134 GEOLOGY. 



3. According to the planetesimal theory, the core of the earth (Fig. 35) 

 is made up of planetesimal matter, perhaps corresponding somewhat 

 in composition to meteorites. After aggregation, the planetesimal 

 matter was probably recrystallized under the influence of the heat 

 and pressure which the aggregation involved, the resulting rock being 

 essentially igneous in its nature. Outside the central core there should 

 therefore be (1) a thick zone made up largely of planetesimal matter, 



Fig. 33. — 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 {JR). This, according to one view, might represent the Archean 

 rock. 



but partly of igneous rocks erupted from below, and partly of sedimentary 

 rocks. The planetesimal matter is assumed to predominate in the 

 lower and major part of this zone; igneous rock, eruptive and irruptive, 

 is assumed to have a somewhat irregular distribution within it; while 

 the sedimentary rock increases in importance above, though remaining 

 throughout a very subordinate constituent. This zone records the 

 growth of the earth from the initiation of volcanic and atmospheric 



