BY G. D. OSBORNE. -l-yO 



Cainozoic Basalt. 



Flows of Cainozoic basalt occur on Mt. Douglas and to the east thereof, and 

 at Red Hill, near the head of Dunn's Creek. The thickness of the basalt varies 

 from 200 feet to 60 feet, the maximum being at Mt. Douglas, and its occurrence 

 as residual cappings to the highest hills points to the existence of former wide- 

 spread flows, which were poured out upon a peneplain surface, this being evident 

 from the record of the aneroid baxometer readings, which showed that the base 

 of the basalt was essentially level. In all localities the rocks are considerably 

 decomposed, furnishing excellent land for orchardists. At Red Hill the basalt 

 has existed in columnar form, large prisms being found amongst the red soil. At 

 this place also, the underlying tuft's have been changed by ferruginous material 

 derived from the basalt. In hand-specimen the basalt is aphanitic and almost 

 black in colour, the microscope showing equal amounts of well-formed laths of 

 felspar and irregular augite, together with magnetite and an occasional grain of 

 olivine, the fabric being radiate and ophitic. 



Cainozoic Detrital Deposits. 



In addition to the silts and alluvium along the banks of the creeks and 

 rivers, and the recent wash and unconsolidated talus elsewhere, there occur some 

 interesting rocks, of the nature of conglomerates, boulder beds, and grits, which 

 are clearly of Pleistocene or Recent age. In places these are surrounded and 

 partially concealed by alluvium, and thus are mentioned in the legend of the 

 map (Plate xxvi.), but otherwise they have not been mapped, in spite of their 

 widespread occurrence, since the placing of them upon the map would have 

 further concealed the already obscured plan of the Palaeozoic rocks, as given 

 on the map. These rocks are found on the highest hills and in the lowest valleys. 

 The best development generally occurs as a cloaking to the slopes of small- 

 stream valleys, and at the mouths of streams. They have, in these cases, been 

 formed as talus masses and alluvial fans, cemented together, and subsequently 

 cut into by the streams during further periods of degi-adation. But they are not 

 confined to stream valleys. On almost any gentle slope, masses of the rocks may 

 be found, often in the process of formation. It was interesting to find that a 

 conglomerate consisting of fragments of toscanite had developed on the dip-slope 

 on which Hungry Trig. Stn. stands, where the angle of dip is 18°. The bouldei-s 

 are sometimes well rounded, but in the majority of cases only partially so, while 

 some rocks are composed of completely angular fragments. The size of the con- 

 stituents varies, the larger being the more angular, and there is no evidence of 

 sorting. The cementing material must differ in detail from place to place, but 

 essentially it is of an argillaceous nature. The extensive character of the de- 

 posits has to a large extent been determined by the fact that there is a notable 

 felspathic content in most of the Palaeozoic rocks in the whole area, whether 

 igneous or sedimentary. The types referred to as gTits are free from boulders 

 and seem to be confined to relatively high localities. A typical outcrop of the 

 conglomerates is to be seen immediately south of the road half 'a mile from Pater- 

 son towards Gresford. They also occur on the top of Mt. Gilmore and in the 

 VaUey of Tucker's Creek and in a large number of other places. The boulder 

 beds are best developed in the creeks to the east of the Gilmore Ridge, while the 

 gritty rocks may be seen round about the Dunn's Creek Road a little south of 

 Red Hill. 



