HORN EXPEDITION — SUMMARY. 103 



pai'th-iriovoments." The evidoiicf oKt.iinod points to tlio oruptivn origin of a large 

 part of tlie nietamorpliic group whereas the Camhriaii rocks of Australia, as far as 

 at present known, are entirely sedimentary. 



In regard to the distinct stratitication and definite and determinable dip of 

 the rocks described l)y Messrs. Brown and Ciiewings the conclusion is arrived at 

 that " althougli it may be possible and even in places prol)able tliat tlie planes, 

 which are so strongly developed, coincide with tlie original planes of stratification 

 in any large area where sedimentary rocks may have })een developed, yet as a 

 general rule there can be no doubt that tliese planes represent foliation planes. 

 This statement is greatly strengthened l)y the facts of the coincidence over large 

 areas of tlic strike of these planes, and of tlunr great persistency ; for they are 

 traceable not only through rock-masses, the eruptive origin of which is highly 

 probal)le, but also even thi'ough undoubted intrusi\e dykes. They are therefore 

 planes of foliation, of stratification-foliation — that is of foliation corresponding 

 with the original bedding planes, it may be in places, but elsewhere assuredly they 

 appear to be those of cleavage foliation."* 



In regard to the age of the rocksf it is pointed out that the strong uncon- 

 formity .separating them from the Lower Silurian group sliows them to be either 

 Pre-Cambrian or Cambrian. In lithological character and tectonic structure they 

 diller from the known Cambrian strata of Yorke's Peninsula and Flinders Range, 

 and agree apparently with the Pre-Oaml>rian rocks of the Mount Lofty Range. 

 Wliilst no eruptive dykes have been noted in Central Australia amongst tlio 

 Lower Silurian rocks, they are very numerous amongst the Pre-Cambrian and 

 exhibit different stages of metamorphism in the same district, which tends to show 

 that they have been intruded at different periods. If tlie highly metamorpho.sed 

 rocks were of Candirian age then some of the eruptive dykes which appeared last 

 might have been expected to liave penetrated the Ordovician strata. Lastly, 

 whilst the Cambrian rocks of Yorke's Peninsula, the Flinders Range and the 

 Kiinberley distiict are fossiliferous there is an entire absence of fossils in the 

 metamorphic rocks of the centre. 



('2) Ordovician. 



To this horizon Messrs. Tate and Watt refer the strata forming (with the 

 exception of the Post-Ordovician conglomerate to be mentioned later) the series of 

 ridges which run roughly parallel to one another from east to west across the 



* Part III., Oeoloyv, p. 40. ^ Loc. cit., p. 37. 



»11A 



