IV.—The Classification of the Rocks of the Western Australian Goldfields

Extract

Sedimentary Formations in the Goldfields.—Sedimentary rocks are rare in all the goldfields south of the Gascoyne River, if superficial deposits of Tertiary or Recent age be omitted. It is true that a sedimentary origin has at times been ascribed to the jaspers, and still more commonly to the bands of graphitic schists within the greenstones. Hand-specimens of the latter rocks and of slaty rocks associated with the jaspers are indistinguishable from similar rocks of sedimentary origin, and it is only by the study of the geological occurrence of the bands that their derivation from igneous rocks can be proved. It is probable, therefore, that many of the rocks mapped as ‘older sediments’ by W. D. Campbell in Kalgoorlie and Norseman are not sediments. There are, however, undoubted sedimentary rocks between Kalgoorlie and Coolgardie, viz. the Kurrawang conglomerates. The enclosed pebbles are largely jaspers and porphyries, so that the formation of the conglomerates must be posterior to the intrusion of the granites, and yet the matrix is sub-schistose and the axes of folding are parallel to the general direction of foliation of the district. The junction with the auriferous series cannot be seen. From the photo reproduced by Campbell, it appears that there are similar conglomerates at Norseman.