president's address. — SECTION c. 157 



second and third submergencies as between the first and second. 

 Unfortunately, this interval by which the Miocene and Pliocene 

 marine deposits was separated in time is, so far as definite evidence 

 goes, a geological blank, and its time value can be gauged only on the 

 altered facies of the fauna which appears in the third submergence 

 as compared Avith the second — a difference which, as we have seen, is 

 very great. 



As the Croydon bore supplies the only evidence we possess of the 

 stratigraphical relationship of the marine Pliocene to the underlying 

 strata, it is of great importance in our attempt to trace the order of 

 events in mid-Cainozoic and later times. Following Tate's guidance, 

 we see in the Croydon section — 1st, a bottom series of unfossiliferous 

 sharp sands, sandy clays, and fine-grained sandstone, having a thickness 

 of 581 feet, resting on a Cambrian floor. These are (?) infra-Eocene, and 

 indicate fluvio-lacustrine conditions. 2nd, there follow, in ascending 

 order, 921 feet of fossiliferous lower Cainozoic (Eocene). 3rd, a barren 

 zone of sandy beds, containing only triturated and uncertain evidence 

 of organic remains, equal to about 255 feet. 4th, the Older Pliocene 

 (marine), 320 feet in thickness. 



With respect to this barren zone, separating the clearly-defined 

 Eocene below from the equally well-defined marine Pliocene above, Tate 

 says, " Below the depth of 715 feet no fossils appear till 778 feet, but 

 the nature of the fossils there encountered do not permit of a decisive 

 determination as to age, and this also applies to other occurrences. 

 It is not until the fossiliferous bed at 970-1,000 feet is reached that 

 undoubted evidence of Eocene age is forthcoming."* This intercalation 

 of 255 feet of relatively barren sands, separating the Eocene and 

 Pliocene in the section, may have a special significance. Tate could 

 not detect the presence of a Miocene fauna in the section, but the place 

 where it ought to have been in the section is occupied by this barren 

 zone of 255 feet. May not these almost unfossiliferous sands represent 

 the interval of elevation which must have existed between the laying 

 down of the Miocene and the Pliocene ? If so, then a period of sub-aerial 

 waste preceded the marine Pliocenes, and, we may assume, was suffi- 

 ciently long to remove locally the whole of the Miocene and the upper 

 members of the Eocene. Then, when the earth movements were again 

 reversed, in the direction of depression, the drainage areas would 

 become aggraded, and the rain wash on the surrounding lower 

 Cainozoic sediments would cause, to some extent, the mixing of the 

 derived fossiliferous material with the alluvial sands. At a later stage 

 the depression became sufficiently pronounced to admit the sea, and 

 the laying down of the Pliocene sediments. It is highly probable that, 

 at that time, the rift valley of South Australia had already been defined, 

 and within its protected area, if anywhere, the sediments that belong 

 to the pre-Pliocene interval are most likely to occur. 



* Loc. cit., p. 195. 



