566 THE DISTRIBUTION, ETC., OF ALKALINE ROCKS, 



opportunity last year to visit the volcanic area of Dunedin, whose 

 rocks are so nearly related petrologically to the Warrumbungle 

 ones, and I was struck forcibly with the difference in appearance 

 of the two regions. In New Zealand the alkaline rocks form 

 rounded hills covered with grass; the valleys are V-shaped. In 

 the Warrumbungles the mountains form steep cones or flat- 

 topped tables with precipitous walls, and the valleys widen by 

 the retreat of vertical cliffs. The differences observed are those 

 which distinguish between normal erosion and arid erosion. 



Old rocks, such as granite and schist (Silurian 1) occur at Gil- 

 gandra; at Tundebrine biotite granite underlies the Trias-Jura at 

 no great depth, having been brought to the surface in the lavas. 

 At Goorianawa granite forms the bottom of the borehole. At 

 Scabby Rock true slate — with quartz reefs — apparently of Gympie 

 age has been turned up by the trachyte-plug. 



The geological history of the Warrumbungle area is identically 

 the same for the Nandewar and Dubbo areas. The line of weak- 

 ness passing through these three areas constituted practically the 

 coast-line in late Palaeozoic time up to the end of the Carbonifer- 

 ous. The land lay to the west of it and the sea to the east. In 

 the early Permo-Carboniferous the land-area was extended right 

 up to the western border of New England, over which area, 

 however, sedimentation continued. In very late Permo-Car- 

 boniferous, when the New England uplift took place, a lacustrine 

 area was formed between it and the old Cobar massive. Here 

 sandstones of the Upper Coal Measures were deposited. This 

 area was now drained and a long period of erosion followed. In 

 Trias-Jura time the sea invaded the western portions of the same 

 area, which might appropriately be termed the Gunnedah Basin, 

 from the north. A withdrawal of the sea from most of the basin 

 followed, and in the Cretaceous period the Triassic and Permo- 

 Carboniferous beds of the area were reduced to a peneplain. The 

 Cretaceous sea only overlapped a small area of Triassic sedimen- 

 tation, but extended over a wide area to the west and north. 



A lake-period followed in the Tertiary. It is interesting to 

 note that in Cretaceous times the position of land and sea with 



