704 



NATURE 



[February 27, 1913 



many, when speaking of a building', whether it 

 l)e cottage or cathedral, ever think of anything 

 but the superstructure, the material or methods 

 of construction, Ihe outward form, or the internal 

 plan? And so Prof. Bonney deals with the Alps. 

 Commencing with the materials of which they 

 are made, he goes on to deal with the processes 

 by which they were raised, and the carving of 

 their outward form by rain, rivers, and glaciers, 

 winding up with the vegetation that clothes their 

 surface, the animals that wander over them, and 

 the humanity which frequents them, whether ins 

 permanent inhabitants or temporary visitors. 

 Attractively got up and pleasantly written, it 

 gives Prof. Bonney 's views on all these subjects — 

 views which, as he mentions in the preface, are 

 by no means universally accepted, but which, we 

 may add, are none the less deserving of respectful 

 attention — and will prove of interest not merely 

 to the geologist, but to every intelligent and 

 observant traveller in the Alps. 



(2) Col. Burrard's memoir is of an entirely 

 different character from Prof. Bonney's book. 

 Addressed to the adept, it makes no appeal to 

 the tyro, and, leaving on one side all consideration 

 of the superstructure, deals only with what may 

 be called the foundations of the Himalayas. 

 Geodesists have long known that the attraction 

 exercised by mountains on the plumb-line is much 

 less than that which should result from their 

 visible masses, and the explanation, first suggested 

 by the late Sir G. Airy, has of late years crystal- 

 lised itself in the hypothesis of isostasy, according 

 to which the mountains are supported by a species 

 of flotation, the excess of material in the protuber- 

 ance above sea-level being compensated by a 

 defect of density below. 



The most complete and best-known investiga- 

 tion of this hypothesis is that of Mr. J. F. Hayford, 

 of the United States Coast and Geodetic Survey, 

 who has dealt with it, in the light of American 

 geodetic observations, in an elaborate manner. 

 The form of the hypothesis adopted by him was 

 that the compensation extended to a uniform 

 depth, and was effected by variation of the density 

 of the earth's crust, so that the total downward 

 pressure of a column of rock under the mountains 

 should be the same as that of the lesser thickness 

 under the ocean depths. Assuming this as the 

 method of compensation, he found that the resi- 

 dual differences between the observed and the 

 calculated deflection became least if the depth to 

 which the compensation extended was taken at 

 about ii3'7 kilometres, and with that assumption 

 the residuals became so small that the hypothesis 

 might be accepted as extremely probable. This 

 is not, however, a necessary conclusion, for an 

 erroneous hypothesis may be in accord with a 

 limited number of observations, but fail when 

 these are extended ; and the result of the applica- 

 tion of Mr. Hayford 's explanation to the Indian 

 observations shows that it is inapplicable to that 

 country. 



(3) The facts on which this conclusion is based 

 NO. 2261, VOL. 90] 



are interesting, and are given in detail by Major 

 Crosthwait. At stations within the Himalayas 

 the plumb-line is deflected by about 30" to 40" 

 to the north, along the foot of the hills this has 

 sunk to some 15" or thereabouts, and at dis- 

 tances of more than forty miles it disappears or is 

 replaced by a small southerly deflection. There is, 

 consequently, a rapid variation in the amount of 

 the observed deflection as we cross the limits of 

 the mountain range, and this change is about 

 double as great as it should be on Mr. Hayford 's 

 hypothesis. 



The only other explanation investigated by 

 Col. Burrard is that of foredcep, filled with sedi- 

 ment, and according to his calculations this hypo- 

 thesis gives results which depart even further 

 from observation than Mr. Hayford 's. Col. 

 Burrard offers an explanation of his own — that 

 there is a rift in the subcrust along the foot of 

 the mountains, the gradual opening of which gave 

 rise to the compression of the Himalayas, and 

 which became filled with the alluvium of the 

 Gangetic plains as it was formed. Unfortunately 

 he confesses that he is not geologist enough to 

 elaborate this hypothesis, and it is diflficult to see 

 how it can lae brought into accord with what is 

 known of the geology of the Himalayas and of 

 the country to the south of them, lior how it 

 differs from Prof. Suess's foredeep. 



Moreover, Col. Burrard appears to have over- 

 looked an important paper, published by Rev. O. 

 Fisher in The Philosophical Ma^asine of 1904, in 

 which he investigates the effect of the Himalayas 

 on_ the plumb-line in the light of an hypothesis 

 of isostasy radically different from Mr. Hayford 's. 

 According to tiiis the crust is of uniform density, 

 the isostatic compensation beinq- obtained by a 

 variation in thickness, and on this hypothesis' he 

 finds that the attraction of the visible range, com- 

 bined with the negative attraction of the down- 

 ward protuberance, should give a northerly 

 deflection of about 24" at the foot of the hills, 

 of 2" at sixty miles away, and a southerly de- 

 flection of about 2" at the farther edge of 

 the plains. These results appear to be in very 

 fair accord with observations in the region of 

 the great Gangetic plain of upper India, where the 

 conditions resemble those postulated in the calcula- 

 tions ; beyond this region, in the Punjab and in 

 Bengal, the ^•ariations are greater than in the 

 central area, but there the conditions are compli- 

 cated by the fact that geology suggests, and 

 gravitation measurements indicate, the presence 

 of denser rock at a small depth below the alluvium. 



It must be acknowledged that Mr. Fisher's 

 investigations do not give a complete explanation 

 of all the variations observed, but this is inevitable 

 in the case of any hypothesis which assumes — as 

 must be done for purposes of calculation — that 

 the crust and the underlying material have every- 

 where the same density. All that can be said is 

 that it seems to be more closely in accord with 

 the Indian observations than Mr. Hayford 's, and 

 it is to be hoped that Col. Burrard will bo able 



