498 W. N. BENSON. 



stone is capped by Wianamatta shale lying on an eroded 

 surface. This may be well seen in the first railway cutting 

 north of Hornsby station. 



The soft nature of the breccia has allowed comparatively 

 rapid erosion, and the neck is shown physiographically by 

 a trench three hundred feet deep in the peneplain level, 

 which extends the whole length and breadth of the volcanic 

 area. It runs in a south-westerly direction, and is drained 

 into Tunk's Creek to the north by a narrow gorge from 

 near its southern end. It receives the drainage of deep 

 narrow gullies from the north, east, and south. The last 

 gully has a curious course, it flows north towards the out- 

 let, then bends sharply to the east and then joins the main 

 creek, flowing west into the outlet, thus leaving a narrow 

 tongue separating the two creeks. This is possibly due to 

 a local hardening of the sandstone, the effect of the volcanic 

 intrusion. Of interest also is an example of incipient 

 domestic piracy ; Pirate Creek will soon capture the nor- 

 thern portion of the Old Man Valley drainage system. 



The volcanic rock filling the neck is a breccia, the inclu- 

 sions of which will be described below. Briefly it consists 

 of grey kaolin, carbonated, and chloritic material, with 

 quartz grains and pebbles, fragments of sandstone, plentiful 

 inclusions of a peculiar trachytic basalt in various stages 

 of decomposition, occasionally small patches of bitumen, 

 many calcite veins, and very rarely small fragments of 

 essexite, peridotite and serpentine. The best exposures 

 are in a small quarry on a tributary creek in the centre of 

 the neck, and at another small quarry in the bend of the 

 main creek to the north-west of the cultivated area. After 

 careful search along every water course, no sign could be 

 found of any intrusion of basalt into the breccia. The soil 

 of the volcanic area is good ; it is cultivated in the northern 

 and southern portions of the neck, and where uncultivated 



