SUMMARY AND CONCLUSIONS. 137 



it is based on the hypothesis of a solid heated, cooling globe, but 

 differs in recounising that this hypothesis necessitates the existence 

 of a zone of tension underneath the outermost layers of the crust. 

 The fact that this consequence follows inevitably from the hypo- 

 thesis, was first pointed out by Mr. Mellard Reade, 1 and once stated 

 is so axiomatic that it immediately met with general acceptance. 

 If we suppose a cooling globe, in which the cooling has only reached 

 a certain distance down from the surface, we have an outer shell, 

 which is contracting and so reducing its circumference, surrounding a 

 central core, which remains unaltered in dimensions, and in those 1 

 circumstances the outer shell must be thrown into a state of tension. 

 Only in the outermost layers will the general reduction of the bulk 

 of the globe, under the layers which, being already fully cooled, 

 are incapable of further contraction, lead to the existence of com- 

 pression, and it has been abundantly shown that the zone of tension 

 must be of much larger dimensions than that of compression. 2 

 Sir Sidney Burrard, however, makes a somewhat different use 

 of this deduction from his predecessors, and considers that the 

 depression of the trough was produced by a withdrawal of material 

 towards the Himalayas, and the range to have been produced by 

 the invasion of the material so withdrawn. Such, eliminating the 

 details of the mechanism invoked, is the essential character of 

 the hypothesis ; it seems to involve a greater tensile strength in 

 the zone of extension than can easily be granted, greater cer- 

 tainlv than that, of any known rocks, as met with near the surface 

 of the earth ; but so little is known, or can be known, of the physica] 

 properties of the material of the earth, when subjected to the tem- 

 peratures and pressures which exist in its interior, that we cannot 

 summarily reject the explanation, on this ground alone. 



In developing his explanation Sir S. G. Burrard is as insistent 

 on the direction of movement as Prof. Suess, though he insists 

 on the exact opposite, and maintains that the Himalayas are due 

 to an underthrust of the sub-crust from the southwards, instead 

 of an over-thrust of the upper layers from the north. This matter 

 has been dealt with already and it has been shown that the distinc- 

 tion is meaningless, so far as the processes which have taken place 

 within the raime itself are concerned ; but it is not meaningless 



1 Origin of Mountain Ranges* 188fi, p. 12I5. 



2 Of. O. Fisher, Physics of the Earth's Crust, 2nd ed., 1889, ch. VIII, for a discussion 

 of this matter and references to earlier literature. 



[ 285 1 



