140 GEOLOGY. 



earlier Formative eon to the later Gradalional eon (p. 119). According 

 to the modified form of the gaseo-molten hypothesis also, the Archean 

 rocks may be referred to the Extrusive eon. 



General Characteristics of the Archean. 1 



The Archeozoic era was characterized by great volcanic activity, 

 as shown by the nature of the formations, and by great cliastrophism 

 and metamorphism, as shown by the deformations and alterations which 

 [he rocks had suffered prior to the Proterozoic era (Fig. 36). The 

 rocks of no later era are so largely igneous, so notably deformed, or so 

 highly metamorphic. Because of these characteristics, the interpreta- 

 tion of these rocks is difficult, and such views of their classification 

 and correlation as are now entertained are to beheld subject to emenda- 

 tion as investigation proceeds. 



Subdivision. — Recent investigations seem to indicate that the 

 oldest parts of the known Archean consist of altered surface flows 

 of lavas, and pyroclastic rocks. This implies a preexisting solid 

 surface on which the flows took place, and therefore indicates that 

 these are not the earliest rocks that were formed. 



Associated with the metamorphosed surface lava-sheets and pyro- 

 clastic formations there are occasional beds of metamorphosed con- 

 glomerate and shale, and teds of jasper and iron ore, all of which imply 

 contemporaneous water-action. Some metamorphosed limestones and 

 sandstones are probably also to be included in the system. It is 

 probable also that arkoses and wackes, produced by partial weathering 

 and wash, are to be included. Such formations may have been abun- 

 dant, though not now recognized, for after they are metamorphosed 



1 The Archean was formerly thought to be composed entirely of igneous and meta- 

 igneous rocks; see Bull. 86, U. S. Geol. Surv., pp. 478-484, and 16th Ann. Rept., 

 U. S. Geol. Surv., Pt. II, pp. 747-756. The character of the Archean as set forth in 

 these publications needs some modification in the light of more 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. Rept., U. S. Geol. Surv., pp. 317 

 and 402-404; Mono. XXVIII, U. S. Geol. Surv., pp. 186-188; Am. Geol., Vol. XXVIII, 



1901, pp. 14-19; Jour. Geol., Vol. X, p. 71; Rept. of Bureau of Mines of Ontario, 



1902, pp. 155-162; Geol. Surv. of Canada, Vol. Ill, Pt. I, F, 1887-99; Am. Jour. 

 Sci., Vol. 50 (1895), pp. 5S-60. Trans. Roy. Soc. Canada, 1899 and 1901. In the 

 reference to Canadian localities, the rocks now classed as Archean (U. S.) are 

 described under the name of Lower Huronian. 



