RELATIONSHIP OP AUSTRALIAN TERRESTRIAL FORMATIONS. 303 



So fur there is complete accord, but when the same author 

 goes on to describe the difference between homotaxial and con- 

 temporaneous relationsliip of rocks of diii'erent counti'ies he makes 

 use of an ambiguous phrase, which cannot be accepted without 

 much qualification. He states, " In line, in every country where 

 the fossiliferous geological formations are well displayed, and have 

 been properly examined, the same yeneral order of organic suc- 

 cession can be made out among theui. Tlieir relative age within a 

 limited geographical area can be demonstrated by the law of 

 superposition. When, however, the formations of distant countries 

 are compared, all that we can safely affirm regarding them is tliat 

 those containing the same, or a represetUaiive assemblage of orgastic 

 remains belong to the same epoch in the liistory of biologiccd progress 

 in each area. They are homotaxial ; but we cannot assert that 

 they are contemporaneous, unless we are prepared to include 

 within that term a vague period of many thousand years." The 

 value of this statement entirely depends upon what may be 

 deemed to be the true meaning of the phrase " containing the 

 same, or a representative assemblage of organic remains." Does it 

 mean such close relationship embracing not merely a chai^acteristic 

 assemblage of genera, but also complete identity as regards the 

 principal characteristic species of each genus of the assemblage 1 ; 

 or, does it mean a similar assemblage of organisms identical so far 

 as the genera are concerned, but differing with respect to the 

 number and identity of their respective species? Upon the 

 answer to these two queries depends the possibility of accepting 

 Dr. Geikie's statement. 



Where in the rocks of two regions far apart, both genera 

 and their species are absolutely identical within a very large and 

 characteristic assemblage, thei'e would prima facie be good 

 grounds for classing homotaxial relationship ; and consequently it 

 might be allowed that the formations in which this identical 

 assemblage of organisms occurred, belonged to the same great 

 epoch ; but if the characteristic assemblage, however great, only 

 involved identity so far as genera are concerned, it would not of 

 itself form sufficient evidence to prove either homotaxis or con- 

 temporaneity; for such an assemblage of particular genera might 

 possibly occur in inverted order in rocks of widely separated 

 regions belonging to different great epochs or systems. 



It is but right to state that this is partly admitted by 

 Dr. Geikie, for, in the same work, (p. 619) he observes : — 

 " A species may have disappeared from its primeval birthplace, 

 while it continued to flourish in one or more directions in its out- 

 ward circle of advance. The date of the first appearance and 

 final extinction of that species would thus differ widely according 

 to the locality at which we might examine its remains." And 

 again he clearly announces this view by observation. 



