130 OLDHAM : THE STRUCTURE OF THE "HIMALAYAS, ETC. 



support by flotation, and the range would sink, carrying with it 

 the crust on either side till a condition of equilibrium was 

 attained. This explanation carries with it the necessity of a 

 depression on both sides of the range ; it renders the elevation ? of 

 the marginal deposits almost impossible, and is in contradiction 

 to the excess of support which is actually found in the central 

 Himalayas. The latter condition could, however, easily be met 

 by putting the neutral zone at a higher level. If placed so that 

 the amount of the crust below were ten times that above, which 

 would correspond to a depth of about two and one-third of a mile 

 in a crust of 25 miles in thickness, the downward protuberance 

 would exceed the upward one in just about the proportion necessary 

 to provide a small excess of flotation. 



In some respects a neutral zone so near the surface would be 

 welcome, for some of the complicated structures, which have been 

 revealed by geological survey of the more highly disturbed regions 

 of the earth, certainly seem easier of explanation if we can 

 consider the relief from compression as having taken place in a 

 downward, rather than an upward, direction, and it is equally 

 easier to accept these structures as having been brought upwards 

 from a depth of a couple of miles than from five times that 

 depth. On the other hand, a neutral zone so near the surface 

 seems to give an inadequate cover for the production of a com- 

 plicated folding of hard rocks, such as could only take place, without 

 crushing and fracture, under a heavy superincumbent load of rock. 

 A more important objection to this explanation is the fact that, 

 though it would provide an adequate amount of support, it would 

 not provide for the alternate defect and excess of compensation, 

 which is revealed by observation, for, so long as the neutral zone 

 is maintained at the same absolute level, preserving the same 

 proportion between the thickness of crust above to that below it, 

 the relative dimensions of the upward and downward protuberance 

 would remain unchanged, and the hills would be uniformly over- 

 or under-compensated, as the case might be. 



A relief from this difficulty may be obtained in several directions. 

 In the first place if the neutral zone maintained a nearly constant 

 depth from the surface, instead of the same fraction of the total 

 thickness of the crust, the downward protuberance under the 

 central range would be developed in greater proportion to the 

 upward one, and the excess of buoyancy attained. The distri- 



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