260 GREAT SERPENTINE-BELT OF NEW SOUTH WALES, vi., 



Oakey Creek, which has partly stripped them off the dolerite, 

 exposing the upper surface of the sill, which is quite coarsely 

 grained. The slope of this surface is thus clearly visible, and is 

 bent into a fairly sharp angle near Oakey Creek Gorge, sloping 

 south-west on the one side, south-east on the other. Minor 

 transgressions of the dolerite into the overlying niudstone are 

 well seen in a little eastern tributary of Oakey Creek. To the 

 west, the face of the basalt has been exposed and its underlying 

 gravels. 



There is some variation in the dolerite throughout this mass, 

 more jDarticularly in the porphyritic charactei' of the narrow, 

 sill-like parts compared with the granitic texture of the thick 

 mass. There appears some evidence of differentiation in the 

 latter, but nothing definite was proved. There is little evidence 

 to show where was the vent from which the magma issued, but 

 as the laccolite is so thick at the northern end and tails off so 

 gradually southwards, it is probable that the vent was near Blue 

 Knob, and that the northern limb of the laccolite was extremely 

 short, perhaps terminated by a fault, similarly to the Mt. Mar- 

 cellina, asymmetric laccolite in Colorado (39). The period when 

 the intrusion took place must remain unknown until the relation 

 of the dolerite-masses to the faults traversing the rocks they 

 invade has been carefully studied. That the dolerite is of late 

 Palteozoic age is most probable. 



East of this intrusion, a small mass of dolerite-porphyrite occur, 

 on the Cobbadah-Barraba Road, and, between here and the Blue 

 Knob, the Barraba Series, at the liead of Sheep Station Creeks 

 passes up into a series of peculiar tuffaceous rocks already men- 

 tioned (p. 241). 



The areas north of Cobbadah Creek have been studied by 

 traverses along Oakey Creek and Anderson's Creek. In Oakey 

 Creek, east of the fault figured (Text-fig. 7), about two miles 

 above its junction with Cobbadah Creek, the stream lies in a 

 deep meandering canyon cut into gently dipping mudstones and 

 tuffs, containing numerous but obscure plant-stems. East of 

 this, the dip changes sharply, with much shattering and faulting. 



