10 W. Boivie — Relation of Isostasy to 



material close to the station, its attractive effect will be 

 less than if we should have material of normal density 

 occupying the same space. This opinion of the writer 

 was accepted by Col. Burrard, former Superintendant of 

 the Trigonometrical Survey of India. He has recently 

 published a very important book dealing with this subject, 

 entitled ^'Investigations in Himalayan and Neighbouring 

 Eegions'' (Professional Paper No. 17, Trigonometrical 

 Survey of India, 1917 Sup. 285, February 1921 of this 

 Journal). The writer had stated that possibly the 

 columns under the Cenozoic formation are in isostatic 

 equilibrium, for it is inconceivable that the load of sedi- 

 mentation added to the column should make it have less 

 material than normal. Col. Burrard made computations 

 to show whether the idea of isostatic equilibrium of the 

 columns under sedimentary areas is a reasonable one. 

 He concluded that the negative gravity anomalies can be 

 caused by the presence of Cenozoic material near the 

 gravity stations and that, probably, the Indo-Gangetic 

 plain is in isostatic equilibrium. 



Relation between isostatic anomalies and the Pre-Camhrian 



formation. 



The writer has found that there is a very definite rela- 

 tion between the sign of the gravity anomaly and the 

 Pre-Cambrian formation. Here we can account for the 

 positive character of these anomalies by the presence of 

 extra heavy material close to the gravity stations which 

 is probably compensated for by a deficiency in the column 

 below. I shall not take time to discuss the Pre-Cambrian 

 formation and its relation to the gravity anomalies, but 

 I have dwelt upon the relation of the gravity anomalies 

 to the Cenozoic formation because it is important in 

 throwing light on some phases of mountain formation. 



The datum used for isostatic investigations. 



A subject that I should like to discuss at some length 

 is the datum used for computations of the effect of topog- 

 raphy and compensation, but I shall not take the time to 

 do so here, because of more important phases of isostasy 

 that must be considered. I may say that it makes very 

 little difference in the conclusions arrived at from isos- 

 tatic investigations whether we use some other datum 

 than mean sea-level for the computation of the effect of 



