THE ARCHEAN ROCKS 317 



This lowest and oldest group of rocks is very complex, embracing 

 lava Hows, volcanic tuffs, igneous intrusions and sedimentary rocks, 

 all more or less metamorphosed and deformed. Distinct fossils 

 have not been found in them, but the presence locally of (i) carbo- 

 naceous slates similar to younger slates containing carbon of organic 

 origin, and (2) occasional formations of limestone and chert, are 

 thought to imply the existence of life, and to warrant placing the 

 era when these rocks were formed in the zoic group of eras (p. 313). 

 The time during which, or during the later part of which, this oldest 

 system of accessible rocks was made, is the Archeozoic era. 



Under the planetesimal hypothesis, the oldest known rocks may 

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

 hypothesis, rocks of organic origin and rocks containing organic 

 products were not only mingled with all series that are accessible, 

 but with great thicknesses of rock below, since life is supposed to 

 have originated long before the earth acquired its present size. 

 The oldest formations known also may be Archeozoic under the 

 modified form of the nebular hypothesis (Fig. 292); but under the 

 original form of the hypothesis, the original crust cannot be Archeo- 

 zoic, since it antedated life. The term Archean (Archean system,' 

 Archean complex) is applied to the formations here referred to the 

 Archeozoic era. This term is applied to the oldest group of accessi- 

 ble rocks, whatever their origin, and whether contemporaneous with 

 life or antedating it. 



Delimitations of the Archean. The bottom of the Archean sys- 

 tem is assumed to be inaccessible. Its upper limit has been fixed 

 differently by different investigators. As first defined, the Archean 

 (\ery old) included all rocks below the Cambrian (p. 323); but 

 later it became clear that the rocks below the Cambrian should be 

 differentiated into two great groups, the upper of which consists of 

 several great systems of dominantly sedimentary or meta-sedi- 

 mentary rocks, unconformable with one another, while the lower 

 is dominantly igneous and meta-igneous. The term Archean is now 

 generally restricted to the latter. The upper limit of the Archean 

 is therefore the base of the oldest dominantly sedimentary system, 



GENERAL CHARACTERISTICS OF THE ARCHEAN 1 



As now defined, the Archean includes two great classes of forma- 

 tions, (i) a great schist series, and (2) a great granitoid series. 



1 Van Hise and Lcith, Mono. LII. U. S. G. S. Chapter XX, and i6th Ann. Kept. 

 U. S. G. S., Pt. I. pp. 744-759. 



