HOR\ EXPEDITION — GENERAL GEOLOGY. 37 



{b) Ase. 



In view of the present limited state of our knowledge of tlie.se rocks it is 

 evident that it is not advisable to enipioy such terms as Archsean, Azoic, etc. 

 The first implies a greater knowledge of the relation of those rocks to those called 

 Archaean elsewhere than we are in possession of, while the second has been shown 

 to be inadniissable by the discovery of fossils in Pre-Candjrian rocks iu North 

 America and France. The best term, if a local name be not desirable, and that 

 strongly recommended by Sir A. Geikie, is Pre-Camljrian. It may, however, in 

 the present case be objected that we liave no reliable e\idenee that these highly- 

 altered rocks in Central Australia are older than the Candjrian, in which case a 

 still more general term such as Pre-8ilurian would be nece.ssary. We base our 

 belief that there is, nevertheless, every reas(jn to consider their age to be Pre- 

 Candjrian on the following considerations : — 



1. In the first place the fact that a very strong unconformity separates them 

 from the Lower Silurian group of rocks makes it evident that they must be either 

 Cambrian or Pn^-Cand^rian. 



2. We favour the latter alternative liecause : — 



(a) The enormous differences in the lithological characters and tectonic 

 structures of these two groups of rocks implies a vast lapse of time 

 between their respective dates of formation. 



(/i) In the above-mentioned features they differ from the known Cambrian 

 strata of Yoike's Peninsula, and from those of Flinders Kange in 

 South Australia. 



(y) Their appnicnt litliolugical and structural similarity to the Pre-Cainbrian 

 rocks of South Austialia, as developed in the Mount Lofty Range. 



(S) No eruptive dykes have? been noted in Central Australia amongst the 

 Lower Silurian rocks, whereas they are \ery numerous amongst the 

 Pre-Cand)rian, and exhiliit ditlei-ent stages of nietamorpliism in the 

 same district, which tends to show that they have been intruded at 

 different periods. Some were ])robably intru<led during the Cambrian 

 era. The process of nietamorpliism was undoul)tedly a very slow 

 one, extending over a lengthy period. As it progrf>ssed intrusive 

 dykes made their appearance from time to time, the latest being, of 

 cour.se, acferis paribus, least altered. ]f th(m the highly metamor- 

 phosed rocks of Central Australia were of Cambrian age, surely 



