CHAPTER III 

 THE NATURE OF THE MATERIALS IN THE LITHOSPHERE 



The rigid quality of our planet. For a long time it was sup- 

 posed that the solid earth constituted a crust only which was 

 floated upon a liquid interior. This notion was clearly an out- 

 growth of the then generally accepted Laplacian hypothesis of 

 the origin of the universe, which assumed fluid interiors for the 

 planets, the crust being suggested by the winter crust of frozen 

 water upon the surface of our inland lakes. To-day the nebular 

 hypothesis in the Laplacian form is fast giving place to quite 

 different conceptions, in which solid particles, and not gaseous 

 ones, are conceived to have built up the lithosphere. The analogy 

 with frozen water has likewise been abandoned with the discovery 

 that frozen rock, instead of floating, sinks in its molten equivalent. 



Yet even more cogent arguments have been brought forward 

 to show that whatever may be the state of aggregation within the 

 earth's core and it may be different from any now known to 

 us it nevertheless has many of the properties recognized as 

 belonging to solid and rigid bodies. Provisionally, therefore, we 

 may regard the earth's core as rigid and essentially solid. It was 

 long ago pointed out by the late Lord Kelvin that if our litho- 

 sphere were not more rigid than a ball of glass of the same size, it 

 would be constantly passing through periodic six-hourly distortions 

 of great amplitude in response to the varying attractions of the 

 moon. An equally striking argument emanating from the same 

 high authority is furnished by the well-known egg-spinning demon- 

 stration. For illustration, Kelvin was accustomed to take two 

 eggs, one boiled and the other raw, and attempt to spin them 

 upon their ends. For the boiled, and essentially solid, egg this is 

 easily accomplished, but internal friction of the liquid contents of 

 the raw egg quickly stops any rotary motion which is imparted to 

 it. Upon the same grounds it is argued that had the earth's 

 interior possessed the properties of a liquid, rotation must long 

 since have ceased. 



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