J. A. Thomson — Rock^ of Western Australia. 211 



schistose rocks of the auriferous series.^ The relationship of the two 

 series has not been worked out, but from a consideration of the map it 

 appears probable that the basic schists are either contemporaneous 

 volcanic rocks or later intrusions. The relationship could be most 

 easily solved by examining the sedimentary series for traces of contact 

 action, and determining the nature of the pebbles in the conglomerates. 



Another highly folded but probably later foi-mation occurs in the 

 Nullagine District, Pilbara Goldfield, and has been termed the Mosquito 

 Creek Series by Maitland.^ The rocks consist of grits, sandstones, 

 shales, and fine conglomerates, all considerably sheared. ^Nevertheless, 

 the series must be younger than the auriferous series, for the 

 conglomei'ates contain numerous pebbles of the laminated jaspers 

 of the latter, in this resembling the Kurrawang conglomerates 

 between Kalgoorlie and Coolgardie. An observation of great interest 

 is that the Mosquito Creek Series is intruded by a granite and forms 

 the matrix of numerous auriferous quartz reefs. 



In the West Pilbara, Ashburton, and Gascoyne Goldfields highly 

 inclined and sub-schistose sedimentary rocks are met with at 

 Bangemall, TJaroo, Westons, and Roeburne.^ Besides quartz-schists, 

 arkose, conglomerate, and phyllites, they include a large number of 

 beds of limestones, dolomites, and cherts. At Roeburne they are 

 intruded by dykes of amphibolites after gabbros and gabbro- 

 pegmatites in every way comparable to rocks of the auriferous 

 series, and probably belong, therefore, to the Warrawoona Series. At 

 Mt. Phillips, near Bangemall, they rest unconformably upon a granite. 

 All these rocks form the matrices of gold and copper lodes. 



These highly folded beds are covered unconformably in the Pilbara 

 Goldfield by rocks of plateau type to which the name of the Nullagine 

 Series has been given.* Though thrown into a series of gently 

 undulating folds the series is on the whole approximately horizontal. 

 Besides sedimentary members such as sandstones, grits, conglomerates, 

 and limestones, there is a well-defined volcanic series of lavas, ashes, 

 and conglomerates. The age of these beds has been variously 

 ascribed to Cambrian and Devonian, from lithological resemblances 

 to fossiliferous beds of those ages in the Kimberley District. The 

 probabilities are in favour of a Devonian age, for the known Devonian 

 has also a volcanic series well developed. 



The basal conglomerates of the Nullagine Series consist chiefly of 

 fragments of pre-existing conglomerates, cherts, grits, and shales of 

 the Mosquito Creek Beds, and of reef quartz, identical in character 

 with that forming the auriferous deposits of the underlying strata.^ 

 They appear thus to be posterior to the main deposition of gold, but 

 the conglomerates are themselves in places auriferous, although 

 whether the gold is authigenous or allothigenous is a matter of 

 uncertainty. 



1 A. Gibb Maitland, Bull, xx, pp. 57-61, 1905 ; Bull, xxiii, pp. 77, 79, 1906. 

 * Bull. XX, pp. 28-30, 1905 ; Bull, xxiii, pp. 77-9, 1906. 

 ^ Maitland, Bull, xxxiii, pp. 42-5, 66-7, 75-6, 99-100, 1909. 

 •* Maitland, Bull, xv, pp. 10, 11, 25, etc., 1904; Bull, xx, pp. 23-8, 1905; 

 Bull, xxiii, pp. 40-3, 77-8, 1906 ; Bull, xxvi, pp. 50-2, 1907. 

 ^ Bull. XX, pp. 24-5, 1905. 



