438 GEOLOGY 



major, part of this zone. Igneous rock, eruptive and irruptive, is 

 assumed to have a somewhat irregular distribution through it, 

 while sedimentary rock increases in importance above, but remains 

 throughout a subordinate constituent. This zone records the 

 growth of the earth from the initiation of volcanic and atmospheric 

 processes to the close of the period of notable growth by accretion. 

 The central core and this thick zone about it represent the Forma- 

 tive Eon (p. 435). (3) The next zone, relatively thin, is assumed 

 to be made up largely of extrusive igneous rocks, with subordinate 

 amounts of sediment and matter gathered in from space. This 

 zone represents the Extrusive Eon (p. 435). (4) On the outside 

 lies the superficial zone in which sedimentary rocks predominate, 

 though associated with not a little rock of igneous origin. The 

 first two zones outside the core are assumed to be universal, while 

 the outermost one fails to completely cover the globe. 



Conjectures concerning the oldest accessible rocks, With these 

 theoretical sections in mind, it is pertinent to inquire what might 

 be the nature of the oldest rock formations accessible, on the 

 various hypotheses which the diagrams represent. The deepest 

 excavations yet made in the earth are but little more than a mile 

 in depth, and while, by reason of deformation and erosion, rocks 

 once at greater depths have been exposed, the maximum thickness 

 of rocks open to observation is but a few miles. Definite knowledge 

 of rock formations and structures is therefore limited to some such 

 thickness. 



(1). According to the gaseo-molten hypothesis in its simple 

 form, we might hope to reach the original crust; for it is not 

 rational to suppose that the original crust, the principal source 

 whence sedimentary rocks were derived, is everywhere covered 

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

 (2). According to the modified form of the gaseo-molten theory, 

 the oldest accessible rock should be the zone of mingled extrusive 

 and sedimentary rocks between the original crust and the domi- 

 nantly sedimentary formations above. (3). On the planetesimal 

 hypothesis, the oldest rocks to which we might hope to gain access 

 would be those referred to the Extrusive Eon (p. 435), during which 

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



