440 GEOLOGY 



but under the simpler form of the hypothesis of a molten earth, the 

 original crust cannot be called Archeozoic, since it antedated life. 

 The term Archean (Archean System, Archean Complex) is often 

 applied to the formations here referred to the Archeozoic era. 

 This term is applied to the oldest group of accessible rocks, what- 

 ever their origin, and whether contemporaneous with life or ante- 

 dating it. 



Delimitations. The lower limit of the Archean system is 

 assumed to be inaccessible. Its upper limit has been differently 

 fixed 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 no longer appropriate for the rocks to which they were 

 applied. As first defined the Archean included all rocks below the 

 Cambrian (p. 476); but progressive study has shown that there 

 are several great systems of sedimentary or meta-sedimentary 

 rocks (with much igneous rock), unconformable with one another, 

 between the Cambrian system above and the Archeozoic system 

 below. The systems of pre-Cambrian rocks which are dominantly 

 sedimentary, should be separated from the dominantly igneous 

 or meta-igneous complex below, and 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 understood, the Archean is made up of two great classes 

 of formations: viz., (1) a great schist series, and (2) a great granitoid 

 series. 



1 Bull. 86, U. S. Geol. Surv., pp. 478-484, and 16th Ann. Kept., U. S. Geol. 

 Surv., Pt. II, pp. 747-756. The interpretation of the Archean as set forth in 

 these publications needs some modification in the light of recent discoveries. 

 The sedimentary character of parts of the system is set forth in the following: 

 Mono. XLV, U. S. Geol. Surv., pp. 129, 172-212; 21st Ann. Kept., U. S. Geol. 

 Surv., pp. 317 and 402^04; Mono. XXVIII, U. S. Geol. Surv., pp. 186 L88j 

 Am. Geol., Vol. XXVIII, 1901, pp. 14-19; Jour. Geol., Vol. X, p. 71 : K'|>t 

 of Bureau of Mines of Ontario, 1902, pp. ir>r)-102; Geol. Surv. of r:m:il:i. 

 Vol. Ill, Pt. I, F, 1887-99; Am. Jour. Sci., Vol. 50 (1895), pp. 58-69. Tr:m-. 

 Roy. Soc. Canada, 1899 and 1901. In the references to Canadian localities. 

 some of the rocks classed as Archean (U. S.) are described under the name 

 of Lower Huronian. 



