BY W. N. BENSON. 593 



Whether the basalt on Black Jack (about 100 acres in area) is 

 part of a flow from a distant vent, or surrounds a local pipe, can- 

 not yet be stated. 



Three types of basalt are developed. There is a smooth, aphani- 

 tic type, which has a grey surface on weathering, streaked with 

 etched-out flow-lines ; and there is also a darker, not so very finely 

 grained type, with a rough hackly surface, and a habit of breaking 

 into small pellets when rather decomposed. An intermediate 

 variety is the commonest rock, and is frequently prismatic, but the 

 extreme types sometimes occur interbanded in the same mass, either 

 in the necks or in the basalt-flows. 



A third type is thoroughly scoriaceous and largely decomposed. 

 This occurs in a flow extending north and south of the Dams on 

 Burnt Hut Creek, by Hanging Rock. No basalt- tuft's or breccias 

 have yet been found in the necks or between the lava-flows. 



(b)Nepheline-basanite occurs, forming the upper 300 feet of 

 Square Top Hill, two miles west of Nundle. This has been shown 

 (24) to be a member of a varied series of rocks of a basic alkaline 

 character, that occur intruding into the Tertiary basalts. The series 

 includes coarsely granular theralites and teschenites, nepheline- 

 basanites, and coarse dolerites with large purple augites, with or 

 without analcite. The mode of occurrence of the Square Top rock 

 is as yet uncertain. 



A small amount of vesicular olivine-basalt has been found 

 around the base of the basanite, and it has been noted that the 

 lower portion of the latter is coarser-grained, and richer in augite- 

 phenocrysts than the upper portion. No gravel was found below 

 the basalt, but only the eastern face has been studied as yet. Pro- 

 bably, as elsewhere, this was a sill-like intrusion through a basalt- 

 flow, possibly it was a mass of the mamelon-type. 



Rocks of this alkaline group occur in great amount, as boulders 

 in the Peel River, evidently derived from the Liverpool Range. 

 They appear to occur in situ on Wombramurra Creek, and it is 

 probable that the very striking cone, Wombramurra Peak, is of 

 this character, to be correlated with Mount Warrawalong, near 



