62 HORN EXPEDITION — GENERAL GEOLOGY. 



V. — Upper Cretaceous. 



General Characters. — The supter-sLructure of the lowest levels afound Lake 

 Eyre has long been kuuwu to be ai'gillaceous, and to contain marine fossils, as at 

 Mount Margaret, Primrose Springs, and Dalliousie. The fauna was at first referred 

 to the Jurassic period, but has in late years been recognised as contemporaneous with 

 tliat of the Rolling Downs series of Queensland. This formation is surmounted 

 by a hard tlinty ijuartzite or chaloedonised sandstone, varying up to fifty feet in 

 thickness, which forms the topmost bed of the table-land country, and will be 

 herein referred to as the Desert Sandstone. Messrs. Jack and Etlieridge, 

 " Geology of Queensland," (p. 390), assign the Rolling Downs series to the Lower 

 Cretaceous epoch ; but the facies of the fauna is more akin to that of the 

 European Upper Cretaceous, while the palifiontological differences between it and 

 the Desert Sandstone are too slight to justify the application of the terms Lower 

 and Upjjer to them respectively, therefore we substitute Upper Cretaceous and 

 Supra-Cretaceous in their place. 



Artesian Ijores around the south and west sides of Lake Eyre have proved 

 the Upper Cretaceous to be essentially argillaceous. By the courtesy of the 

 Conservator of Water, we have had the privilege of inspecting the bore material 

 from the Oodnadatta boi'e and publishing whatever may be desirable relating 

 thereto ; the boring was near completion on our return to that place. The site of 

 the bore is approximately at the same level as the railway station, namely, 397 

 feet above the sea; the bore was discontinued at a depth of 1571 feet, where a 

 good supply of water was tapped. The whole thickness is essentially argillaceous, 

 varying from clay, shale, to marly clay, intercalated with which are thin 

 argillaceous limestones and some sand-beds ; these latter occur at various horizons, 

 and the chief supply of water was obtained in the basal sands of the section. Thus 

 the general character of the strata passed through is like that of other bore 

 sections in the Lake Eyre basin. The deeper seated shales and limestones are 

 sparingly fossiliferous, and ;it Oodnadatta the only fossil-record is at 1070 feet in 

 depth ; whilst a fairly rich fauna lias been collected from the outcrop of the top- 

 most limestone at the various localities previously named. Up to the present no 

 distinctive rock-formation or fossil-zone has been recognised by which it would be 

 possible to correlate one bore section with another, and so to arrive at some know- 

 ledge of the physical structure of the basin and the probable source of its artesian 

 waters. 



The Oodnadatta bore-section is continued above the surface in the low bluffs 

 to the northward of that township by a limited thickness of argillaceous strata. 



