ABSTRACTS 



Authors of scientific papers are requested to see that abstracts, preferably 

 prepared and signed by themselves, are forwarded promptly to the editors. Each 

 of the scientific bureaus in Washington has a representative authorized to for- 

 ward such material to this journal, and abstracts of official publications should 

 be transmitted through the representative of the bureau in which they originate. 

 The abstracts should conform in length and general style to those appearing in 

 this issue. 



GEODESY. — I nter'pr elation of anomalies of gravity. Grove Karl 

 Gilbert. U. S. Geological Survey Professional Paper 85-C. Pp. 

 37. 1913. 

 Hayford and Bowie, in discussing the deflections of the plumb line 

 and the variations of gravity determined by the geodetic work of the 

 U. S. Coast Survey, have introduced the following hypothesis: At a 

 certain depth below the earth's surface, called the depth of compensa- 

 tion, stresses are uniform. Each vertical element above that level has 

 the same mass, its density (assumed uniform) being low in proportion 

 as its length is great. Under this hypothesis they find the most prob- 

 able depth of the level of compensation to be 122 kilometers. The 

 hypothesis approximately satisfies the requirements of gravity determi- 

 nations for the United States but leaves certain residuals called anom- 

 alies of gravity. These have been ascribed to imperfection in the com- 

 pensation. It is now pointed out that this explanation does not accord 

 well with geologic data as to loading and unloading. Two alternative 

 explanations are suggested, and computations indicate that each is 

 quantitatively adequate: (1) The anomalies may be occasioned by 

 inequalities in the vertical distribution of density above the level of 

 compensation; (2) some of them may be occasioned by local inequalities 

 of density within a rigid nucleus assumed to exist below the region in 

 which mobility permits isostatic adjustment. These suggestions are 

 in accord with a speculation as to the general structure of the earth, — 

 that it includes a zone or layer of relative mobility, separating a super- 

 ficial zone of less mobility from a highly immobile nucleus. The zone 

 of relative mobility is the site of volcanic initiative, and is an important 

 factor in those transgressions by which superficial tracts are tangentially 

 shortened, as in the Appalachians, or tangentially extended, as in the 

 Basin ranges. G. K. G. 



192 



