SUMMARY AND CONCLUSIONS. 133 



He next investigated the possibility of an expansion of the crust 

 by the injection of dykes ; this process seems just about able to 

 produce the amount of force required, but here again the range 

 of motion is inadequate. He finally suggested tlie existence of con- 

 vection currents, rising under the ocean beds, flowing outwards 

 along the under surface, of the crust towards the continents, and 

 giving rise, by a drag on the under surface of the crust, to compres- 

 sion in the continental areas. This cause, granted the existence 

 of the currents, would produce, an ample range of movement, but 

 it is doubtful whether it could produce sufficient force to give rise 

 to a yielding and compression of the crust. The drag exerted by 

 such a current on the underside of the crust would be proportionate 

 to three factors, the co-efficient of friction, the rate of flow, and the 

 length of the tract along which the How takes place ; of these the 

 first would be small, the second probably also small, but the third 

 would be some hundreds of miles, and therefore large, so that the 

 stress, accumulated along a length of the crust, might attain a 

 magnitude sufficient to give rise to compression of the weaker 

 portions of the crust. 1 It seems that, granted the existence of the 

 currents postulated, the effect might be produced, but the con- 

 clusion is by no means established, and the postulate has by no 

 means been accepted, very largely on account of the nomenclature 

 adopted. 



The notion of convection currents connotes, and was certainly 

 intended to imply, a degree of fluidity which appears difficult to 

 grant, but it is important to observe that similar movements might 

 take place in a material which exhibits none of the properties 

 associated with a fluid, as it exists on the surface of the earth. 

 A material having the properties of the asthenosphere of Prof. Barrell, 2 

 would have sufficient power of yielding, to long continued stresses, 

 to permit of the existence of movements analogous to convection 

 currents, and so provide the motive power required. Though 

 possible, however, this explanation can hardly be regarded as 

 probable, or even satisfactory, but it is at least a feasible one, and 

 not more unsatisfactory than any other which has been offered as yet. 



Of these the first to be considered is that of Prof. Suess 

 which, being incorporated and developed in his great work on the 



1 Physics of the Earth's Crust, 2nd ecL, 1889, p. 320. 



2 Journal of Geology, XXII, 1914, p. 666. 



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