564 THE DISTRIBUTION, ETC., OF ALKALINE ROCKS, 



The Canoblas stand on the southern flank of the Permo-Car- 

 boniferous basin which stretches, with a few interruptions of 

 residuals of older rocks, as at Wellington, from Orange via Mudgee 

 to Narrabri and Moree. The Triassic sea lay to the north and 

 north-west, but its sediments have been removed by erosion, if 

 any ever were deposited in the Canoblas district. The definite 

 relation of the Australian line of alkaline rocks to the area of 

 Triassic sedimentation is here lost. 



Probably the Canoblas sit on a fracture which crossed the 

 land-tongue which joined the New England mass to the Goulburn 

 mass, and which escaped Triassic sedimentation. 



Per mo-Carboniferous sediments were probably deposited on a 

 portion of this land-tongue across the Cassilis Geocol.* Indeed 

 the Gunnedah Coal Measures, which extend as far south-west as 

 Dubbo, belong entirely to the Upper Freshwater Coal Measures, 

 and were deposited probably later than those of Newcastle. It 

 is highly doubtful that these two basins were ever continuous. 

 Again, the Triassic of Dubbo, Coonabarabran, and the western 

 plains does not appear to be Hawkesbury, but rather Trias-Jura, 

 the same as the Clarence River and Ipswich Beds. 



In the Canoblas so little petrological work has been done on 

 the alkaline rocks that little or nothing is known regarding the 

 magmatic differentiation. 



The next locality at which foyaitic rocks are found in New 

 South Wales is at Dubbo. Near Dubbo, 5 miles south-west 

 between the Peak Hill and Oberley Roads, there is a hoary mass of 

 alkaline trachyte, locally known as " The Gib." Jt consists of a 

 fine-grained holocrystalline solvsbergite which has for the most 

 part a microgranitic fabric. This is undoubtedly the plug of a 

 volcano. Dykes of trachyte intersect the Trias-Jura strata in 

 the neighbourhood. Specimens of a beautiful arfvedsonite 

 trachyte have also been collected by Mr. C. E. Murton, Geological 

 Surveyor, in the Parish of Dungarry. These rocks have been 



* Term used by T. G. Taylor, Proc. Linn. Soc. N.S.Wales. Vol. xxxi. 

 Part 3. 



