Page:Cyclopedia of Western Australia, volume 1.pdf/37
sandy flags. Very little is known of these Cambrian strata at present. Their superficial area, however, would seem to be extensive, for they have been proved to extend in a north-east and south-west direction from the Burt Range and for some distance to the southward of Mount Dockrell. The strata have been tilted in such a way that the principal axis of folding is north-west and south-east.
No estimate has yet been made of the thickness of these the oldest fossiliferous beds yet found in Western Australia. Despite the poor localization of Mr. Hardman's fossils it may be considered as proved that Cambrian strata do occur somewhere in Kimberley about 18° southern latitude. The discovery of Olenellus and Salterella in the limestones of the Daly River, in the Northern Territory, is of considerable geological importance, indicating as it does a somewhat wide distribution of Cambrian strata in the northern portion of Australia.
Nullagine Series.—Age undetermined. A formation which has been designated the Nullagine Series, and which cannot be exactly correlated with any defined geological horizon in Western Australia, has a very wide distribution in the North-West Division of the State. The series consists of a great thickness of sandstones, grits, conglomerates, and limestones, some of which are magnesian, together with a series of lavas, ashes, and agglomerates. The formation is of some economic importance, by reason of the fact that the basal members of the series have proved to be auriferous in two localities, viz., Nullagine and Just-in-Time.
The beds of the Nullagine Series make a very prominent feature in the landscape in the districts in which it is developed, and play a very important part in the geology of the north-west. The series may be followed from the Oakover River, across the upper reaches of the Nullagine, the Coongan, and the Shaw Rivers, as far as the western boundary of the Pilbara goldfield on the Yule River, near Cangan Pool, from which locality it can be followed without a break to Roebourne, and southward to the Fortescue River. The same series constitutes the Hamersley Range, which contains Mount Bruce, the highest summit in the State. The Nullagine beds are continuous as far south as the Lyons River, where at Mount Phillips the conglomerate at the base of the series is seen to rest with a violent unconformity upon an uneven floor of ancient crystalline rocks.
In its lithological characters and general behaviour the Nullagine Series bears a very strong resemblance to the quartzites, etc., which constitute that continuous formation which extends from Wyndham to Mount Hart, a prominent summit in the King Leopold Range in the Kimberley Division. This formation as developed in Kimberley consists of a series of quartzites, sandstones, fine conglomerates, and shales, disposed in a series of broad anticlinal folds. Associated with the sediments is a series of bedded and intrusive igneous rocks, some of which are distinctly amygdaloidal, and contain zeolites and agates in the cavities. Beds of volcanic ash and breccia are common in certain localities. In certain portions of the King Leopold Plateau are many excellent sections showing the intrusive nature of some of the igneous rocks. The sandstones are sometimes altered into hard compact quartzite, portions of which have been caught up in the body of the igneous rock. Other sections indicate quite clearly that some of the igneous rocks have found an easy passage along the bedding planes of the sedimentary rocks, and evidently occur in the form of sills. The lavas are traversed by almost vertical dykes of epidosite, which are traceable across country for long distances; while both the sedimentary and igneous rocks are intersected by numerous segregation veins of quartz, some of which are of considerable size and horizontal in extent.
The recognition of the position of the beds of the Nullagine Series in the stratigraphical succession is of considerable importance; the entire absence of fossils throughout the series renders correlation extremely difficult. The beds forming the King Leopold Plateau have been claimed by previous observers as being of Cambrian age. If the lithological and stratigraphical resemblance between beds of the King Leopold Plateau and those of the Nullagine Series, as developed in Pilbara and the Ashburton, should prove to possess greater significance than at present appears, a Cambrian age for the Nullagine Series would have strong claims to acceptance.
Silurian.—The occurrence of Silurian rocks in Western Australia has been more inferred than proved. Mr. F. J. Gregory described in 1861 certain rocks occurring in the Mount Barren Range as being probably of Silurian age, though the evidence upon which this determination was based was not given. The strata forming the Stirling Range, which lies about 50 miles north of Albany, have been claimed as Silurian. The beds consist of quartzites, sandstones, and shales, the whole being traversed by quartz veins; they are highly folded, contorted, and faulted in places. According to the researches of Mr. H. P. Woodward the rocks at the western end of the range, near Mondinup, have been thrown into three sharp anticlinal and synclinal folds, in a distance north and south of about ten miles, by a lateral compression from the south.
The strata chiefly developed in the Leopold and Mueller Ranges of Kimberley have been provisionally classed by Mr. E. T. Hardman as Silurian, more on account of their lithological character than on any stratigraphical or palæontological evidence. The beds