Geology of Cobvnj Area. 170 



states that the water is in too small a quantity to splash about. 

 During storms this is not so, but usually; however, the quantity i* 

 only small, and as the run off after a storm dies away, the water 

 merely trickles over the edges, and then undoubtedly surface tension 

 (■onus into play. However, surface tension alone can only explain 

 the formation of a vertical face, and it is difficult to explain the 

 complicated system of watercourses at Coburg by any other agent 

 than running water. Erosion as pictured by Dr. Leach is neces- 

 sarily a slow process, and it is most probable that the water running 

 over the canyons after a heavy fall of rain, does more erosive work 

 than six months of water just trickling over. During some periods- 

 of the year several months may elapse during which no heavy rain 

 occurs. During this time surface tension is steadily at work after 

 showers of rain, and certainly gives the walls of the canyon those 

 characteristics enumerated by Dr. Leach. These, however, are 

 more or less obliterated after the next heavy fall of rain. 



The exact, stratigraphieal horizon of this sandstone series is 

 difficult to determine, for they are unfossiliferous. One has to leave 

 Coburg and examine neighbouring localities. At Koyal Park, to- 

 the South, a similar series appear to overly a fossiliferous bed 

 outcropping in the railway cutting. This latter series is generally 

 regarded as Kalimnan in age. Also at Keilor in Green Gully 

 thick, unfossiliferous sands overly thin beds of highly fossiliferous- 

 limestones of Barwonian and Kalimnan a«je. The horizon of the 

 Coburg sands then appears to be post-Kalimnan and pre-newer 

 basaltic. 



Newer Basalt. — This occurs in the West of the area. Petrologi- 

 cally it can be divided into two types — (a) Low level, (b) High level. 



(a) Low Level Type. — The low level basalt is chiefly found filling 

 up the pie-basaltic depressions, such as river beds, etc. In the 

 hand specimen the rock is compact and medium fine grained, with 

 a few small phenocrysts of olivine. In the bed of the Merri just 

 East of Pentridge, it exhibits columnar jointing, seen in a basalt 

 pavement. A study of this particular pavement shows that the 

 cracks radiate in threes from a centre, and at various angles to 

 one another. The angle between cracks in this limited exposure is 

 always greater than 90 (leg., so that the prisms in plan are never 

 less than five-sided. The centres frequently seem to be joined by 

 a crack, making the other cracks symmetrical about it. The result 

 is that the five cracks resemble the arms of, say, Tetragraptus quad- 

 ribrachiatus. The distance between centres varies from 2 inches. 

 to 24 inches. 



