6 OLDHAM: THE STRUCTURE OF THE HIMALAYAS, ETC. 



affords a tolerably secure indication. The total thickness of the 

 Siwaliks, in the Kumaon and Garhwal districts, was estimated by 

 Mr. Middlemiss at an average of about 16,500 feet * ; Mr. Medlicott 

 estimated the thickness of the Siwaliks north of Hardwar at 15,000 

 feet, 2 and the whole thickness is not exposed on this section. We 

 may therefore take it that the depth of alluvial deposits, being 

 the continuation of these Siwaliks, is not likely to be materially 

 less than 15,000 to 16,000 feet at the northern limit of the plains, 

 and we may safely say that the alluvium at the northern edge 

 of the plains is very improbably much greater or less than about 

 three miles in depth. 



(5) At the southern edge of the alluvial plain the thickness is 

 small, the boundary is irregular, following the contour of the much 

 denuded surface of the older rocks of the Peninsular area, which 

 crop out, near the boundary, in numerous isolated patches and 

 hills, rising from the surrounding spread of alluvium. All the 

 features, in fact, suggest a gradual encroachment of the alluvium 

 on an 'old land surface of rock, and a gradual southward growth 

 of the depression in which the Gangetic Alluvium has been 

 deposited. 



Besides these well established conclusions, there arc certain 

 others of a more conjectural character, which need confirmation, 

 or greater amplification, than the present state of geological know- 

 ledge — or in some cases any conceivable advance in it — can afford. 

 Of these the following seem capable of elucidation by the data 

 to be dealt with : namely — 



(1) The question of whether the elevation of the Himalayan 

 range was caused, or merely accompanied, by its compression. 

 The natural conclusion would be that they were related to each 

 other as cause and effect, but in which direction cannot be re- 

 garded as proved. Were the elevation due to a simple process 

 of tumefaction, or swelling up, of the material underlying the range, 

 this would set up internal strains in the elevated mass and a ten- 

 dency to spread, which might result in compression and folding. 

 This hypothesis has in fact been proposed and experimentally illus- 

 trated on the small scale, 3 but it has never been tested by actual 



* Memoirs, XXIV, p. 87. 



2 Memoirs, III, pt. 2, p. 118. 



»E. Rayer, Nature XLVI, p. 224 (1892). 



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