HORN EXPEDITION — GENERAL GEOLOOY. 51 



being seen from the. south in their strike-plane, present the appearance of an 

 uneonfonnaljle capping. 



In 1891, in liis paper entitled "Descriptions of some South Australian 

 Silurian and Mesozoic Fossils," Mr. 11. Etheridge, Junr.,* described the following 

 Lower Silurian fossils : — Raphisloina bro7Vint\ Orthoccras sp. ind., and Or/his 

 levioisis. ^\\ a note in the same report Mr. Brown describes the discovery of these 

 fossils in Septeniljei-, 1890. In this report, and two suljsequent ones, ]\Ir. 

 Etheridge assigned the fossils to a Lower Silurian age. A complete account of 

 the fauna will be given in Part V. — PAL.EONTOLor.Y. 



As has been mentioned already, we eliminate Candjrian from our classification 

 of the rocks occurring within tiie region under consideration (from Oodnadatta to 

 the McDonnell Ranges) for the reasons that have been detailed. We include in 

 the Ordovician system all the strata lying between Mount Burrell Cattle Station 

 on the south and the McDonnell Ranges on the north, with the exception of the 

 conglomerate which was observed on the north side of Rudall's Creek, and on the 

 banks of Ellery's Creek north of the Lutheran Mission Station (llermannsburg). 



(/') Extent. 



In the journey north from Oodnadatta the first outcrop of Ordovician 

 strata was seen about seven miles north of Francis Well on the Hugh River, 

 and about the same distance south on the ^Mount Burrell Cattle Station, 

 which is also on the Hugh. The southern boundary of the Ordovician area 

 extends westei'ly from this point, pas.ses a few miles south of Chandler's Range, 

 and thence sweeps round the southern faces of Levi and Ceorge (till Ranges, and 

 perhaps cuntinues for many miles further west. East of Mount Burrell the 

 southern Ijoundary is not known. The northern limit of this area is partly formed 

 by the prominent (luartzite ridge, in which arc the Heavitree, Emily and Jessie 

 Gaps ; but at Blount Sonder, and to the west of it, there are beds wiiich intervene 

 between this (juartzite and the Pre-Caud^rian rocks. 



The above-mentioned ridge is traceable for about twenty-five miles east of the 

 first-named gap, beyond which it sweeps suddenly northwards ; so that in 

 journeying due east we find the Pre-Cambrian rocks overlain by a northern 

 extension of the Ordovician strata. The northei'n boundary of the (Jrdovician 

 rocks north of Mount Benstead, and east of that point, has not been traced. In 

 journeying ncai-ly due east of Mount Benstead, and at a distance of about twenty- 



* Kt'ports on Coal-licurintj Area in Nei^'hourhooii of Leij^h's Creek, Ailclaide, L>'J1, pp. !)-ll, i>I. i.-iii. 



