An Unrecognised Agent in the Deformation of Bocks. 395 



on. All below this zone is ready at the slightest strain to give way 

 and flow in the direction of pressure, and this while the whole is 

 entirely solid. From the rate of propagation of earthquake waves 

 through the centre of the earth, it is found that the effective rigidity 

 is nearly twice that of steel,'" so that we must reject the idea that the 

 centre of the earth is a molten mass and the crust is a thin skin formed 

 on cooling. The present state of our knowledge indicates the reverse, 

 namely, that the earth solidified from the centre outwards, and the 

 apparent fluidity of the interior to be due to the weakness of the 

 material to withstand the forces in operation at great depths. 



T am not a sufficient mathematician to go into the question of 

 how this agrees with the results of astronomical observations on the 

 attraction of the moon on the earth, and the amount of distortion 

 caused by the revolution of the earth ; I only want to put the purely 

 geological view of the case. 



I do not wish also here to examine the effects of these three 

 elements of distortion in combination. It is sufficiently obvious 

 that any two combined must lessen the depth at which deformation 

 can take place, but the amount effected will not have any bearing on 

 my argument. One point, however, has already come out, namely, 

 that time is an element which has to be taken into account. In the 

 question of solution this was obvious, and in the case of earthquakes, 

 although the centre of the earth is in a state of perfect plasticity, a 

 sudden shock finds it more rigid than steel. 



I will now endeavour to show that time may produce certain 

 effects with agents so insignificant, that they would ordinarily be 

 considered wholly insufficient to accomplish. 



The secular bending of marble and alabaster is often compared 

 with the flow of viscous materials, like sealing-wax and pitch, but 

 the two cases are not analogous. Instances of this bending are 

 known from many places. At the Alhambra f alabaster jambs of 

 doorways have curved inwards, and in churchyards slabs of marble 

 supported only on their ends have slowly bent downwards.]: 



In the case of the marble we have a material composed of grains 

 of calcite crystallised in rhombohedra and twinned parallel to a 

 rhombohedron face. There is no question of heat or solution, for 

 the same effects have been observed in old churches when the slabs 

 have remained dry and at an equal temperature for one or two 

 centuries. 



* C. S. Knott, Scottish Geographical Journal, 1899 ; J. Milne, Geographical 

 fTournal, 1903, p. 7. 



t S. Pickering, Nature, 1902, vol. 65, p. 81.- 



I Sir A. Geikie, Roy. Soc, Edinb. ; T. J. J. See, Nature, vol. 65, 1902, p. .%. 



