172 F. L. St'dlweU: 



be found in greatest quantity along those directions which were 

 joint planes, and this is actually the case. The limestone in the 

 main creek, whose direction has not been determined by a joint 

 pla.ne, has accumulated partly by the main process and partly 

 as a secondary product derived from material deposited in joint 

 planes. G. H. F. Ulrich's statement, then, that the rock in the 

 bed of the creek is, most likely, secondary, is probably correct in 

 part. The original limestone in the big joint planes, while being 

 washed by streams and rain, would be partly diss&lved and re- 

 deposited in other parts of the stream course, and in this way the 

 shells of organisms living in the stream, many of the basalt 

 boulders, and quartz and felspar grains have become buried in it. 

 The rock thus formed is of the same age as the river alluvium. 

 It is probable that the hard, compact, grey rock, which may be 

 observed to weather to a dull white rock, is the original lime- 

 stone, and the whitish rock along the main creek is mostly 

 a secondary deposit of the same age as the alluvium, and derived 

 from the former either by weathering or by solution and re- 

 deposition. 



That the limestone has been deposited from solution, and, 

 further, that the process is still going on, is evident from the 

 fact that many of the stones in the creek have incrustations 

 of lime, and that there are boulders of fresh basalt, up to three 

 and four feet in diameter, partially cemented into the rock. We 

 also find this, where the main stream flows directly over the 

 Silurian bedrock, filling up cracks and joint planes in a manner 

 which could only result from deposition from solution. 



Correlation with SimiJar Victorian Deposits. 



I have not been able to find an account of a limestone of 

 exactly similar nature in Victoria. An outcrop of a tertiary 

 limestone occurs six miles away at Green Gully, Keilor, but this 

 ia marine (4). A fresh-water limestone at Malmsbury is re- 

 ported in the Catalogue of Rocks and Minerals (9), and is stated 

 to contain much magnesium carbonate and to represent a pro- 

 duct of the decomposition of basalt. A magnesian limestone 

 of fresh water origin occurs at Coimaidai, near Bacchus Marsh 

 (14), but differs from the Broadmeadows occurrence in the thin 



