158 JOSEPH BARRELL 



compensation of the topography to between limits of one square 

 mile and one square degree with the added suggestion of a radius less 

 than 18.8 km., which has been advanced on other pages by the 

 authors/ These figures merely show that, to the outer limit of 

 zone M, radius 58.8 km., and probably to outer limit of zone N, 

 radius 99 km., one method is as good as another for purposes of 

 computation, which is not true in nature. The errors introduced by 

 observation and computation, the errors introduced by the lack of 

 recognition necessary in the preHminary hypothesis regarding the 

 irregularities in the depth and distribution of compensation — -these 

 produce effects which overshadow the small systematic differences 

 due to the hypotheses of local versus regional compensation. For 

 the outer limit of zone O, radius of 166.7 km., a real distinction 

 does, however, begin to appear in the data for the two groups of 

 mountain stations. It is, however, very small and based upon a 

 rather too limited number of stations to give quantitative reliability 

 to the mean. Furthermore, as discussed in detail under a later 

 heading, there is quite possibly a real difference between the limits 

 of regional compensation and depth of compensation in the moun- 

 tain regions of the West compared to other parts of the continent. 

 Evidence drawn from the Cordillera cannot, therefore, be applied 

 safely to the other portions of the United States. 



Let the assumption be introduced that the limits of regional 

 compensation are variable, ranging from 100 to 500 km. in radius. 

 Such variable limits may well exist because of several factors; first? 

 because of a real variabihty in the strength of the crust; second, 

 because the greater vertical stresses could be carried only by smaller 

 areas. In regions of mountainous relief due to folding, or of high 

 anomalies due to great irregularities of density, the mean size of 

 unit areas should therefore be less. On the whole the anomalies as 

 well as the relief appear to be somewhat greater over the western 

 United States. Third, in regions of recent block faulting or warping 

 the stresses have presumably been lessened from what they were 

 immediately before the movement. Such diminution of strain 

 could take place by the breaking-up of a large unit area of crust 

 into smaller units with dift'erential movement among them, as 



^ Hayford and Bowie, 191 2, p. 102. 



