Geology of Coburg Area. 177 



Victorian Tertiary alkalic basalts. In this connection F. L. Still 

 well, 1 M.Sc. has suggested that this alkalir basalt horizon is the 

 age of the monchiquite dykes, which come up along the anticlinal 

 axes of Uendign. Nowhere in the Melbourne district are dykes 

 similar to that at Coburg found penetrating the Tertiary sands, so 

 that the upper age limit of these dykes is certainly the horizon ol 

 the sands. As to the lower Unlit it is obviously Silurian. There is 

 the possibility of the Devonian being the age of the dyke. The 

 evidence of the other areas is against this, however, e.g., at Beildigo 

 the quartz reefs are connected with the Devonian granodiorite to 

 the South, and these reefs are frequently cut across by younger 

 monchiquite dykes. 



Tertiary Sands. — This series is composed for the main part of a 

 system of unfossiliferous, iron-stained sandstones. Similar sand 

 •deposits are also found capping the Silurian hills in many parts 

 around Melbourne, e.g., Studlev Park, Kew and Hawthorn. At 

 Coburg the finer grained beds have grains as much as 3 millimeters in 

 diameter. Occasionally a coarser band of quartz pebbles occurs, 

 pebbles up to .'3 cm. in diameter being frequent. The series now 

 recurs capping the hills. Its lower limit is about 1G<> feet above 

 .sea level. The series was laid down in pre-basaltic time, the area 

 uplifted and the sands partly eroded away before the outpouring of 

 the basalt. Whether they are of marine or fresh-water origin still 

 remains a problem. Many of the grains are angular, indicating 

 a source near at hand. 



The basalt filled up the low level portions of the area, which 

 portions were low lying, partly because their burden of sands had 

 been eroded away. Hence it is difficult to get the relation between 

 the basalt and the sand series. However, in places, e.g., on the 

 banks of the Menu North id' Pentridge, basalt is seen to overly a 

 thin deposit of quartz pebbles, which again, rest on Silurian. 

 This pebble bed is probably re-sorted Tertiary sands, and the 

 busalt a later formation than the sands. 



A sample of the fine sand deposit was taken and boiled in 

 hydrochloric acid to get rid of the ferruginous coating. The sand 

 was then washed in water and agitated, so getting a division into 

 coarse sand and fine sand. This latter was then dried and 

 examined under, the microscope. It consists almost entirely of 

 quartz grains, but there is a very slight content of a black mineral. 

 Some of this is strongly magnetic, and hence is magnetite. Other 



1. Proc. Hoy. Soc. Victoria, vol. xxv. (n.s.), 1913, p. 1. 



