THE VICTORIAN NATURALIST. 155 



THE GEOLOGY OF ARN HEM'S LAND, N.A. 



By the Rev. J. E. Tenison-Woods, F.G.S., F.L.S. 



(Hon. Member.) 



{Read before the Field Naturalists' Club of Victoria, xoth 



December, 1888.) 



In 1886 T furnished to the South Australian Government a report 

 on the geology of the Northern Territory. As this report is 

 mainly occupied with the mineral riches of the colony, a little 

 additional information on the geology of the country and its 

 physical geography may be of interest to the Society. 



Seven principal formations are found in Arnhem's Land, 

 which may be thus enumerated in the ascending order : i. 

 Granite, pegmatites, &c. 2. Crystalline schists, slates, and gold- 

 bearing reefs, with other minerals. 3. Limestones. 4. Con- 

 glomerates (waterworn). 5. Volcanic remains and trap-rocks. 

 6. Fluviatile sandstones and conglomerates, 7. Desert sand- 

 stone. 



From this enumeration it appears that the cretaceous deposits 

 so extensively developed in Western Queensland have dis- 

 appeared, and there does not seem to be any formation 

 between the crystalline schists and the desert sandstone. 



Granite. — This is extensively developed in belts, all of which 

 are of an eruptive character. It is of two kinds, pink and red 

 granite and blue granite. Instances of the red granite in extensive 

 tracts may be seen along the telegraph line, at the Finnis, where 

 it forms a narrow belt, extending a long distance to the eastward ; 

 and at Yam Creek, where it forms an unbroken mass as far as the 

 Driffield. It is coarse-grained, with large crystals of orthoclase 

 felspar, such as is called granite porphyry. At M 'Minn's Bluff 

 and other places a remarkably fine-grained granite takes the 

 place of the granite porphyry, probably forming dykes or elvans 

 belonging to a subsequent intrusion. The blue granite is exten- 

 sively represented to the eastward of the telegraph line. This is 

 also an orthoclase felspar, with Muscovite mica. Both the blue 

 and red kinds rise into isolated hills, seldom exceeding 100 feet 

 in height. 



Usually, the granite country is level, forming plains, out of 

 which great bosses and boulders continually project. The soil is 

 of a light sandy description, only able to support small trees and 

 poor grasses. It is arid and waterless in the winter, and in the 

 summer or rainy season very boggy. One may know at once, 

 from the aspect of the country, when the underlying rock is 

 granite. It is unmistakably an eruptive rock which has broken 

 up the ancient formations, and therefore a newer intrusion in the 

 geologic history of the continent. The pegmatite is a granite 

 without mica, occupying an extensive tract at the head of the 



