138 GEOLOGY. 



be confidently referred to the Archeozoic era, for, according to this 

 hypothesis, rocks of organic origin and rocks containing organic prod- 

 ucts were not only mingled with all series that are accessible, but with 

 an indefinitely deep series below, since life is supposed to have originated 

 long before the earth acquired its growth. The oldest formations 

 known may also be Archeozoic under the modified phase of the molten 

 hypothesis which has been presented (p. 88) ; but under the simpler 

 form of the hypothesis of a molten earth, the original crust cannot be 

 placed in the Archeozoic system, for its formation antedated the appear- 

 ance of life. It must therefore be assigned to an earlier "azoic" or 

 "lithic " era (see p. 83). Since recent investigations render it extremely 

 doubtful whether any accessible rocks can be referred to the supposed 

 original crust, there is little ground for referring any known rocks to 

 an azoic era antedating the Archeozoic. 



To avoid all difficulties arising from doubt as to the downward 

 extension of organic rocks, the non-committal term Archean (Archean 

 system, Archean complex) is commonly applied to the formations 

 here referred to the Archeozoic era. This term is properly applicable 

 to the oldest group of accessible rocks, whatever their origin, and 

 whether contemporaneous with life or antedating its introduction. 



Delimitations. — The lower limit of the Archean system is assumed 

 to be inaccessible. Its upper limit has been fixed at different horizons 

 by different authors. The term Archean (very old) was originally 

 introduced to displace the older terms Azoic (without life) and Eozoic 

 (dawn-life), whose etymological meanings made them sometimes 

 inadmissible and sometimes of uncertain application. Originally 

 the term was made to include all rocks below the abundantly fossil- 

 bearing (Cambrian and later) systems; but progressive study has shown 

 that there are three or more great systems of sedimentary or meta- 

 sedimentary rocks (with inter bedded igneous sheets), unconformable 

 with one another, between the Cambrian sedimentaries above and the 

 great complex of igneous and metamorphic rocks below. It has been 

 thought by leading geologists especially engaged in the investigation 

 of these formations, that the pre-Cambrian systems of dominantly 

 sedimentary rocks should be separated from the dominantly igneous 

 or meta-igneous complex below. 1 Following their lead, these pre- 



1 Irving, Seventh Ann. Rept., U. S. Geol. Surv., pp. 448-454, and Van Hise, Bull. 86, 

 U. S. Geol. Surv., pp. 491-493. 



