THE GEOSPHERES 235 



the continental areas of the lithosphere lighter 

 than the sub-oceanic areas, because of the 

 lower specific gravity of these siliceous 

 deposits. 



If the surface of the earth was originally a 

 molten mass, we may assume that all the 

 silica (SiOg) was originally in combination 

 with bases, and that the primeval rocks were 

 basic rather than acid in composition. With 

 the first precipitation of rain on this primeval 

 crust, many substances were doubtless washed 

 down from the atmosphere, and many new 

 compounds were formed. When the surface 

 cooled and the primeval crust formed, the 

 processes which we now see in operation 

 would soon be established. As at the present 

 time, carbonic acid being in aqueous solution 

 would attack the felspars and other silicates, 

 carry the bases away in solution, and a con- 

 siderable part of the silica would be left on the 

 continents to form vein quartz. When this 

 rock was again melted and re-formed, it would 

 contain more silica than the original one, and 

 would be more acid in composition. With a 

 repetition of this process the rocks on the con- 

 tinents would become on the whole more and 

 more acid — would approach the composition 

 of granite and gneiss. 



The processes here indicated would ulti- 

 mately result in a great accumulation of 



