CHAPTER III. 

 THE ARCHEOZOIC ERA. 



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

 in the last chapter are at best only inferential, since they are not based 

 on facts definitely recorded in any part of the earth's structure which 

 is open to observation; yet they form, none the less, an indispensable 

 preparation for the most intelligent study of that part of the earth's 

 history which is recorded in accessible rock formations. As a brief 

 resume in a more concrete form, Figs. 33, 34, and 35, which represent 

 diagrammatic radial sections illustrative of the different conceptions 

 of the constitution of the earth's interior, arc here introduced with 

 the following summary: 



1. According to the current conception of the history based on the 

 gaseo-molten hypothesis (Fig. 33) there should be pre-sedimentary 

 igneous or meta-igneous rock at all points below the prevailing surface 

 sedimentary rocks. The plane of demarkation between these two 

 classes of rock should be a sharp one, except in so far as obscured by 

 the extrusions and intrusions of igneous rock which cross it, and by 

 the infolding of sedimentary rocks in the lower mass. The fact that 

 both the infolded sedimentary and the extruded and intruded igneous 

 rocks may have become metamorphic, might make the separation of 

 the pre-sedimentary rocks from the rocks of lesser age difficult, locally; 

 but taken as a whole, the distinction should be recognizable. 



2. According to the modified form of the gaseo-molten hypothesis, 

 suggested in Chapter II (p. 88), there would be no sharp plane of 

 demarkation between the original crust below and a dominantly sedi- 

 mentary or meta-sedimentary series above, for between them there 

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

 or their metamorphic equivalents (Fig. 34). This intermediate zone, 

 in turn, might not be sharply differentiated either from the original 

 crust below or the sedimentary group above. 



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