164 Proceedings of the Royal Society of Victoria. 



This surface drift appears too extensive to be attributed to 

 fluviatile action alone, and I am inclined to regard it as of 

 combined fluviatile and littoral origin, especially as it shows 

 evidence of having been subjected to a great deal of attrition, 

 whereby all the softer rocks, such as argillaceous slates and shales, 

 have been worn completely away, and only the harder siliceous 

 rocks left. This refers of course to that portion of the drift 

 which comprises sedimentary rocks foreign to the locality, thus 

 excluding local basalts. 



It would, therefore, appear as if the surface di'ift had been laid 

 down along the shore of a shallow sea, the bed of which consisted 

 •of hard basalt while this basalt itself had flowed over a thin 

 bed of coarse and fine sediments, which also had been deposited 

 along a shore line, but in deeper water. 



This suggests the probability that a shallow sea existed 

 in the locality at a former period, in which Mounts Mary and 

 Cotterill, as well perhaps as some other high points, stood as 

 islands in a state of fairly energetic eruption ; that their lava 

 flows, running in a northerly and westerly direction respectively, 

 were poured over a sea-floor, and thus were submarine flows 

 which gradually shallowed the sea ; that during cessations in 

 •eruptions or deviations in flows from these stated directions, 

 the deposits of siliceous pebbles were brought down by a large 

 river from the Ordovician highlands above Bacchus Marsh ; 

 that tlie finer sediment on the upper portions of this intercalated 

 bed, which consists of an intimate mixture of rounded and sub- 

 angular fine quartz grains embedded in a rather harsh, greenish- 

 yellow clay, is possibly partly of sedimentary and partly of 

 volcanic origin ; that another lava flow spread itself over this 

 one, advancing probably nearer to the shore line than the former 

 one ; that cessation or deviation again took place long enough 

 for a thicker deposit of pebbly gravels to be formed ; that during 

 the whole of this time the coast line was gradually rising, and 

 now came above the surface while the volcanic forces wei'e 

 gradually becoming extinct ; that as the land continued to rise 

 and the sea to recede the latter carried away the outer margin of 

 the littoral deposits, and continued to do so at the same relative 

 rate as the land was rising, thus precluding the probability of 

 any patches of it being left on the normal level of the underlying 

 basalt. 



