2 OLDHAM: THE STRUCTURE OF THE HIMALAYAS, ETC 



had the paper not seemed to imply that the geodetic evidence 

 necessitated the existence of a deep and comparatively narrow 

 rift along the edge of the hills, filled with rock of lesser density 

 than that on either side— with some suggestion of actual cavity— and 

 at one place the figure of 20 miles was given for the depth of this 

 rift. 1 Such at least was the interpretation which the paper seemed 

 to bear, and the figure, mentioned for the depth, implied a diaclasm 

 so far transcending in magnitude anything which has been estab- 

 lished from observation, in the Himalayas or elsewhere, that its 

 acceptance would have necessitated the revision of what had been 

 regarded as well founded deductions from geological evidence. 

 Diaclasms of three miles in depth are well established, and even 

 five miles is not impossible, and these are the figures which had 

 been regarded as the probable, and the extreme possible, limit 

 of the faults along the southern boundary of the Himalayas. The 

 assertion that the geodetic evidence pointed to, if it did not 

 necessitate, the existence of a diaclasm of four times the extreme 

 magnitude of which we had any indication in geological observa- 

 tion, naturally attracted attention, led to an examination of the 

 grounds on which the assertion was based, and gave rise to a some- 

 what extensive literature, which, being mainly controversial, was 

 mostly unprofitable. 



I have no intention or desire to add to this literature ; his main 

 rift has been placed further south, and the figure of 20 miles 

 has been explained away, 2 thereby removing the contradiction 

 which appeared to exist between the geodetic and geological 

 observation, yet the original statement cannot be regretted for it 

 has drawn attention to the geodetic work of the Trigonometrical 

 Survey and led to an examination of the light thrown by it on 

 some interesting and doubtful points of geological structure, which 

 it had not been possible to elucidate by geological observations 

 alone. In the course of this examination I shall have occasion 

 to refer to theories which have been offered in explanation of the 

 origin of the Himalayas, but only so far as to indicate the influence 

 which they might have on geodetic observations, and so afford 

 a guide to the directions in which these should be examined ; for 

 the object of this investigation is not advocacy, or attack, of any 



1 hoc. cit p. 11. "A rift in the sub-crust south of Mussooree and 20 miles deep would 

 explain the largo deflections in the interior of the Himalayas." 



2 Proc. Roy. Soc, Series A. XCI, 1915, p. 229. 



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