2IO - JOSEPH BARRELL 



compensation and the degree to which such variability may give 

 rise to anomalies and residuals without signifying incompleteness 

 of compensation in the column as a whole or regional departures 

 from isostasy. 



In order to show the limits of variation in density which are to 

 be expected, the specific gravity of rocks is first considered. Figures 

 are computed for the mean specific gravity of igneous rocks and 

 the three types of sediments. It is shown that the range of varia- 

 tion is an important factor. Under the subject of the relations 

 between mass and the distance of mass upon anomalies, the effects 

 are computed of unit masses at various depths and extending 

 various distances.^ This lays the basis for considering the influence 

 of the specific gravity of the surface geologic formations upon the 

 difference between the mean anomalies for stations on pre-Cambrian 

 and those on Cenozoic areas. It is found that the greater density 

 of the older rocks accounts for a part and another part is accounted 

 for by their resistance to erosion. This still leaves, however, large 

 outstanding regional variations not related to surface geology or 

 topography and requiring some other explanation. To that end 

 criteria are discussed for the recognition and separation of the 

 effects of mere variable vertical distribution of compensation on 

 the one hand, from partial regional absence of isostasy on the other. 

 It is concluded from the application of these criteria that the 

 anomalies are in large part caused by real regional departures 

 from isostasy extending over broad areas. The results are thus 



' A paper by Gilbert has recently appeared entitled "Interpretation of Anomalies 

 of Gravity" {Part C, Professional Paper 8j, U.S. Geological Survey, 1913). This 

 did not reach the present writer until after Parts III and IV of this article were in 

 galley proof, so that his results cannot be as fully interwoven into the discussion as 

 would otherwise have been the case. On pp. 30, 31, Gilbert considers the interpre- 

 tation of anomalies on the assumption of vertical heterogeneity of the crust and shows 

 clearly that moderate variations of density in a vertical direction could explain them. 

 From this he infers that the anomalies may be due in part to such irregularities. This 

 is the topic which is treated in Part III of the present article under the title "Inter- 

 pretation of Anomalies in Terms of Mass and Depth." The method of reasoning is 

 somewhat different, but although the conclusion reached is the same, the calculations 

 given here are intended to bring out in addition the limitations of area and mass within 

 which that principle applies. It is concluded as a result of the following examination 

 of the evidence that although vertical variations of density are a real cause they are 

 not the major cause of anomalies. 



