238 PROGRESS OF SCIENCE IN THE CENTURY. 



Sollas suggests, a localisation of the water in primi- 

 tive faint depressions (anti-cyclonic areas), and a 

 corresponding reduction of pressure on the incipient 

 continental areas, there might result an expansion of 

 the underlying rock of these areas, ^' for a great 

 change of volume occurs when the material of igneous 

 rocks passes from the crystalline state to that of 

 glass." In some such way, the ocean basins might be 

 deepened and the continental areas raised. The hot 

 water of the primeval ocean would act energetically 

 on the silicates of the primitive crust ; it would begin 

 to be " salt " with saline solutions and to precipitate 

 deposits. Since the condensation of the oceans, 

 Prof. Joly suggests a lapse of eighty to ninety mil- 

 lions of years. 



To sum up dogmatically ivould he absurd, hut it 

 may he said that a nebular mass probably gave rise 

 to a rapidly rotating molten planet; that after central 

 solidification, this may have given birth to the moon; 

 and that as cooling slowly continued, there followed 

 the consolidation of the crust and the beginning of 

 the distinction between ocean basins and continental 

 areas. 



Through phases more or less like those outlined 

 above the Earth has reached its present state. The 

 vast nucleus or '^ centrosphere," is practically solid, 

 the melting-point of the metals and metalloids being 

 raised by the immense pressure. Outside the cen- 

 tral mass there is " a shell of materials bordering 

 upon fusion,'' that which Sir John Murray calls 

 the ^^ tektosphere." On this plastic shell there rests 

 the heterogeneous and wrinkled crust or lithosphere, 

 always slightly pulsating. 



Wrinkling of the Lithosphere. — How the crust or 

 lithosphere has come to be elevated into continental 



