Colonel Burrard — The Origin of Mountains. 387 



values of ff that have been met with on islands and coasts. Fisher's 

 theory does not explain these anomalies.' 



6. Although Mr. Hayford has shown that continents and mountains 

 are generally compensated, yet we have no grounds for assuming that 

 mountains maintain a constant height. The author of the theory of 

 isostasy said that it was not intended to account for the origin of 

 mountains, and that it was only put forward to explain how existing 

 mountains were maintained. It has even been argued by others that 

 as material is removed from peaks and crests by denudation the 

 mountains become lighter and in consequence higher. This seems 

 to me to be an unwarranted assumption based on no data whatever. 

 In a similar way it has been stated that the rock floors of river valleys 

 continue to sink as silt is deposited upon them, the underlying idea 

 being that the weight of every additional layer of silt deposited is too 

 much for the solid crust to support, and that the latter yields in 

 consequence. Deep boreholes have been sunk in the deltas of certain 

 rivers, and bones, shells, and remains of plants have been found at all 

 levels, however deep the boreholes have been taken ; the conclusion has 

 been drawn that these remains were originally deposited at sea-level. 



But let us consider the case of the Plains of Northern India. 

 These extensive plains are deposits of silt brought down by rivers 

 from the Himalayas. Boreholes have been sunk and remains of 

 organic life found buried at great depths in the silt, but these 

 observations do not justify the assumption that the rock-floors 

 underlying the Indus-Ganges Valleys have been continually sinking 

 under the increasing weight of the deposits. Both on the west and 

 on the east long, deep, narrow, submarine troughs extend out into 

 the oceans in continuation of the Indus and Ganges Valleys. The 

 waters of the Indus and Ganges are continually pouring silt into 

 these deep troughs ; amongst the silt are remains of organic life that 

 once flourished at sea-level — plants, shells, bones — and these are beings 

 deposited at great depths in the troughs. With this going on before 

 our eyes how can we presume to argue that the remains found at 

 great depths in boreholes were originally deposited at sea-level ? 

 The simplest explanation is that the plains of Northern India are 

 concealing a sub-crustal crack, that the submarine troughs are con- 

 tinuations of this crack, and that as the crack has opened and grown 

 deeper the deposits filling it up have been continually sinking 

 to lower levels. 



7. The problem of isostasy that is now requiring solution may be 

 stated as follows: "Continents and mountains have been found to 

 be compensated by underlying deficiencies of density; how has this 

 condition resembling hydrostatic equilibrium arisen upon a solid 



^ Although Hayford' s theory is the nearest approach to truth that has yet 

 been made, it does not afford a complete explanation of every observed anomaly. 

 In my paper on the origin of the Himalayas I corrected all geodetic results in 

 accordance with the Hayfordian system, and I then regarded the uneliminated 

 residuals as indications of actual departures from isostasy. Mr. Fisher calls 

 attention to a change in the observed value of g at Dehra Dun that has taken 

 place between 1870 and 1904. This change is, however, apparent only ; it is due 

 to the recent introduction of a correction for the vibration of the brick pillar 

 upon which the pendulum is swung. 



