BY E. M. JOHNSTON, F.L.S. 53 



kind. The direction of these upheaved tracts, whether deter- 

 mined, as Mr. Darwin suggests, by the effects of internal 

 distortion or by some original features in the structure of the 

 crust, would be apt to be linear. The lines, once taken as 

 lines of weakness or relief from the immense strain, would 

 probably be made use of again and again at successive 

 paroxysms or more tranquil periods of contraction. Mr. 

 Mallet has ingeniously connected these movements with the 

 linear direction of mountain chains, volcanic vents, and 

 earthquake shocks. If the initial trend to the land masses 

 were given as hypothetically stated by Mr. Darwin we may 

 conceive that after the outer parts of the globe had attained a 

 considerable rigidity, and could then be only slightly influnced 

 by internal distortion, the effects of continued secular action 

 would be seen in the intermittent subsidence of oceanic basins 

 already existing, and in the successive crumpling and elevation 

 of the intervening stiffened ridges." To overcome the diffi- 

 culties which arise out of one or other of these hypotheses 

 various modifications of the contraction theory have been 

 generally adopted, the greater number still adhering to the 

 view that the main cause of terrestrial movements must be 

 sought in secular contraction. The theory which seems to be 

 most favoured next in importance to the Contraction Theory 

 is: — 



The alleged expansion and contraction of the underlayers 

 resulting from a rise or fall of temperature caused 

 by the loading or unloading of the areas affected. 



Great uncertainty still exists as regards the present condi- 

 tion of the earth's interior ; some conceiving it to have a fluid or 

 viscous central core ; some again incline to the view that a 

 viscous intermediate layer separates the solid crust from the 

 solid nucleus ; while others have given reasons for the view that 

 on the whole it has now attained a rigidity equal to that of 

 glass or steel. 



Expansion and Contraction of the Undeelayees 



Bestjlting eeom a Eise oe Fall oe Tempeeatuee. 



Babbage seems to have been the first to suggest this theory. 

 In his memoir (1834) on the Temple of Serapis, "Besides 

 recognising the relabions of isothermal planes, and the effect 

 upon them by the surface changes, whether removals of rock 

 material, or accumulation, the memoir accounts for changes of 

 level caused through the expansion or contraction caused by 

 changes in the subterranean heat or in the position of these 

 isogeothermal planes." Mr. Mellard Beade has lately, ' ; Origin 

 of Mountain Banges," strongly insisted on this factor as an 

 important if not the main factor in producing subsidence and 

 elevation. Similar views have also been strongly advocated 



