H. D. Oldham — Essays in Theorciical Geology. 75 



We liave now to consider what modifications are required to fit 

 the purely mathematical theory to the more complex conditions of 

 actuality. In the first place the elements of rigidity and rotation 

 being abandoned, we need not consider the left-hand side of the 

 diagram, and may redraw it in greater accordance with the conditions 

 of the Himalayan region. (See Fig. 2.) We have now an elevated 

 region A A subjected to denudation, and adjoining it an area 

 extending to B,, on which deposition is taking place, the deposits 

 being contributed by the elevated ground A to the north, and the 

 waste of the rock area to the south. The tract being supposed to be 

 in equilibrium, as A is lightened by denudation, the surplus floating 

 power of B will cause it to rise, and the load thrown on D will cause 

 it to sink, especially in the neighbourhood of A, where the load is 

 greatest, till the magma displaced by the lower surface of the crust 

 is sufficient to float the load. The result will be, firstly an extension 

 of the depression in a direction away from the elevated tract A, 

 and secondly a strong tendency to either fracture or flexure of the 

 crust at the junction of A and D. 



As we may take the crust to be infinitely yielding to long-continued 

 stresses, there is no reason why that produced by the lightening 

 of the one area and the loading of the other should not be relieved 

 simply by the sinking of the latter and the rising of the former 

 on either side of a separating plane. But denudation and deposition 

 are not the only forces at work ; for, to bring the case into connection 

 with that of the Himalayas, we must suppose compression to be 

 continually at work. This will be relieved partly by an additional 

 elevation of A, but also by the compression and consequent elevation 

 of the marginal deposits of D, which would not offer the same 

 resistance as the already consolidated beds of A. In this way the 

 deposits on the edge of the depression D would gradually come 

 to form part of the tract A, whose boundary would advance towards 

 JB, but not to the same extent as the shifting of the outer boundary 

 of the depression towards B! . 



This must not be regarded as a modification, but rather as an 

 amplification of Mr. Fisher's theory ; it is a more detailed investiga- 

 tion of a part of the process which does not lend itself to mathe- 

 matical treatment. The elevation of the marginal deposits of the 

 depression is of a different nature to that referred to by Mr. Fisher, 

 an elevation which belongs to the period of decadence of the range, 

 and is unaccompanied by disturbance. The Himalayas have only 

 just completed their growth, if they are not still growing, and the 

 elevated marginal deposits with which we have to deal owe their 

 elevation to compression, and belong to the period of growth of 

 the range. 



Taking this amplification of the theory, we find that a mountain 

 range which has completed or nearly completed its growth, but not 

 entered on the period of its decadence, should show the following 

 features : — 



I. There should be a region continuously exposed to denudation 

 and simultaneously to elevation. 



