Dr. Harold Jeffreys — Causes of Mountain Building. 219 



crust would then prevent oceanic tides from reaching any noticeable 

 size; earthquake waves could not be transmitted through the fluid 

 portions ; further, isostasy would be perfect, which is not the case. 

 Below the layer of compensation, at a depth of probably some 

 300 kilometres, there is a layer of weakness known as the 

 " asthenosphere ", where flow appears to be produced much more 

 easily than in the outer portions and most of the isostatic adjustment 

 takes place. The properties of earthquake waves nevertheless show 

 that it is very rigid. 



The laws of elastico-viicosUy and Jirmo-viscosttt/ that I have used in 

 previous papers are precise mathematical expressions of particular 

 types of plasticity and elastic after-working respectively. 



In the above discussion nothing has been said about the mechanism 

 that causes imperfection of elasticity, and the argument is independent 

 of this. Barrell believes that adjustment in the asthenosphere takes 

 place by progressive local melting or solution under shear; the 

 melted parts immediately flow till the shear is reduced to zero, and 

 thus the shears are always kept small. In crystals it may take 

 place by sliding on the cleavage planes ; such bodies as pitch may be 

 deformed by molecular displacement without anything resembling 

 fracture ; brittle substances may be crushed to powder and then 

 spread out by the stress ;'but in any of these cases the recovery after 

 the stress is removed is incomplete and the general description of 

 plasticity applies. Elastic after- working is to be attributed to 

 iiitermolecular friction. 



Mr. Deeley discusses at much length the question whether the 

 liquid and solid states pass into each other continuously or discon- 

 tinuously. The sudden change from one to tlie other at a definite 

 temperature is a characteristic property of pure substances; the 

 impure aggregates with which geologists have for the most part to 

 deal may be expected to pass through a pasty state when heated, in 

 which flow becomes steadily more easily produced. The question is 

 not, however, a vital one; there is little liquid within the earth, and 

 when it does occur it is probably produced, not by heating, but by 

 local release of pressure. Most of the interior is probably at a 

 liigher temperature than the melting-point of ordinary rocks at 

 ordinary pressures, and is only kept solid by the high pressures there 

 existing. A local fracture would release this and render melting 

 possible. 



Keferences. 



Bareell (Joseph). Journal of Geology, vol. xxii, 1914 ; vol. xxiii, 1915. 



Holmes (Arthur). " Radio-activity and the Earth's Thermal History " : 

 Geol. Mag., February-March, 1915; June, 1916. " Eadio-activity and 

 the Measurement of Geological Time": Proc. Geol. Assoc, vol. xxvi, 

 1915. 



Jeffreys (Harold). " The Compression of the Earth's Crust in Cooling" : 

 Phil. Mag., vol. xxxii, pp. 575-91, 1916. " The Viscosity of the Earth " 

 (Third Paper): Monthly Notices of R.A.S., vol. Ixxvii, pp. 449-56, 1917. 



Love (A. E. H.). Proc. Roy. Soc, vol. IxxxiiA, pp. 73-88, 1909. 



