president's address SECTION c. 191 



The Strathhogie and Whitfield Areas. — Mr. Summers's work in 

 the northern part of the Strathbogie area (93) has shown that near 

 Mount Samaria there appears to be a gradual passage from gi'anitic 

 rocks through granite-porphyries to quartz-porphyry. In the same 

 paper he states that in the Tolmie and Toombullup Ranges the por- 

 phjTies are clearly older than Carboniferous, since sandstones of that 

 age rest on the porpliyries and show no signs of contact metamor- 

 phism. Some of the hypersthene^bearing dacites of the Strathbogies 

 contain abundant garnets, and in the Whitfield area Kitson has 

 described (74) porjjhyries, also garnetiferoas, and correlated them with 

 the Snowy River porphp-ies. Their geographical and petrological 

 relations suggest, however, that it would be safer to group them with 

 the dacites and porphyries of the northern part of the Strathbogies, 

 and I have therefore refei'red to them in this place. 



Relations of the Dacites and Quartz-Porphyrites to the 

 Palceozoic Sediments. 



All observers are in agreement that the dacite series is younger 

 than the Ordovician and Silurian sediments with which it comes into 

 contact. Prof. Gregory states tliat evidence of contact metamorphism 

 near the junction of dacite and Ordovician was sometimes found in 

 the Macedon area, but sometimes the OrdoAdcian rocks were quite 

 unaltered. He referred the alteration where seen to the intrusion of 

 the granodiorites, and hence regarded the dacites as rocks of super- 

 ficial origin. 



Mr. Summers and I have found evidence of contact meta- 

 morphism wherever we have seen the sediments in contact with the 

 dacites in the Macedon area, and sometimes several miles away from 

 the outcrop of granitic rock at the surface. While it is possible the 

 gi'anitic rock may be concealed beneath a thin cover, we are inclined 

 to regard some of the contact effects as due to the dacite, and to 

 regard it as in part of intrusive character. 



I have not studied the contact with the sediments in the Dande- 

 nong area except south-east of Bayswater, where no granitic rock 

 occurs and the sedimentary rocks are altered near the contact. 



In Marysville township for a considerable distance from the con- 

 tact with the dacites the palceozoic sediments are considerably altered 

 and indurated. No gTanitic rocks occur at the surface near here, and 

 I think a good deal of the Marysville dacite must be regarded as 

 intrusive rather than effusive. 



The Age of the Dacites and Quartz-Porphyrites. 



Selwyn described this series as Pal;x:ozoic '"' Traps " passing 

 gradually into granite. Prof. Gregoiy regards them not only as later 

 than the Silurian and Ordovician sediments, but also as far younger 

 than the granodiorites and possibly of Lower Kainozoic age. 



For the reasons above given I believe them to be younger than 

 the Silurian and Ordovician sediments, but slightly older than the 

 granodiorites. The evidence Mr. Siunmers has obtained in the Strath- 

 bogie Ranges is important as showing that Lower Carboniferous con- 

 glomerates and sandstones rest upon the northern development of the 

 dacite-quartz-porphpy series. This fixes their age in that locality 



