THE ARCHEOZOIC ERA 439 



this hypothesis, as on the preceding, no sharp line of demarkation 

 would be expected between dominantly sedimentary rocks above, 

 and dominantly non-sedimentary rocks below. 



The oldest rocks known. The rock-formations now most widely 

 exposed at the surface are sedimentary, and were formed during 

 the great Gradational Eon (p. 435). In not a few places, however, 

 diverse formations which are predominantly extrusive (igneous or 

 meta-igneous) are found, either beneath the prevailing sedimentary 

 rocks, or projecting up through them in such relations as to show 

 their greater age. In many cases these lower and older rocks 

 were thoroughly metamorphosed, and in essentially their present 

 condition, before the deposition of the overlying beds. These 

 dominantly igneous and meta-igneous formations which antedate 

 the oldest known series of dominantly sedimentary rocks are the 

 oldest formations known, and the era during which they were formed 

 is the first era of which there is definite record in the accessible 

 formations of the earth. 



This lowest and oldest group of rocks is very complex, em- 

 bracing lava flows, volcanic tuffs, igneous intrusions of various 

 types, and sedimentary rocks, all more or less metamorphosed and 

 deformed. Distinct fossils have not been found in them, but the 

 presence locally of (1) carbonaceous slates similar to younger 

 slates which derived their carbon from organic sources, and (2) 

 occasional formations of limestone and chert which, as a class, 

 are usually the products of organisms, are thought to imply the 

 existence of life, and to warrant placing the era when these rocks 

 were formed in the zoic group (p. 435). The era 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 confidently referred 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 a deep series below, since life is supposed to have origi- 

 nated long before the earth acquired its present size. The oldest 

 formations known may also be Archeozoic under the modified 

 phase of the molten hypothesis which has been presented (p. 430) ; 



