VOLCANIC NECK AT THE BASIN, NEPEAN RIVER. 



117 



morphosed Hawkesbury sandstone. In some of the less 

 altered sandstone fragments examples of " injection 

 breccia" have been observed: along the bedding planes 

 the breccia has been forced giving a characteristic appear- 

 ance. 



The foreign and cognate inclusions comprise peridotites, 

 rhyolitic rocks, granitic representatives and a calcareous 

 sedimentary rock of decided Mesozoic facies. These types 

 will be described in Part II. Another interesting occur- 

 rence in the breccia is that of a large mass of coal-like 

 material, which appears to be the cindered remains of a 

 tree-trunk. Similar occurrences at Hornsby are invariably 

 associated with aragonite and silica. At the Basin smoky 

 calcite is associated with this coaly material. 



The breccia has been intruded by at least three dykes 

 and an irregular mass of basalt. These are shown diagram- 

 matically on the map (Fig. 1). 



The dykes are numbered on the map, and their respective 

 features are tabulated below: — 



Dyke. 



Strike. 



D 



Amount. 



ip. 

 Direction. 



Width. 



Rock 

 Inclusions. 



No. 1 



S. 421° E. 



71° 



W. 421° S. 



2 ft. 



abundant. 



No. 2 



E. 33£° S. 



80° (?) 



S. 331° W. 



5 ft. 6 in. 



absent. 



No. 3 



E. 261° S. 



Indeterminate. 



1 ft, 4 in 



absent. 



Dyke No. 1 has sent out an offshoot intruding the adjacent 

 breccia in a joint plane parallel to the parent dyke. It has 

 also produced a sympathetic set of joints in the breccia 

 parallel to itself. 



Relation of the neck to the tectonics of the area. — From 

 the map it will be seen that the vent breaks through the 

 Hawkesbury rocks close to and even at the axis of the 

 monoclinal fold. The sandstone is dipping at 32° in a 

 general easterly direction at the eastern end of the neck. 



