﻿part 1] PROCEEDINGS OF THE GEOLOGICAL SOCIETT. 



difference of density : this group of hypotheses, therefore, may 

 be regarded as one of support by flotation. They involve 

 a migration of matter from outside to beneath the range, they 

 allow of a considerable local departure from exact balance between 

 load and support (or topography and compensation), so long as the 

 defect in one tract is balanced hj an excess in an adjoining one, 

 and they provide for an adjustment of any disturbance of this 

 balance. 



The geodetic observations in the Himalayas show that there is 

 a defect of compensation 1 in the outer hills, which increases in 

 amount until, at some -50 miles from the edge of the hills, it reaches 

 an equivalent to an overload of about 2000 feet of rock. In the 

 interior of the Himalayas the only observation yet published shows 

 that, at about 140 miles from the edge of the hills, this overload 

 has disappeared, and compensation is in excess. The variation in 

 the balance between topography and compensation points to one 

 of the second group of hypotheses, to a support of the range by 

 flotation, and to the conclusion that the growth of the support 

 has been more rapid than that of the range. The primary problem 

 then becomes, not as to how the Himala} r as are supported at their 

 actual height, but why they are not even loftier : in other words, 

 the problem is carried one stage farther back, from the origin of 

 the range to the origin of its ' root.' 



This result of the examination of the geodetic data simplifies 

 the explanation of some difficult geological questions. It affords 

 an easy explanation of the indications which are found in the 

 interior .of the Himalayas, and of other similar ranges, of simple 

 vertical uplift without disturbance, and also of the manner in 

 which the contorfed and faulted strata, the disturbance of which 

 must have taken place under the pressure of some thousands of feet 

 of rock, have been brought up to a level where they are exposed to 

 denudation and their structure revealed ; but it brings us very little 

 nearer to an explanation of the ultimate origin of the range. It is 

 a distinct step forward in illustration of the mechanism of the 

 production of mountain-ranges of the type of the Himalayas and 

 the Alps, but we are as far as ever from an understanding of the 

 power by which this mechanism is driven. 



A short discussion took place, and the cordial thanks of the 

 Fellows present were expressed to Mr. Oldham for his lecture. 



