May 8, 19 13] 



NATURE 



H5 



deficiency of mass, when transferred to the subcrust 

 under the plains, is S". The significant fact is not 

 so much the reduction of deficiency below the moun- 

 tain range as the location of a deficiency under the 

 plains between the foot of the slope and the southern 

 station of observation. In Fisher's hypothesis this is 

 the important feature which brings about a more 

 rapid variation of deflections than follows from the 

 assumption of simple compensation. 



The results of the calculation upon the second 

 hypothesis give 20" as the variation of deflection be- 

 tween Dehra Dun and Kaliana, with an average 

 deflection of 12", against the observed 30" and 16". 

 The calculated variation and average deflection for 

 the interval between the foot of the hills and the 

 southern edge of the plains are, respectively, 24" and 

 10", the observed values being 43" and 16". The 

 differences between Fisher's quantities, based on the 

 second hypothesis, and those observed, approach, on 

 an average, 35 per cent, of the observed values. 



Both hypotheses, that of simple and that of general 

 compensation, fail to give results in accord with 

 observation, 'when Fisher's numerical values are used. 

 In his second hypothesis his assumption of a three- 

 mile depression of the crust is inadequate. As pointed 

 out by Col. Burrard in his paper, to explain Hima- 

 layan deflections by a hidden synclinal, we must 

 assume the latter to be seventy to eighty miles wide 

 and six miles deep under Siliguri, seven miles south 

 of the foot of the slope, and two miles deep under 

 Jaljaiguri, thirteen miles south of Siliguri, the rock 

 composing the synclinal basin to have a density of 

 27, and the sediment filling the synclinal to have a 

 density of 1-9. As Col. Burrard says, it is doubtful 

 whether the density of sediment, when under a pres- 

 sure of a vertical column six miles high, would re- 

 main as small as i-g; any increase in its value will 

 require the depth of the supposed synclinal to be 

 increased. 



In connection with Fisher's investigation, there is 

 an interesting point. Putting aside his computed 

 figures, we see that both he and Burrard agree in 

 considering that the observed facts cannot be ex- 

 plained by only the visible Himalayan mass and its 

 vertically underlying root. Both investigators are 

 forced to conclude the existence of a third factor, a 

 source of negative attraction under the plains at the 

 foot of the hills. Fisher prefers to adopt the idea 

 of deficiency extending under a relatively wide belt of 

 the crust due to the depression of the latter into the 

 liquid substratum, the outer surface of this depressed 

 tract being brought up to sea-level by the deposition 

 of the Siwalik beds and alluvium. This hypothesis 

 leads to the assumption of very doubtful values of 

 some of the unknown quantities, as has been shown 

 above. Burrard's hypothesis differs from Fisher's in 

 that he would localise the deficiency in a rift in the 

 crust subsequently filled in by deposits. 



H. M. Cowie. 



Dehra Dun, U.P., India, April 3. 



Surely Col. Burrard and Major Cowie have mis- 

 read the review; it did not dismiss Col. Burrard's 

 speculations, but pointed out that he had himself 

 dismissed, with what appeared to be inadequate 

 examination, an hypothesis which seemed fully 

 capable of explaining the facts. The sentence 

 which has elicited "their letters was intended 

 to refer solely to the memoir under review-, 

 and had no application to other publications 

 by the same author. The memoir did not, in fact, 

 contain any detailed investigation of an hypothesis 

 which, if tested numerically and in its completeness, 

 appears to be at least as capable of affording an 

 explanation of the facts as that propounded by Col. 

 NO. 2271, VOL. 91] 



Burrard. The reviewer may point out that the 

 limited amount of space at his disposal compelled the 

 omission of reference to many points of which he was 

 well aware, and had fully considered, but in view ot 

 the publication of these letters he may be permitted 

 to amplify the argument of the paragraph in the 

 review which has called them forth. 



Mr. Fisher's investigation assumes an isostasy by 

 flotation, and, what is an almost inevitable conse- 

 quence, that the flotation is not confined to the area 

 of the range, but that, as an iceberg has generally 

 an under-water extension helping to support the 

 visible mass, so the lighter "crust" under the plains 

 is borne down into the denser "substratum " or "sub- 

 crust" by the weight of the mountain range. This 

 interpretation is in accord with the evidence of the 

 pendulum, which shows that the defect of gravity 

 under the mountains is continued under the 

 plain, and only gradually decreases with in- 

 creasing distance from the range ; it is also in 

 accord with conclusions drawn by the Geological Sur- 

 vey long before the observations of variations in the 

 force of gravity and of deflection of the plumb-line in 

 the neighbourhood of the foot of the hills were pub- 

 lished, and the constants used by Mr. Fisher, so far 

 as they are special to the Himalayas, were taken from 

 these reports. 



According to the hypothesis, a station near the 

 edge of the hills, such as Kurseong, would be affected 



(1) by the positive attraction of the visible masses; 



(2) the negative attraction of the "root" or down- 

 ward thickening of the "crust" into the "sub- 

 stratum " ; (3) by the negative attraction of the sub- 

 merged portion of the "crust" under the plains, re- 

 placing denser "substratum"; and (4) — though Mr. 

 Fisher did not separately consider this — by the nega- 

 tive attraction of the alluvial deposit of the plain, the 

 mean density of which is less than that of average 

 rock. Of these (1) is the same whatever hypothesis 

 of isostasy is adopted ; (2), it appears from Mr. Hay- 

 ford's investigation of the effect of an isostasy pro- 

 duced by compensation limited to a ten-mile stratum, 

 between twenty-five and thirty-five miles depth from 

 the surface, would somewhat increase the deflection 

 at a station situated on the edge of the hills (e.g. 

 Kurseong), and make but little alteration at a station 

 twenty or thirty miles out in the plain (e.g. Jalpai- 

 guri) ; (3) and (4) would both produce their maximum 

 effect at a station situated like Kurseong, and have 

 comparatively little influence at one situated like Jal- 

 paiguri. Here we have three separate corrections, 

 all working in the same direction, and all attaining 

 their maximum at the same station, and it is not 

 inconceivable that together they might afford an ex- 

 planation of the peculiarities noticed by Col. Burrard. 



It is obviously useless, at the present stage of our 

 knowledge, to enter into detailed calculations of an 

 imaginary range, but some approximate calculations 

 made by the reviewer indicate that the increase 

 in the difference of deflection as between Kurseong 

 and Jalpaiguri due to (2) would be of the order of 4", 

 to (3) of the order of 8", and to (4) of not less than 

 q', or a total increase in the calculated difference of 

 deflections amounting to more than 21", as com- 

 pared with Col. Burrard's unexplained anomaly of 

 30". These figures have no value, except as indicat- 

 ing that there is another hypothesis, besides that of 

 the "rift," which would account for a change in the 

 amount of deflection near the foot of the range, of the 

 same character and order of magnitude as that actually 

 observed. 



It must be added that this explanation can only 

 be taken as applying to the Himalayas; the conditions 

 in the Vindhya are entirely different and require to 

 be considered apart. Thk Reviewer. 



