THE ARCHEOZOIC ERA 



437 



Primitive igneous 

 rock. 



Primitive igneous 

 rock. 



Planetesimal matter 

 predominant; igneous 

 rock abundant; sedi- 

 mentary rock - a minor 

 constituent, increasing 

 toward the surface. 



Planetesimal 



matter with 



more or less 



igneous 



rock. 



Fig. 337. 



Fig. 338. 



Fig. 339. 



Fig. 337. A diagrammatic sector of the earth illustrating its structure 

 according to the Laplacian hypothesis. The great body of the earth 

 is made up of the original igneous rock. Sedimentary rocks, together 

 with some extrusive rocks, make but a thin coating, represented in the 

 diagram by black, outside the great igneous interior. The original 

 igneous rock is represented as appearing at the surface in some places 

 (S). This, according to one view, might represent the Archean rock. 



Fig. 338. Diagrammatic sector of the earth illustrating its composition 

 on the modified form of the Laplacian hypothesis. As in the preceding 

 figure, the great body of the earth is made up of the original igneous rock. 

 Outside this original igneous mass, there is, according to this hypothesis, 

 a zone of extrusive material, with perhaps some sedimentary rock inter- 

 mingled. This is represented by zone 2 in the diagram. The material 

 of this zone is represented as coming to the surface at two points (jl). 

 Outside this zone, there is a third zone made up primarily of sedimentary, 

 but subordinately, of extrusive rocks. According to one interpretation, 

 the material of the second zone might constitute the Archean rock. 



Fig. 339. Diagram representing the structure of the earth according to the 

 planetesimal hypothesis. The material of zones 1 and 2 is indicated 

 ' in the diagram. Zone 3 of this figure corresponds to zone 2 of Fig. 338, 

 and zone 4 of this figure corresponds to zone 3 of Fig. 338. 



