30 OLDHAM: THE STRUCTURE OF THE HIMALAYAS, ETC. 



as possible, as they not only give a closer approximation to the 

 absolute values of the anomalies but also to the differences between 

 them. The other values of the anomaly of gravity, which are 

 available for all stations, are comparable with each other, though 

 not directly comparable with the Hayford anomaly, and afford 

 an indication of the nature, and approximately of the amount, 

 of the difference in the anomaly at any two stations and in this 

 way will be utilised, as far as seems practicable. 



So far, attention has been confined to the fact of compensation 

 and the effect of variation in the depth at which it takes place ; it 

 will now be necessary to devote some space to a consideration of 

 the manner in which compensation may be brought about, and 

 to the cognate concept of isostasy. 



The word isostasy was introduced by Major C. E. Dutton, 1 and 

 by etymology implies merely the statical condition that the mass 

 — or, more correctly, weight — of matter under every considerable 

 portion of the earth's surface of equal area is the same, irrespective 

 of the elevation of that area above or below sea level. The state- 

 ment is not intended to apply to every small protuberance or de- 

 pression in the surface of the land or bed of the sea, but to the 

 general level, and involves, of necessity, a lesser density of the 

 matter under an elevated region, such as a great mountain range, 

 than under the plains at its foot, and a greater density under the 

 depressed floor of the ocean. This leads to the same result as 

 the concept of compensation, but the two are not synonymous, 

 for the elevated regions of the dry land are continually suffering 

 a loss of weight by denudation, while the material removed is 

 deposited on the lowlands, and especially on the bed of the sea ; in 

 this way the load on one area is diminished, that on the other is 

 increased and isostasy is destroyed, till the strain set up by this 

 shift of load causes an underground flow of matter from the over- 

 burdened to the lightened area and so isostasy is re-established. 

 From this it will be seen that the two principles of isostasy and 

 compensation are related to each other in as much as the former 

 necessitates the latter, and further that, whereas compensation 

 merely expresses a static fact, isostasy, in spite of its name, implies 



1 Bull. Phil. dor. WashiiK/ton, XI, p. 53J1889). 



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