THE ARCHEOZOIC ERA. 135 



processes to the close of the period of notable growth by accretion. The 

 central core and this thick zone about it represent the Formative eon 

 (p. 119). (2) The next zone, probably a relatively thin one, is assumed 

 to be made up dominantly of extrusive igneous rocks, with which would 

 be associated subordinate amounts of sedimentary matter and matter 



*<g 



Fig. 34. — Diagrammatic sector of the earth illustrating its composition on the modi- 

 fied 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 intermingled. This is represented by zone 2 in 

 the diagram. The material of this zone is represented as coming to the surface 

 at two points {At). Outside this zone, there is a third zone made up primarily 

 of sedimentary, but subordinately, of extrusive rocks. According to one inter- 

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



gathered in from space. This zone represents the Extrusive eon (p. 

 119). (3) Outside it lies the superficial zone in which sedimentary rocks 

 predominate, though associated with not a little rock of igneous origin. 

 The first two zones outside the core are assumed to be universal, while 

 the outermost zone, being composed primarily of material washed 

 down from the land and deposited in the sea, fails to encircle the globe. 

 The oldest accessible formation. — With these ideal sections in mind, 

 it is pertinent to inquire what might be the nature, of the oldest rock 



