Igneous Rochft. 288 



area, determining the age of the beds as probably Tangilian. 

 Poorly preserved corals and ci'inoids, occur in conglomerates from 

 near Narbethong. 



2. The vulcanieity in this area commenced in 1 Devonian 

 times, after the folding and uplift of the sediments. Extrusion of 

 rhyolites, accompanied by minor explosive outbursts, inaugurated 

 the cycle of volcanic activity. The expiring rhyolitic vulcanieity 

 was marked by the passage of hydrothermal solutions, containing 

 alkaline carbonates and sulphides, in solution, through the pre- 

 viously consolidated igneous rocks. 



3. A great thickness of dacitic rocks, including quartz free 

 andesites, biotite dacites, and hypersthene biotite dacites suc- 

 ceeded the rhyolites and pyroclastic rocks, and covered up most 

 of them. The latter rocks are now only visible around the 

 periphery of the dacites, where denudation has been greatest. 

 There is good reason to believe that the dacitic magma was very 

 viscous. Steam cavities are almost entirely absent, and mineral- 

 isers which would have decreased the viscosity were apparently 

 ecaroe. 



Later still, granodiorite was intruded at several points. Subse- 

 quent denudation, which has been greatest around the margins 

 of this volcanic pile, has exposed certain of these intrusions. 



The volcanic activity appears to have been entirely sub-aerial. 

 There is no evidence present anywhere in the area of lacustrine 

 or marine sediments associated with the igneous rocks. 



4. It is. very probable that all the igneous rocks described are 

 congeneric. The more complete evidence available from other 

 areas in Victoria, e.g., Macedon and Dandenong, where somewhat 

 similar rocks are present, supports this belief. The sequence of 

 eruption was apparently one of increasing basicity. 



5. Magmatic fusion and assimilation of aluminous rocks by cer- 

 tain of the dacites has taken place, and as a result cordierite, 

 sillimanite and, possibly, garnet, have been formed. 



6. The origin of the garnet remains unsolved, but it seems 

 very likely that it is a primary mineral in the rocks. Its exten- 

 sive occurrence, in granites, porphyries and dacites elsewhere in 

 Victoria, supports this contention. 



In conclusion, the writer desires to express his indebtedness to 

 Professor "Watts, for many facilities granted him in the carrying 

 out of this research at the Royal College of Science, London ; and 

 to Professor Skeats, for advice and suggestions. 



