74 R. D. Oldham — Esmys in Theoretical Geology. 



would be perfectly symmetrical, we may assume that the watershed 

 would be nearer one side than the other, and, consequently, a larger 

 amount of debris removed by denudation would be deposited on the 

 opposite side of the ridge, with the result that the centre of gravity 

 would be shifted towards that side on which the greater quantity of 

 sediment is deposited, that is, to the right of the diagram, Fig. 1. 

 In this state the centres of gravity and of flotation, are no longer 

 in the same vertical, and equilibrium is only restored by a rotation of 

 the disturbed tract accompanied by an extension of the depression 

 to the right and a diminution and ultimate extinction of that to the 

 left. Along with this rotation there would be a general shifting of 

 the tract to the right, which would expose the left-hand side to 

 tension, " which may possibly open fissures downwards on the 

 western side of the range." 



Fig. 2. 



Fig. 1. 



Fig. 1. — Fisher's Physics of the Earth's Crust. Diagram on p. 186. 2nd edition. 



Fig. 2.— Diagram to illustrate the theory of the elevation of the Himalayas. 



scale : hor. about 60 miles, vert, about 30 miles to 1 inch. 

 A. — Massif of the Himalayas. 

 B. — " Root " of the same. 



C. — Earlier marginal deposits, compressed and elevated. 

 c. — Continuation of the same, depressed and undisturbed. 

 D. — Subsequent deposits overlapping C. 

 b. — Sinking of lower surface of crust due to C and D. 



The extinction of the Nummulitic sea of the central Himalayas 

 and the great volcanic outbursts which accompanied it may represent 

 the extinction of the depression on the left-hand side of the diagram, 

 but the greater part of the elevation of the Himalayas having taken 

 place since this, we are at once encountered by the difiicultj'^ that 

 the hypothesis requires a rotation of the whole of the Central Asian 

 plateau. We have no reason to suppose that so large a mass of the 

 earth's crust would have a rigidity sufficient to allow of its rotation 

 as a whole ; on the contrary, there is every reason to suppose that 

 it would yield infinitely to any long continued stress. In Mr. 

 Fisher's investigation it is, however, assumed that all lateral pressure 

 is relieved before the action of denudation commences. Such a 

 simplification of the conditions is essential to a mathematical in- 

 vestigation ; but as in the case of the Himalayas the compression 

 has lasted up to the recent period, while there are not wanting 

 indications that it is still in progress, we may take it that the 

 state of things which the latter part of Mr. Fisher's investigation 

 deals with has not yet been reached, and we may consequently 

 neglect all considerations connected with the rotation of the Hima- 

 layan tract of elevation. 



