whole. 



To find the cause for the remaining portion of the anomaly, two 

 hypotheses may be considered: first, that it is due to a slight 

 regional excess of density extending to a depth of 114 km., the 

 'hypothesis favored by Bowie; or, second, that the Archean areas 

 on the average stand higher than the Cenozoic by virtue of resistance 

 to erosion. 



The geologic evidence as it is at present understood is against 

 the first hypothesis and in favor of the second. This statement 



