3i6 ARCHEOZOIC ERA 



nearly its present size. The central core and this thick zone about 

 it represent the Formative Eon (p. 313). .(3) The next zone, rela- 

 tively thin, is assumed to be made up largely of extrusive igbeous 

 rocks, with subordinate amounts of sediment, and matter gathered 

 from space. This zone represents the Extrusive Eon (p. 313). 

 (4) On the outside lies the superficial zone in which sedimentary 

 rocks predominate, though associated with not a little rock of igne- 

 ous origin. This fails to cover the globe completely. 



The oldest rocks. The deepest excavations yet made in the 

 earth are little more than a mile deep. Because of deformation and 

 erosion, rocks once at much greater depths are now exposed; but 

 the maximum thickness of rocks open to observation is no more than 

 a few miles. Definite knowledge of rock formations and structures 

 is therefore limited to some such thickness, (i) According to the 

 gaseo- molten hypothesis, we might hope to reach the original crust; 

 for it is not to be supposed that this original crust is everywhere 

 covered so deeply by material derived from it as to be inaccessible. 

 (2) According to the modified form of this hypothesis (Fig. 292), the 

 oldest accessible rock should be in the zone of mingled extrusive 

 and sedimentary rocks between the original crust and the domi- 

 nantly sedimentary formations above. (3) On the planetesimal 

 theory, the oldest rocks which we might hope to reach would be 

 those referred to the Extrusive Eon (p. 313, zone 3, Fig. 293), during 

 which more or less sedimentary rock was mingled with the volcanic. 

 On this hypothesis, as on the preceding, the line of demarkation 

 between dominantly sedimentary rocks above, and dominantly 

 non-sedimentary rocks below, would not be sharp. 



The rock-formations now most widely exposed at the surface are 

 sedimentary, and were formed during the Gradational Eon (p. 313). 

 In many places, however, diverse formations which are predomi- 

 nantly 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 (Fig. 302). 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, older than the oldest known 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. 



