CHAPTER XIII 

 THE ARCHEOZOIC ERA 



Though the preceding sketches of the early stages of the earth's 

 history are but hypothetical, they afford a helpful introduction to 

 the study of that part of the earth's history recorded in the rocks. 

 Figs. 291-293 represent diagrammatic radial sections which illus- 

 trate the different conceptions of the constitution of the earth. 

 The following summary should make the figures clear: 



1. According to the Laplacian hypothesis, there should be pre- 

 sedimentary igneous or meta-igneous rock everywhere below the 

 prevailing sedimentary rocks of the surface. The plane of demark- 

 ation between these two sorts of rock should, as a rule, be distinct. 



A modification of the Laplacian hypothesis, so far as applied to earth, pos- 

 tulates that much gas and vapor were occluded in the molten earth, instead of being 

 all in the atmosphere. On this assumption, it is conceived that there might have 

 been a period of great vulcanism after the formation of a crust, and that the 

 original crust was covered deeply with extruded rock (Fig. 292). If this were the 

 case, the original crust might not be accessible. On the meteoritic hypothesis of 

 the earth's origin, the conditions would have been much as on the planetismal 

 hypothesis so far as concerns the oldest rocks accessible. 



2. According to the planetesimal theory, (i) the core of the 

 earth (Fig. 293) is made up of planetesimal matter. After aggrega- 

 tion, this matter was probably re-crystallized under the influence 

 of the heat and pressure which the aggregation involved, the result- 

 ing rock being essentially igneous in its nature. Outside the central 

 core there should be (2) a thick zone made up largely of planet- 

 esimal matter, but partly of igneous rock erupted from below, and 

 partly of sedimentary rocks formed at the surface at all stages of 

 the earth's growth after the hydrosphere came into existence. 

 The planetesimal matter is assumed to predominate in the lower and 

 major part of this zone. Igneous rock is assumed to have a some- 

 what irregular distribution through it, while sedimentary rock 

 increases in importance above, but remains throughout a subordinate 

 constituent. This zone records the growth of the earth from the 

 beginning of volcanic and atmospheric processes, until it reached 





