May 8, 1913] 



NATURE 



243 



confronted by the geodetic problems which they pre- 

 sent. The highest geological authorities express 

 •doubts ai to how mountains have been upraised, and 

 geological theorists differ widely. 



In my recent paper I suggested, with diffidence, 

 the hypothesis that the long belt of Indus-l I 

 alluvial plains was concealing a deep crack in the 

 subcrustal shell of the solid earth, and that the Hima- 

 laya mountains had been crumpled up by the opening 

 of this crack in the solid globe. 



When a large mass of heated rock, or ore, or glass 

 is cooling, its surface is apt to crack ; we never see 

 the core of any such mass shrink away from the 

 outer shell and leave the outer shell too large and 

 unsupported, as is often assumed to be happening in 

 the case of the earth. 



Are geologists quite sure that the earth's outer 

 shell has not been cracking, and that the cracks are 

 not hidden from our sight by silt? Would not the 

 cracking of a solid globe provide a sufficient mechan- 

 ical force to elevate mountains? 



The earth's rotation is slowing down; the equa- 

 torial protuberance of rock is constantly straining to 

 move polewards. 



Throughout the whole length of the Indus-Ganges 

 belt of plains earthquakes are frequently occurring, 

 and what can be causing these earthquakes, if not 

 the splitting asunder of the solid globe beneath? 



At both extremities of the Indus-Ganges belt deep 

 narrow submarine canons exist extending far out 

 to sea; they are known to sailors as "swatches." 

 What are these swatches, if not the surface indications 

 of a subcrustal crack? 



In the review in Nature my hypothesis is dismissed 

 without any specific objection to it being raised. I 

 am not wedded to it, and I should welcome its dis- 

 missal, if I thereby learnt its errors. But I am 

 disappointed to see it rejected merely because my critic 

 has himself accepted the " floating crust " hypothesis. 



The reviewer has accepted as true the hypothesis 

 of the Rev. Osmond Fisher. According to this hypo- 

 thesis the solid crust of the earth is of limited thick- 

 ness and floats upon a liquid magma of greater 

 density. This assumption of a liquid substratum 

 appears to me to be opposed to the views of the great 

 majority of geological writers. So far as I am able 

 to judge, the weight of the evidence seems largely 

 in favour of a solid globe. 



Furthermore, Mr. Fisher has to assume that as the 

 central core of the earth cools down, the outer crust 

 is left unsupported owing to the core's contraction 

 ("Geology of India," R. D. Oldham, p. 471). This 

 assumption of a cooling core contracting away from 

 its shell seems to me to be more difficult to justify 

 than the assumption of a cooling shell becoming too 

 small for its core. 



But let me descend from these great assumptions 

 to actual geodetic figures. 



Mr. Fisher assumes that the crust floats in the 

 dense liquid, just as ice floats in water. Each moun- 

 tain rising from the upper surface of the crust has 

 a corresponding protuberance extending downwards 

 from the lower side of the crust. The buoyancy of a 

 protuberance suffices to support the weight of the 

 corresponding mountain above it. 



If 7i = height of a mountain, and if d = depth to 

 which its protuberant root extends downwards into 

 the liquid, then, according to Fisher, d = 9-6xfe. 



Now let me apply this hypothesis to the mountains 

 of India. All these mountains are assumed to be 

 buoyed by subcrustal protuberances of lower density 

 than the mafma, but the protuberances extend down- 

 wards to different depths, which are proportional to 

 the several mountain heights. 



NO. 2271, VOL. rjl] 



Let us suppose a plumb-line to be suspended near 

 the foot of a mountain, and let us suppose that the 

 mass of this mountain is compensated by a defii i< ncy 

 of density underlying it below the crust. If that 

 deficiency of density be wholly concentrated near sea- 

 level, it will entirely compensate the attraction of 

 the mountain mass, and the plumb-line will hang 

 vertically. But if that deficiency of density be dis- 

 tributed to a great vertical depth, it will not com- 

 pensate the mountain's attraction, first, because of its 

 greater distance in depth from the plumb-line, and 

 secondly, because its resultant action is more in- 

 clined to the horizontal. 



If a mountain (Himalayan) is four miles high, and 

 if its protuberance extends downwards to a depth of 

 thirty-eight miles, the geodetic observer would re- 

 port : — "There is distinct evidence of compensation, 

 but the compensation is by no means complete." ' 



If, however, a mountain (Vindhyan) is o-6 mile 

 high, its compensation would take place within five 

 miles of the crust, and the observer would report : — 

 " The compensation is here more complete than in 

 the case of the higher mountain." 



Similarly a pendulum observer at a station (Hima- 

 layan) two miles high will (according to the Fisher 

 hypothesis) not find the same degree of compensation 

 as he will at a station (Vindhyan) half a mile high. 

 The underlying deficiency of density will in each case 

 have a retarding effect on the pendulum, but at the 

 Himalayan station the deficiency reaches downwards 

 nineteen miles into the liquid, whilst at the Vindhyan 

 station the deficiency only extends five miles down- 

 wards. 



According, then, to the "floating-crust" hypothesis 

 our plumb-line and pendulum observers should find 

 the attraction of small mountains more completely 

 compensated than the attraction of high mountains. 

 But what are the results of actual observations? Both 

 the pendulum and plumb-line observers find the 

 attraction of the Himalayas to be largely compen- 

 sated, whilst the Vindhyan mountains are not com- 

 pensated at all. Actual results of observation are in 

 direct opposition to the " floating-crust " hypothesis. 



Mr. Havford has stated that the " floating-crust " 

 hypothesis is not true for the LInited States of America 

 (" Figure of the Earth and Isostasy," p. 164), and in 

 my opinion the evidence is sufficient to show that this 

 hypothesis is not true for India. 



S. G. BURRARD. 



Survevor-GeneraFs Office, Dehra Dun, March 29. 



As an officer of the Survey of India, employed for 

 many years in determining deflections of the plumb- 

 line and variations in the intensity of gravity, I was 

 interested to find in the article which appeared under 

 this heading in Nature (No. 2261, vol. xc, February 

 (27) reference to the hypothesis suggested in 

 1004 by the Rev. O. Fisher as to the nature of moun- 

 tain compensation, and the statement that this hypo- 

 thesis goes far to explain the deflections of the plumD- 

 line observed at the foot of the Himalayas and in the 

 Gangetic plain. The article states that, according to 

 Mr. Fisher's hvpothesis, "the crust is of uniform 

 densitv, the isostatic compensation beinc obtained by 

 a variation in thickness," and that, on this hvpothesis, 

 Mr. Fisher " finds that the attraction of the visible 

 range combined with the negative attraction of the 



