46 HORN EXPEDITION — GENERAL GEOLOGY. 



III. — Ordovician, 



(a) Introductory. 



If the group of metamorphic rocks just described be considered Pre-Canibrian, 

 then the Cambrian is altogether unrepi'esented ; and either sedimentation never 

 took place over the area now exposed, or if it did, the materials so deposited were 

 entirely denuded off from this area during the long interval separating the con- 

 clusion of their deposition and the disappearance of this region beneatli the waters 

 of the Ordovician sea. 



Messrs. Brown and Chewings, however, have at various times argued, on what 

 appears to us insufficient evidence, that the age of certain strata flanking the 

 McDonnell Ranges on the south is Cambrian. Thus Mr. Brown (x., p. 2) in 1890 

 examined in the vicinity of Heavitree Gap, to the south of Alice Springs, a series 

 of rocks resting unconformably upon the •' Metamorphic and Plutonic Primary 

 rocks "' of the McDonnell Ranges, and called them "Primary rocks (Cambrian?)." 

 He included in this series "quartzites (sometimes conglomeratic), dolomitic lime- 

 stone and occasionally clay-slates." "They vary," according to this author, "in 

 dip and strike, are often much twisted, contorted, and dislocated. . . . And 

 the granite and other dykes and quartz reefs do not extend into these rocks, nor 

 have fossils been observed in this formation." 



In 1891 Mr. Chewings (xi., p. 219) did not include Cambrian in his classifica- 

 tion of the rocks of this district, the rocks of Mr. Brown's Cambrian being partly 

 included by Mr. Chewings in his Pre-Silurian, and perhaps partly also in his 

 Silurian. 



In the same year Mr. Brown (xii., pp. 12, 13) says: — "Lying unconformably 

 on the crystalline, metamorphic, gneissic, and granitic Archsean rocks of the 

 McDonnell Ranges, there are two other rock systems unconformable to each other. 

 The lower of these two system.s," he continues, " consists of quartzite, quartzite 

 conglomerate, dolomitic limestone, limestone, sandstone, and slate, striking east 

 and west. They are lithologically similar, and doubtless of the same age as the 

 quartzite, dolomitia limestone, limestone, sandstone, and slate series of the Flinders 

 and other ranges in the northern part of South Australia proper (in the limestone 

 of which fossils of Cambrian age have been identified by Mr. Etheridge . . . ), 

 the Dennison and Peake, Mount Dutton, and ranges to the west of the Musgrave 

 and in the neighljourlioud of Mount Burrel. . . ." 



