Geological Age of the Fossils. 



Unfortunately there is a want of agreement in the opinion 

 of the few geologists who have had the opportunity of 

 examining the older rocks of Central Australia. Mr. H._ Y. 

 L. Brown, (9) the late Government Geologist, as well as Dr. 

 Chewings/ 10 ^ include a Cambrian series between the Pre- 

 Cambrian and the Ordovician of the MacDonnell Ranges; 

 whilst Tate and Watt do not recognize any rocks of Cambrian 

 age in the MacDonnell Ranges, and affirm that the Ordovicians 

 rest directly on the Pre-Cambrian bed-rock. This interpre- 

 tation of the geological order is strongly maintained by these 

 authors (in opposition to the views of Brown and Chewings) 

 in their Report of the Horn Expedition to Central Australia. 



It is in this disputed borderland, between the Ordovician 

 and Pre-Cambrian, that Dr. Chewings obtained the fossils 

 described in this paper. Tate and Watt would probably in- 

 clude the fossiliferous horizon in their Ordovician, while 

 Brown and Chewings place it in the Cambrian. As bearing 

 on this question it is important to note that undoubted Cam- 

 brian fossils have been found in Central Australia. An 

 Olenellus (0. browni) was obtained by Mr. Brown at Alex- 

 andra station, situated between Tennant Creek and the 

 Queensland border; and Agnostus and Microdiscus were ob- 

 tained at Elkedra station, 150 miles south of the preceding 

 locality, and in a north-east direction from the MacDonnell 

 Ranges. 



In America the genus Gryptozoon appears to have a 

 rather extensive vertical range — Etcheminian. Cambrian, and 

 Lower Ordovician. It is, therefore, of little value in the 

 present case for determining the geological horizon. The 

 matrix of the Gryptozoon fossils in the MacDonnell's does not, 

 however, resemble the typical limestone of the Ordovicians of 

 that region. The latter are "grey, yellow, and red/' while 

 the Gryptozoon limestones are of a bluish tint and granular 

 structure, having a marked similarity to many of the mag- 

 nesian limestones of the Cambrian of the Flinders Ranges, 

 more to the south. The question of age must be left as a 

 doubtful point at present, with the weight of the evidence, 

 probably, in favour of them being Cambrian. 



(9) Mr. Brown's views are contained in various official reports, 

 published by Authority, and also in his Geological Maps of the 

 MacDonnell and associated regions. 



(10) Geological Notes on the Upper Finke River Basin, Trans. 

 Roy. Soc, S.A., vol. xiv., p. 247: also Notes on Sediment. Rocks 

 in the MacDonnell and James Ranges. Ibid., vol. xviii., p. 197. 



