536 



SCIENCE 



[N. S. Vol. XL. No. 1033 



unexplained by any of the obvious phe- 

 nomena at the command of the geologist. 

 G. K. Gilbert now suggests that, while it 

 may be true that the product of the length 

 of the unit column by its mean density 

 may be the same, the density variations 

 within the column may be such as to give 

 rise to different effects on the pendulum. 

 If, for instance, one considers two columns 

 of the same size and of exactly the same 

 weight, with, in one case, the heavy ma- 

 terial at a high level and in the other case 

 with the heavy material at a low level, the 

 center of gravity of the former column, be- 

 ing nearer the surface, will manifest itself 

 with a greater pull on the pendulum ; these 

 columns would be, however, in isostatic ad- 

 justment.'' 



Gilbert's hypothesis thus differs slightly 

 from the conception put forth by Hayford 

 and Bowie ; for Gilbert assumes that there 

 is still appreciable heterogeneity in the 

 more deep-seated parts of the earth, while 

 Hayford and Bowie's hypothesis assumes 

 that in the nuclear mass density anomalies 

 have practically disappeared, and that 

 there is below the depth of compensation 

 an adjustment such as would exist in a 



5 It is interesting to note that the idea sug- 

 gested by G. K. Gilbert in 1913 was partly antici- 

 pated by Major H. L. Crosthwait in 1912 (Survey 

 of India, Professional Paper No. 13, p. 5). Major 

 Crosthwait in discussing the similar gravity 

 anomalies in India remarks parenthetically : ' ' Ar- 

 suming the doctrine of isostasy to hold, is it not 

 possible that in any two columns of matter ex- 

 tending from the surface down to the depth of 

 compensation there may be the same mass, and 

 yet that the density may be very differently dis- 

 tributed in the two columns? These two columns, 

 though in isostatic equilibrium, would act differ- 

 ently on the plumb-line owing to the unequal dis- 

 tribution of mass. 



' ' The drawback to treating this subject by hard 

 and fast mathematical formulae is that we are in- 

 troducing into a discussion of the constitution of 

 the earth's crust a uniform method when, in real- 

 ity, probably no uniformity exists." 



mass composed of homogeneous concentric 

 shells. 



In order to make the Indian observa- 

 tions comparable to those of the United 

 States as a test of the theory of isostasy, 

 Major H. L. Crosthwait® has adopted Hay- 

 ford's system of computation and has ap- 

 plied it to 102 latitude stations and 18 

 longitude stations in India. He finds that 

 the unexplained residuals in India are far 

 more pronounced than they are in the 

 United States, or, in other words, it would 

 appear that isostatic conditions are much 

 more nearly realized in America than in 

 India. 



The number of observations considered 

 in India is still too small for the forma- 

 tion of a detailed map of anomalies, but 

 the country can be divided into broad 

 areas which show that the mean anomalies 

 are comparable to those of the United 

 States only over the Indian peninsula, 

 which, being a mass of rock practically 

 undisturbed since early geological times, 

 may be regarded safely as having ap- 

 proached isostatic equilibrium. - To the 

 north of the peninsula three districts form 

 a wide band stretching west-north-west- 

 wards from Calcutta, with mean residual 

 anomalies of a positive kind, while to the 

 north of this band lies the Himalayan belt, 

 in which there is always a large negative 

 residual. 



Colonel Burrard'' has considered the Him- 

 alayan and Sub-Himalayan anomalies in a 

 special memoir, and comes to the conclusion 

 that the gravity deficiency is altogether too 

 great to be due to a simple geosynclinal de- 

 pression filled with light alluvium such as 

 we generally regard the Gangetic trough 

 to be. He suggests that the rapid change 



6 Survey of India, Professional Paper No. 13, 

 1912. 



'Survey of India, Professional Paper No. 12, 

 1912. 



