R. M. Deeley — Mountain Building. 113 



Some experiments recently made by the author of this paper/ 

 showed that when a heavy viscous layer of sealing-wax loaded with 

 sand rested upon a layer of pitch, the heavy wax sank into the lighter 

 layer below, in such a manner as to simulate the effects of com- 

 pression. This point was noticed by Mr. G. W. Lamplugh, who 

 called my attention to it, and he inquired whether the same con- 

 ditions could be applied to the folding of mountain chains. If such 

 were the case the folding would result in local tension, not general 

 compression. 



The experiment above referred to was made for the purpose of 

 showing that when a heavy bed of sandy gravel, or clay with stones, 

 rests upon a soft clay or brickearth, if the two deposits were brought 

 into a viscous condition after frost, then the heavy overlying bed 

 would settle into the lighter bed below in a manner which would 

 produce a structure verycloselyresembling the " trail and underplight" 

 of Spurreir. 



That the crust of the earth is flexible, and that it, therefore, can 

 sink and rise, as denudation and deposition take place, is now 

 generally conceded. In the case of the Gangetic trough Oldham,'' 

 though not considering that the trough owes its origin to the weight 

 of the alluvium, remarks : " But though the weight of the sediment 

 cannot have been the originating cause of the depression of the 

 Gangetic trough, it may have had considerable influence in deter- 

 mining the magnitude of its dimensions, for if there had been some 

 other cause capable of forcing down the level of the crust to a given 

 depth before the resistance to further movement became equal to the 

 force, then the addition of a load of alluvium would enable the same 

 force to lower the level to ^a greater extent than if the hollow had 

 been left empty or only filled with water. The amount of this extra 

 depression would depend on the balance between the force and the 

 resistance ; if both remained appreciably constant, within the limits 

 of the movement involved, the weight of the alluvium would enable 

 this to be carried about five times further than would otherwise be 

 the case, so that the Gangetic trough, taken as 15,000 feet deep, 

 would only liave a depth of about 3,000 feet had it not been filled 

 with alluvium as fast as it was formed." 



The above reasoning is based upon the fact that wherever mountains 

 occur it has been found that the crust beneath them is of low density 

 and that the mountains float upon this lighter material. SimHarly, 

 beneath the deep seas the crust is of high density and the land level 

 is caused to sink to great depths. As changes in the level of the land 

 have been numerous, and of great magnitude during geological time, 

 it is clear that changes in the density of the lower portions of the 

 earth's crust ai-e continually but slowly taking place. As long as 

 such areas of high or low density persist there will continue to be 

 mountains or deep seas. Denudation alone cannot reduce a mountain 



' E. M. Deeley, "Trail and Underplight" : Geol. Mag., Dec. VI, Vol. Ill, 

 pp. 2-5, 1916. 



^ "The Structure of the Himalayas": Geol. Survey of India, vol. xlii, 

 pt. ii, p. 122. 



DECADE VI. — VOL. V. — NO. III. 8 



