250 SEE— FURTHER RESEARCHES ON [April 24, 



The structure of the crust is so complex, and the resistance offered by it 

 to the pressure is consequently so varied, that abundant cause is furnished 

 for almost any diversity in the forms and distribution of the wrinkles into 

 which it is thrown. It is evident, however, that the folds have tended to 

 follow a linear direction. In North America, from early geological times, 

 they have kept on the whole on the lines of meridians. In the Old World, 

 on the contrary, they have chosen diverse trends, but the last great crumplings 

 — those of the Alps, Caucasus, and the great mountain ranges of central 

 Asia — have risen along parallels of latitude. 



" Mountain chains must therefore be regarded as evidence of the shrink- 

 age of the earth's mass. They may be the result of one movement, or of a 

 long succession of such movements. Formed on lines of weakness in the 

 crust, they have again and again given relief from the strain of compression 

 by undergoing fresh crumpling and upheaval." 



Geikie's views may be considered the accepted view.s of geolo- 

 gists generally, and it will be seen that they rest on the theory of 

 contraction due to sectilar cooling. 



On the constitution of the globe Geikie qttotes (" Geology," 

 p. y^) from the paper of Arrhenius, " Zur Physik des Vulcanismus " 

 (1900), the following theory of the illustrious Swedish physicist: 



" If the rocks at the earth's surface have a density half that of the 

 globe as a whole, and if the density continues to hold good for the magma 

 that arises from the melting of these rocks, we must conceive the existence 

 of a much denser substance in the earth's interior. On various grounds, 

 such as the preponderance of iron in nature, both in meteorites and in the 

 sun, and the phenomena of terrestrial magnetism, it may be inferred that 

 this substance is metallic iron. In consequence of its greater density this 

 iron will naturally be deeper than the rock magma, and on account of the 

 high temperature must exist in a gaseous condition. Somewhere about a 

 half of the planet therefore should consequently consist of iron, and of 

 other metals mingled with it in smaller proportions. The semi-diameter of 

 this gaseous iron-sphere will thus include about 80 per cent, of the earth's 

 semi-diameter. Then will come about 15 per cent, of the gaseous rock magma, 

 next to it the liquid rock-magma for a thickness of about 4 per cent, of 

 the terrestrial semi-diameter, and lastly the solid crust, for which not more 

 than I per cent, may be claimed" (pp. 404-5). 



Referring to the light thrown on the constitution of the interior 

 by the observation of waves propagated by earthquakes, Geikie also 

 adopts the theory of Arrhenius, which is as follows : 



"The density of much the largest part (reckoned linearly) of this 

 interior, amounting, as above stated, to about 80 per cent, of the radius, 

 must be nearly three times higher than that of quartz. Since now the 

 mean velocity of transmission of earthquake waves in tlic interior of the 



