The Compression of the Eartlis Crust in Cooling. 575 



seem, therefore, definitely established that ionization takes 

 place in helium under an applied potential o£ about 20 volts. 

 This fact can be reconciled with Bohr's theory only by 

 making certain new assumptions as to the mechanism of 

 impact ionization in helium. These new assumptions have 

 not yet been subjected to experimental investigation. 



I wish to express my obligation to Professor 0. W. 

 Richardson for valuable advice given throughout the course 

 of these experiments. The quartz tube was constructed by 

 Mr. Reynolds of the Silica Syndicate. 



Wheatstone Laboratory, 

 King's College. 



August 20, 1916. 



LXIII. The Compression of the Earth's Crust in Cooling. 

 By Harold Jeffreys, B.A., M.Sc, Fellow of St. John's 

 College, Cambridge *. 



I. Introduction. 



IT is generally agreed among geologists that the principal 

 cause of the elevation of continents and mountains is 

 that the crust of the earth must be in a state of horizontal 

 compression, under which it frequently gives way, the strata 

 being then folded into a shorter length in the neighbourhood 

 of the point where the weakness has been shown. Such a 

 compression appears to be the only mechanism that has been 

 suggested that is qualitatively capable of producing the 

 observed results. The cause of the compression itself is, 

 however, very uncertain. The contraction hypothesis is the 

 most satisfactory of those that have been offered, but grave 

 doubts have frequently been expressed about its quantitative 

 adequacy. According to this hypothesis, the earth was 

 originally at a very high temperature, and different parts 

 have since cooled by different amounts, changing in volume 

 in consequence. A state of strain is thus set up in the crust. 

 The mathematical aspect of the theory is due originally to 

 Dr. C. Davison f and Sir G. H. Darwin % ; they showed 

 that as the inner layers after a certain time are cooling more 

 rapidly than the outer ones, and therefore contracting more 

 rapidly, the outer layers will have to undergo compression 



* Communicated by the Author, 

 t Phil. Trans. 178 A, pp. 231-242 (1887). 



t Phil. Trans. 178 A, pp. 242-249 (1887); or Sci. Papers, vol. iv. 

 p. 354. 



