48 W. G. Lainjford: 



Jutson^ Ijelioved that the Silurian st'diinents at Wai'raudyte Avere 

 derived solely from a sedimentary series because no pebbles of 

 igneous rock were found by him in the Warrandyte conglomerate. 



Junner,2 in his paper on the Diamond Creek aiea. concludes that 

 the sandstones of that district were derived to a " fair extent " 

 from pre-Silurian igneous rocks. He gives tlie following reasons 

 for his view : — 



1. The abundance of muscovite. 



2. The presence of biotite, plagioclase, and chlorite, which is 



usually derived from unstable iron magnesium minerals. 



3. The occurrence of zircon and lutile crystals in the quartz 



grains in the sandstones may indicate an igneous origin 

 for such quartz. 



4. The constant presence of tourmaline suggests such an origin. 



5. The absence of metamorphic minerals, etc.. .shows that they 



were not derived from metamorphic rocks. 



The last does not show, however, whether the Silurian sediments 

 Avere derived from igneous or sedimentary rocks. Muscovite and 

 tourmaline, whilst suggesting an originally igneous origin, are 

 stable minerals and may easily be handed down from one sediment 

 to another. Similarly, quartz grains containing zircon and rutile 

 may easily have suffered more than one transportation before 

 coming to their final resting place. 



The presence of biotite, plagioclase and chlorite suggests that the 

 rocks were derived to some extent direct from pre-Silurian igneous 

 rocks, especially if the felspar were fairly fresh. 



The writer finds felspar to l>e present in two conditions in his 

 sections; the one much decomposed but showing traces of twin 

 lamellation, and the other quite clear and fresh. The natural 

 inference from this fact is that the decomposed material has suf- 

 fered more handling than the fresh. This would suggest that the 

 clear material has been directly derived from an igneous source, 

 possibly granitic, whilst the decomposed material has been derived 

 from an older sediment. No pre-Silurian granit^es. however, are 

 known near Melbourne with certainty. The You-Yangs granite 

 may be pre-Silurian. 



The writer inclines to the view that both origins are i)robable for 

 the rocks of the Melbourne district. This view is strongly supported 



1. "The Structure and General Geology of the Warrandyte Gold-field, &e." Proo. Roy. 

 Soc. Victoria, vol. xxii., pt. II., 1911. 



2. "General and Mining Geology of the Diamond Creek Area." Proe. Roy. Soe. Victoii.!, 

 vol. XXV., pt. II., 1913. 



