THE ARCHEOZOIC ERA. 137 



3. On the plane tesimal hypothesis, the oldest rocks to which we might 

 hope to gain access would be those referred to the Extrusive eon (p. 

 119), during which more or less sedimentary rock was mingled with 

 the volcanic. On 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 rock-formations now most widely exposed at the surface are 

 sedimentary, and were therefore formed during the time of atmospheric 

 and hydrospheric dominance, that is, during the great Gradational 

 eon (p. 119). In not a few places, however, formations of dominantly 

 extrusive-igneous or meta-igneous origin 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 

 inferior rocks were thoroughly metamorphosed, and in essentially their 

 present condition, before the deposition of the overlying beds, for masses 

 of metamorphic rock derived from them are included in the non- 

 metamorphic series above. These igneous and meta-igneous forma- 

 tions which antedate the oldest known series of rocks made up chiefly 

 of sediments, are the oldest rock-formations known, and the era during 

 which they were formed is therefore the first era of which there is 

 definite record in the accessible formations of the earth. 



These rocks consist of a very complex series of formations, embrac- 

 ing lava outflows, volcanic tuffs, igneous intrusions of various types 

 and exceptional extent, together with some sedimentary deposits, 

 all usually metamorphosed and notably deformed. Distinct fossils 

 have not been found in these rocks, but the occasional presence of 

 (1) carbonaceous shales very similar to younger shales which derived 

 their carbon from organic sources, (2) iron-ore beds similar to those 

 which owe their origin to plant action, and (3) occasional limestones 

 and cherts which, as a class, are usually the products of organic 

 action, are thought to imply the existence of life, and to warrant 

 placing the era when these rocks were formed in the "zoic" group. 

 This view is supported by the theoretical probability that life extended 

 back through a vast period unrecorded by fossils, since the earliest 

 fossils yet found imply a very prolonged antecedent evolution. The 

 era during which, or during the later part of which, this oldest system 

 of accessible rock-formation was made, is the Archeozoic era. 



Under the planetesimal hypothesis, the oldest known rocks may 



