700 WILLIAM HERBERT HOBBS 



The earth's density as a whole being more than double that of 

 the part known to us from observation, we may assume almost 

 any distribution of matter which arranges the concentric shells 

 in inverse order of density from the center to the surface. It is 

 in fact quite as probable that near the surface contacts between 

 successive shells of different density are abrupt as it is that they 

 are gradual. Hayford has assumed, though he does not appear 

 to regard it as an assumption of importance, that down to a depth 

 of 76 miles (122 km.) 1 no shell of notably higher density than that 

 at the surface is encountered. It is, however, entirely within the 

 realm of probability that material in all respects resembling the 

 stone meteorites or stone-iron meteorites may be found within 

 this depth. If, further, the surface of contact between a lower 

 shell of higher density and a higher shell of lower density is 

 not only abrupt but irregular and characterized by notable local 

 prominences, an explanation can be found for most local anomalies 

 of gravity. 



EVIDENCE OF LOCAL IRREGULARITIES IN DISTRIBUTION OF GRAVITY 

 AND MAGNETIC CONSTANTS 



Evidence from Russia and from Southern Italy. — It is proposed 

 now to state certain evidences that local conditions may be responsi- 

 ble for large anomalies in the value of gravity. The evidence now 

 upon record has been drawn from a number of widely separated 

 provinces, in all of which extended series of measurements either 

 of gravity or of deflection of the plumb line have been carried out. 2 

 The great plain of Russia is of special interest in a discussion of 

 isostasy, since any anomalies in gravity which occur do not require 



1 Helmert has distinctly recognized that there are anomalies of gravity not 

 explainable on general conditions, but, using the same assumption of the truth of 

 Pratt's hypothesis, he has determined the depth of the Ausgleichfldche to be 118 ±22 

 km. He has chosen for this purpose the zones of special disturbance above and on 

 either side of the steep slope bordering the continental shelves (F. R. Helmert, "Die 

 Tiefe der Ausgleichflache der Prattschen Hypothese fur das Gleichgewicht der Erd- 

 kruste und den Verlauf der Schwerestorung vom Innern der Kontinente und Ozeane 

 nach den Kiisten," Sitzungsber. d. k. preusz. Akad. d. Siss. z. Berlin, 1909, pp. 1192-98). 



2 F. de Montessus de Ballore, "Sur les anomalies de la pesanteur dans certains 

 regions instables," Comptes Rendus de VAcademie Franqaise, CXXXVI (Paris, 1903), 

 705-7. 



