of the Earth's Crust in Cooling. 



591 



variable value, depending on the distance of the moon from 

 the earth, but always between 10 14 and 10 15 , the process 

 of transference of angular momentum from the earth to 

 the moon in consequence of tidal friction would take place 

 at the greatest rate possible ; and the changes would be 

 complete in about 50 million years. This optimum viscosity 

 must evidently have been attained at some time during the 

 process of cooling, by what has been above stated, so that 

 for a considerable part of the time the conditions must 

 have been suitable for evolution to take place at the 

 maximum rate possible. The truth of the theory of Tidal 

 Evolution thus follows as a natural consequence of the 

 present paper, and must therefore be considered with it. 

 Most of the evolution must have taken place in a com- 

 paratively short time, perhaps about 200 million years, 

 and since then the viscosity must have been too great for 

 much change to take place ; so that now the mean distance 

 of the moon must be nearly constant. The most rapid 

 part of the change must have occurred at a fairly high 

 temperature, probably well above the boiling-point of water, 

 but after solidification. It is thus to be placed before the 

 oldest sedimentary rocks, and the consequent change in 

 the ellipticity can have had little direct effect in the forma- 

 tion of the present mountain range-, which have for the most 

 part been elevated since the Carboniferous period. In one 

 way, however, it may be important. The ellipticity of the 

 eaith would decrease as the velocity of rotation decreased, 

 and the consequent shortening of the equator would form 

 pre-Cambrian mountain ranges with a north and south 

 alignment. On account of the plasticity of the rocks, 

 however, which then extended nearly to the outer surface, 

 these might soon cause fracture below them and disappear. 

 In any case their existence might, however, leave a record 

 in the form of definite lines of weakness, which would 

 tend to localise subsequent folding caused by contraction. 

 The tendency of folding to occur repeatedly along the same 

 line in different geological periods may be regarded as 

 confirming this suggestion. 



In conclusion, the author wishes to express his indebtedness 

 to Mr. Arthur Holmes, whose helpful criticism from the geo- 

 logical point of view has been invaluable. 



I 



