lxxxiv Journal of the Asiatic Society of Bengal. [N.S. , XII, 



theories, I do not believe that it is inconsistent with geological 



observations. 



If the Himalaya had the uncompensated mass which they 

 appear to have, and which the school of geologists who follow 

 Suess ascribes to them, they would attract the waters of the 

 Indian Ocean over India; the plains of Northern India would 

 be a great sea ; this sea would be 300 feet deep above Allaha- 

 bad, 400 feet deep above Lucknow and Gorakhpur, and 800 

 feet deep above Pilibhit and Bahraich. Fortunately those 

 mountains have not the power of attracting the Indian 

 Ocean. 



Mountain Flotation and Isostasy. 



But if the theory of compensation has suffered at the 

 hands of its opponents, it has suffered also from its friends. 

 Pratt's theory of compensation has been stretched into a 

 theory of flotation: an iceberg floats, because ice is lighter 

 than water; an iceberg is compensated in the water by its rela- 

 tive deficiency of density; Sir George Airy, the Astronomer 

 Royal, suggested that mountains were compensated because 

 they were floating upon a heavy subterranean magma. Pratt 

 never went as far as this; he merely said, " the mountains are 

 compensated." Airy went further ; he said, "the mountains 

 are floating. 5 ' Distinguished geologists, Fisher, Dutton, Old- 

 ham, have developed the idea of flotation. 



The theory of flotation lays down that the mountains are 

 supported in their present positions by hydrostatic pressure, 

 just as an iceberg floats upon water. I have no time to dis- 

 cuss this theory at length, but I should like to point out to 

 you that if an iceberg floats upon water, its weight must be 

 compensated by underlying deficiencies of density : the theory 

 of flotation does not state this with regard to mountains; it 

 states the converse, viz., that as mountains are compensated 

 they must be floating. The principle of hydrostatic pressure 

 demands that if any mass is floating it must be compensated ; 

 it does not, however, follow that if a mass is compensated it 

 must be floating. The theory of flotation is based upon the 

 assumption that the compensation of mountains is complete 

 and perfect ; but we have not found complete compensation 

 in India : the outer Himalaya are compensated to the extent 

 of 80 per cent. An iceberg would not float, unless its com- 

 pensation were exactly complete ; the fact that mountain com* 

 pensation is nowhere quite complete or perfect is a serious 

 argument against flotation. This imperfection of compensa- 

 tion differentiates rock from water : it denotes rigidity. 

 What I have been calling the theory of flotation is frequently 

 called the theory of Isostasy. I have however purposely 

 avoided using the word Isostasy, as its exact meaning is open 

 to question. Isostasy is a condition of approximate equili- 



