Page:Cyclopedia of Western Australia, volume 1.pdf/41
three districts, viz., Eucla, the Metropolitan Area, and Gin Gin, and there are good grounds for believing that beds of this age occur in the valleys of the Gascoyne, Lyndon, and the Minilya Rivers.
The results of the recent boring in that enormous limestone plateau which extends from the South Australian border to the neighbourhood of Israelite Bay has proved the occurrence of Cretaceous strata lying beneath the Eucla limestone, which is supposed to be of Eocene age. In bore No. 3, put down at the 337 miles 61 chains peg on the surveyed route of the Western Union Railway, at an altitude of 576 ft. above sea-level, the following strata were passed through:—Eucla limestone, 603 ft.; shales (sometimes glauconitic), 667 ft.; fine and coarse sandstone and conglomerate, 74 ft.; and granite, 28 ft.; the total depth of the bore being 1,372 ft. The beds beneath the limestone have yielded two of the most characteristic fossils found in the lower Cretaceous strata of South Australia and Queensland, viz., Aucella Hughendensis, Eth.; and MacCoyella Corbiensis, Moore, together with portions of a bivalve shell, possibly Fissilunula, which is found in the lower Cretaceous beds of Eastern Australia. There is, therefore, little doubt that the strata pierced in No. 3 bore are the equivalents of the Rolling Downs beds of Queensland.
In view of this discovery it is important to note that Cretaceous rocks occurring as table-topped hills were noticed in 1893, in the country to the north of No. 3 bore, by the geologist attached to the Elder Exploring Expedition.
The Cretaceous rocks of Gin Gin consist of a white chalky limestone without flints, which passes downward into a greenish glauconite marl, and below that into a clay shale. Above the limestone is a bed of very ferruginous sandstone, the whole series dipping to the northward at an angle of 8° to 9°.
A series of fragmentary fossils was collected in 1903, and contained Ostrea, allied to O. vesicularis, Lam.; a crushed Magellania; a doubtful Terebratulina; an undescribed Serpula, allied to S. Bognoriensis; spines of two species of Phyllacanthus, together with a fragment of Placunanomia. There were in addition thirty-seven species of Foraminifera, eight of which are confined to Cretaceous strata elsewhere and four to the upper Tertiaries, while the other twenty-seven are known to have existed in Cretaceous times. A further collection was obtained by Mr. W. R. Philbey in 1897, and although not in good condition is now undergoing critical examination. The collection comprises spines of Echinoderms, three genera of Brachiopoda, fragments of Lamellibranchs, two genera of Cephalopoda, one Crioceras, (?) and a shark's tooth Odontaspis-like in shape. There seems little doubt that the Gin Gin chalky limestone and its associated beds are of the Cretaceous age.
Farther north, in the neighbourhood of Dandaragan, is a belt of white chalky limestone, at the base of which lies a phosphatized bone bed, which there seem some sound reasons for believing to be the northern extension of the Gin Gin beds.
Tertiary.—Owing to the absence of valuable mineral deposits the Tertiary strata have not as yet received much attention at the hands of the Survey.
The basal beds of what are believed to be Tertiary strata are seen to rest unconformably on the eroded surface of the Jurassic beds in the Champion Bay district. The beds consist of limestones, sandy shales, and sandstones, which form a relatively narrow strip of country along the coastline.
Similar strata occur at Shark Bay and to the southward of Perth, extending as far as the South Australian border, where the chalky limestone that bulks largely in the formation contains fossils which seem to point to its being of the Eocene age.
In the south-western portion of the State the Tertiary strata are associated with basaltic lavas as well as basaltic dykes. These basaltic lavas may be seen at Bunbury, at several places in the Blackwood River, at Black Point on the south coast, and near Silver Mount between the Warren and Donnelly Rivers. They were also cut in two of the bores put down in the search for petroleum on the Warren River.
In the Kimberley Division basic lavas and ashes occur in great force. These Tertiary lavas and ashes appear in the valleys of the Ord and the Bow Rivers to have levelled up the depressions formed therein, except certain knife-edged ridges of the older rocks which still protrude above the general level. On the Behn River, just above what is known as the Gorge, a dome or puy of basalt, which apparently formed one of the focii from which some of these lavas issued, has been described.
At Norseman remnants of an extensive dolomitic limestone formation at an altitude of 900 ft. above sea-level and 100 miles distant from the coast have been discovered. This bed at Norseman contains species of Turitella, allied to T. terebra, Pecten, Cardium (or Cardita), Magellania, and fragments of Polyzoa.
Latertte.—No account of the geology of Western Australia would be complete without some reference to that extensive development of residual deposits which occur in nearly all portions of the State and for which the term laterite has been adopted. The laterites consist largely of hydrated oxide of iron and alumina, pro- ducing on the one hand deposits of iron ore, and on the other bauxite. In some localities the deposition of secondary silica in the lateritic deposits produces what are practically quartzites. These, by an increase in the ferruginous colouring matter, pass into a jasperoid form