50 HORN EXPEDITION — GENERAL GEOLOGY. 



extremely gentle angles (usually at an angle of not more than two to three degrees) 

 throughout the area occupied l>y them ; while the Silurian rocks, consisting of 

 sandstones, quartzites, and limestones, unlike the Cretaceous, dip at from 40° to 

 50° in the vicinity of the junction-line of the two formations. Again, on leaving 

 the Lower Silurian area at Heavitree Gap, for instance, the differences, both litho- 

 logical and structural, to be observed in the two sets of rocks ought to attract the 

 notice of even the most casual observer. 



In 1890 Mr. Brown (ix., p. 6) placed tlie quartzites, dolomitic limestone, and 

 clay slates, which are seen to rest unconformably on the Pre-Cambrian rocks at 

 Heavitree Gap and elsewhere, and which appear to be unclassified in the sketch 

 section accompanying his previous report, in a group by themselves, which he calls 

 Primary rocks (Cambrian 1). On a study of Mr. Brown's later reports (xiv. p. 7) 

 it will be observed that he retains this division, but extends the area of the i-ocks 

 included in it as far south as the Lutheran Mission Station, absorbing thereby 

 part of his former Silurian rocks. In 1891, after the determination of the Lower 

 Silurian age by Mr. R. Etheridge, Junr., of the fossils submitted to him by Mr. 

 Brown, his (Mr. Brown's) original Silurian became Lower Silurian, and, as more 

 clearly stated, was made to include the rocks of the George Gill, the James, and 

 Ooraminna Ranges. Thus the rocks which in 1890 were classified as Devonian (?) 

 are now included in the Lower Silurian. 



In 1891 Mr. Chewings (xi., pp. 249-252) called a portion of the system of 

 rocks under consideration Silurian, basing his classification on the determination 

 of the Silurian age of the fossils collected at the head of the Walker River, at 

 Mereenie Bluff, and on the Petermann Creek. This author separates from this 

 system the Ooraminna .sandstone and the conglomerate which flanks the South 

 McDonnell Range. The Ooraminna sandstone, as there is every reason to believe, 

 l)elongs to the Ordovician system, but we have positive evidence of the Post- 

 Ordovician age of the conglomerate. 



In a note to this paper of Mr. Chewings's, Prof. Tate (xi., p. 255) gives a list 

 of the fossils identified by him, and from the meagre evidence afforded by them 

 was inclined to assign an Upper Silurian age to them. Mr. Chewings's Devonian 

 of the George Gill Range is identical with the red sandstones overlying the fossil- 

 iferous Ordovician limestones, which sandstones have there a very low dip, but are, 

 nevertheless, superimposed conformably on the limestones, wliich outcrop in 

 Petermann Creek {7'i^e Fig. 7). Tlie Tertiary deposits of the same author are 

 outliers of the same range, which, because of the low dip and the fact of their 



