CHAPTER XIV 

 THE ARCHEOZOIC ERA 



The sketches of the early stages of the earth's history presented 

 in the last chapter are no more than inferential, yet they afford 

 a helpful introduction to the study of that part of the earth's history 

 which is recorded in the rocks. Figs. 337-339 represent diagram- 

 matic radial sections illustrative of the different conceptions of the 

 constitution of the earth's interior. The following summary 

 should make the figures clear: 



1. According to the conception of the history of the earth 

 based on the Laplacian or gaseo-molten hypothesis, there should 

 be pre-sedimentary igneous or meta-igneous rock at all points 

 below the prevailing sedimentary rocks of the surface (Fig. 342) . 

 The plane of demarkation between these two sorts of rock should, 

 as a rule, be distinct. 



2. According to the suggested modification of the gaseo-molten 

 hypothesis (p. 430), the above distinction would fail; for between 

 the original crust and the sedimentary rocks above, there should 

 be a zone composed of mingled igneous and sedimentary rocks, or 

 their metamorphic equivalents (Fig. 338). This intermediate 

 zone might not be sharply differentiated either from the original 

 crust below or the sedimentary group above. 



3. According to the planetesimal theory, (1) the core of the 

 earth (Fig. 339) is made up of nebular or planetesimal matter. 

 After aggregation, the planetesimal matter was probably recrystal- 

 lized under the influence of the heat and pressure which the aggre- 

 gation involved, the resulting rock being essentially igneous in its 

 nature. Outside the central core there should be (2) a thick zone 

 made up largely of planetesimal matter, but partly of igneous 

 rocks erupted from below, and partly of sedimentary rocks. The 

 planetesimal matter is assumed to predominate in the lower and 



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