128 OLDHAM: THE STRICTURE OF THE HIMALAYAS, ETC. 



Turning; now to the interpretation of these two lines, we see 

 on the extreme left of the figure, that the firm line is below the dotted 

 one, representing the greater depth of " root " required to produce 

 the excess of compensation which exists in this region. To the 

 right, but still within the region of the hills, this excess of com- 

 pensation disappears and we enter a region where the crust is up- 

 lifted, as a whole, by the excess of buoyancy to the left, the hills 

 are still compensated to a large extent, but not completely, and 

 the defect may reach a maximum of about the equivalent of 2,000 

 feet of rock, or one-sixth to one-fifth of the whole amount of what 

 would be complete local compensation of this portion of the range. 

 Further to the right, this uplift gradually dies out and a condition 

 of equilibrium is reached, at a point somewhat beyond the outer 

 limit of the visible hills, but not maintained, for the weight of the 

 tract which has been uplifted by the excess of buoyancy in the 

 central region bears down the crust on the side towards the plains, 

 and causes the crust to be depressed below the level of equilibrium, 

 giving rise to the depression of the Gangetic trough. This depres- 

 sion reaches its maximum limit and then the buoyancy of the crust , 

 further away from the hills, causes it to bend upwards, till a con- 

 dition of equilibrium is again reached, at a point which seems to 

 lie not far from the southern boundary of the alluvium, where it 

 attains its greatest development and width, but to lie south of the 

 boundary in the region of the Aravalli hills, and where the Raj- 

 mahal hills project into the alluvial area west of the Gangetic delta. 

 Further to the right the condition of equilibrium is, once more, nol 

 maintained, but the downward tilt of the crust to the left is continued 

 as an upward tilt to the right, with a corresponding rise of the under 

 surface of the crust, till the weight of the unsupported crust beyond 

 puts an end to this uplift, and the crust bends downwards again into a 

 condition where the influence of the Himalayan range is no longer felt. 



It will be seen that this development of the consequences which 

 would result from the hypothesis of a floating crust, supported 

 on a denser, plastic, but not necessarily liquid, substratum, is in 

 close accordance with the larger features of the structure of the 

 country south of the Himalayas. It provides for the trough, for 

 the elevation of part of the earlier deposits formed from the waste 

 of the hills on the north of this trough, and for a gradual extension, 

 by progressive regular subsidence, to the southwards, as the range 

 itself grew in magnitude ; it provides also for that v belt of positive 



[ 27G ] 



