Physiography of Bulla Area. 327 



Corrosion.— A study of the effect of the volcanoes on that sec- 

 ition of Jackson's Creek north of Sunbury would provide interesting 

 matter, for it is evident that the Sunbury volcanoes in late Kaino- 

 zoic times, formed. an immense, bar across the old Jackson's Creek, 

 and overwhelmed the valleys to the south beneath a flood of lava. 



It will be noticed that Deep Creek flows close to the boundary of 

 the granodiorite, and the basalt. Originally it flowed along the 

 junction. As Deep Creek deepened its bed, the granodiorite be- 

 came exposed on both banks. It is probable that streams such as 

 the Yarra and Deep Creek tend to flow at the junction of basalt 

 and older rock because hill drainage Uelps to form a valley at the 

 junction of the bedrock, and the lava sheet, and because the lava 

 flow is probably depressed at the edges, and thus directs the drain- 

 .age of the area to the line of junction. 



By meandering and deepening the streams have reduced the 

 slope from 60 ft. per mile to 18 ft. per mile. From the creek 

 junction to the most northerly point of Deep Creek, marked on 

 the map, is 7 miles. In this distance it falls from 280 ft. to 150 

 ft., giving a slope of 18 ft. per mile. It is remarkable that Jack- 

 son's Creek from the| north-west boundary on the map to the creek 

 junction flows 11 miles, and falls from 350 ft. to 150 ft., giving 

 again a slope of 18 ft. per mile. The slopes are interesting when 

 compaied with that of the sluggish Mississippi, which ha« a fall of 

 less than 1 ft. per mile. At the Bulla School the creeks are orly 

 half a mile apart, but, owing to the short distance along Deep 

 Creek to the junction as compared with that along Jackson's 

 Creek, the bed of Deep Creek is 60 ft. lower than that of Jackson's 

 Creek. • This illustrates the fact that in river capture the more 

 vigorous stream may be captured by the less vigorous. Jackson's 

 Creek and Deep Creek having the same slope, are of equal vigour, 

 but if their valleys) met Deep Creek would capture the head waters 

 of Jackson's Creek. 



Deep Creek has been superimposed along the whole of its course 

 on the older rocks beneath. Jackson's Creek is still cutting into 

 basalt, though it frequently carves its way across the tops of the 

 old Ordovician and Silurian hills. 



Basalt bars of greater density retard its rate of deepening by 

 checking its velocity. In several parts Jackson's Creek resembles 

 a series of small lakes separated from one another by. basalt bars 

 over Avhich the wafer tumbles in miniature rapids. 



