51 
from More ton Bay to Port Essington. 
ney, particularly as December advanced, and the setting in of the 
rainy season was to be expected every moment. 
I think that the most important results of my expedition are 
the discovery of the Mackenzie, the Isaacks, the Downs of Peak 
Range, and the Suttor; that of a communication between the 
east coast of Australia and the east coast of the Gulf of Car¬ 
pentaria, along the river, with running water through a fine 
country; that of the Nonda Country, and of the Big Plains at 
the east side, and at the head of the Gulf; that of communication 
between Limmen-bight and the South Alligator River, along 
running streams and creeks. The future will show how far the 
country along the Big Rivers between the head of the Gulf and 
Limmen-bight is available. * 
[Dr. Leichardt’s Lectures on the Physical Geography, Produc¬ 
tions, Capabilities, &c. of the Country visited by him, will appear 
in the January Number of the Tasmanian Journal .—Ed.] 
Art. IV. On Dykes of Marble and Quartz, in connexion with 
Plutonic Rocks, on the Upper Wollondilly, in Argyle County, 
New South Wales. By the Rev. W. B. Clarke, M.A., F.G.S. 
The tract of country described by the author in this memoir is 
situated not far from Sydney and Port Jackson, the river Wol¬ 
londilly, whose gorge lays bare the geological structure of the 
district, taking its rise in latitude 34° 26' south, longitude 
149 ° 23'east, and, after receiving the waters of several streams 
running into the Nepean river, and emptying itself into the 
ocean considerably to the south of Sydney. The stratified 
rocks traversed by the remarkable defiles through which these 
rivers flow, belong to the steril upper portions of the carboni¬ 
ferous formation so widely spread in Australia; and these carbo¬ 
niferous rocks are traceable (with occasional interruptions from 
basaltic dykes), from the district in question to the borders of the 
Illawarra region, where they present a lofty mural escarpment. 
* For the Map which accompanies this Article we are indebted to the kindness of W. O. 
Elliston, Esq., the proprietor of the Hobart Town Courier... Ed. 
F, 2 
