534 R. D. Oldham, — Origin of the Himalayas. 



geoclesists must take count of the facts of jjeology, and in either case 

 a distinction must be drawn between the facts of either science and 

 the conclusions of individual workers, and again between those direct 

 and inevitable deductions, whicli have almost the value of observed 

 facts, and the more remote inferences, whicli may represent only 

 one, or a part of one, of the possible explanations. Bearing this in 

 view I propose to review, very briefly, the facts and explanations on 

 either side. 



On the geodetic side the facts may be summarized by taking the 

 two stations Kurseong and Jalpaiguri, 25 miles apart, the first situated 

 on the edge of the Himalayas, the second out in the Gangetic 

 alluvium. At Kurseong the observed deflexion of the plumb-line 

 is 46" to the northwards; the calculated effect of the attraction of 

 all visible masses, after allowance is made for the effect of isostasy, 

 should have produced a deflexion of only 23", leaving an unexplained 

 residual of 23" northerly deflexion. At Jalpaiguri the observed 

 deflexion is only 1" to the northwards; the calculated deflexion 

 should have been 8" to the northwards, leaving an unexplained 

 residual of 7" southerly deflexion. It may be well to point out that 

 the values, both of the observed deflexions and of the unexplained 

 residuals, depend on the assumed dimensions of the earth. The 

 figures quoted are deduced from the dimensions now accepted by the 

 Great Trigonometrical Survey of India as the nearest approach which 

 has been made to exact accuracy, but, although the acceptance of 

 different values for the size of the earth would alter the figures, no 

 admissible variation would make a material change in the difference 

 between them. For instance, if the earlier values, based on the 

 Everest spheroid, are adopted, the observed deflexions at Kurseong 

 and Jalpaiguri become 51 "and 6" to the north and the unexplained 

 residuals 28 " to the north and 2 "to the south, still leaving a difference 

 of 30" as between the two stations, which cannot be explained by 

 the ordinary methods of geodetic calculation, and is only to be 

 accounted for by some local peculiarity, or dep;irture from average 

 conditions. The explanation offered by Colonel Burrard is a deep 

 and narrow rift, filled with material of less density than average 

 rock, and situated between the two stations ; there can be no question 

 that the explanation is a feasible one, so far as the mathematics 

 are concerned,' for the diminished attraction caused by the replace- 

 ment of denser by less dense material would cause an apparent 

 repulsion on either side of the rift, but before accepting this as the 

 only, or even as the probable, explanation, we must see whether 

 another cannot be found, in better accord with the known facts of 

 geology. 



The geological facts, which ai'e pertinent to the question under 

 consideration, may be simply expressed. All along the southern face 

 of the Himalayas runs a great fault, or series of parallel faults, 

 known as the boundary fault, on the north of which lie the older 

 rocks of the Himalayas, and on the southern the upper Tertiary 



^ It may be added that a much lesser depth than 20 miles would not satisfy 

 the conditions. 



