292 JOSEPH BARRELL 



Where ages of uplift and erosion have followed periods of 

 igneous activity there are revealed great bodies of intrusive rock 

 varying in density from granites at 2.65 to gabbros at 3 . o. These 

 great batholiths are of irregular distribution in the crust, both 

 vertically and horizontally. Their abundance increases downward 

 so far as erosion has revealed the evidence. The outer crust of the 

 earth has become vertically and areally heterogeneous by such 

 means and should cause variations and irregularities to an appre- 

 ciable degree in the distribution of isostatic compensation, as noted 

 under the topic of the influence of variable rate of compensation 

 upon gravity anomalies. Here we note in addition the decreased 

 depth of compensation and decreased rigidity at the time of 

 intrusion. 



Hayford notes that the stations classified into geographic groups 

 show as a rule as great contradictions in depths of compensation 

 between adjacent groups as in those which are far apart. This 

 variation between adjacent groups is taken by him as weakening 

 the evidence that there is any real variation in the depth of 

 compensation over the whole area investigated.^ For the reasons 

 outKned previously, the present writer, influenced by the geologic 

 inferences, does not view such irregularity of distribution as proof 

 that the evidence is weak and conflicting. The strength of the 

 evidence must be judged rather by the nature of the residuals. 

 Hayford points out that the depth of compensation in the West 

 seems on the whole to be somewhat less than in other parts of the 

 United States, though he does not regard it as safe to assert that it 

 does exist. On dividing the whole area into four sections, the 

 minimum sum of the squares of the residuals indicates depths as 

 follows as best satisfying the hypothesis of uniform distribution of 

 compensation : 



From all residuals of the central group, 174 km. 

 From all residuals of the northeastern group, 187 km. 

 From all residuals of the southeastern group, indeterminate. 

 From all residuals of the western group, 107 km.^ 



In the 1909 paper Hayford gives a tabulation of the residuals for 

 fourteen geographic groups. The results for the United States as a 

 ' 1909, pp. 58, 59. = 1906, pp. 142-46. 



