282 N. R Junner: 



yet, they appear to be sharply defined from one another. The 

 superposition of the dacites on the rhyolites near the Acheron 

 River, and the inclusion of fragments of the latter rock in the 

 former at certain places, are sufficient to prove the subsequent 

 extrusion of the dacites. The position of the quartz free andesites^ 

 is not certain. They occur marginal to the dacites near Launching- 

 Place, suggesting that the latter rocks overlie them. Numerous 

 xenoliths of andesite occurring in the Blacks' Spur dacite, may 

 also indicate a subsequent origin for the dacite. However, in 

 the absence of mora certain evidence it is better to leave the ques- 

 tion of the age of the andesites unanswered. The evidence brought 

 forward is sufficient to establish the following sequence, from older 

 to newer — 



Rhyolites and rliyolitic tuffs, 



Andesite, 



Dacites, 



Granodiorite. 

 The sequence, viewed broadly, shows the order of extrusion to be 

 one of increasing basicity. 



C- — Origrn of the rocks. 

 Without much doul^t all these rocks have been derived by dif- 

 ferentiation from a connaon magma. Whether the differentiation 

 is of a serial or complementary type, cannot however be deter- 

 mined Avith certainty in the absence of chemical analyses of all 

 these rocks. The qviestion of differentiation has been attacked 

 exhaustively in the Macedon area by Professor Skeats and Dr. 

 Summers, and at Dandenong, by Mr. Morris, and their conclu- 

 sions leave no room for doubting that the granodiorites and dacites 

 in these areas are consanguineous. If any further evidence i» 

 needed in the Healesville area to establish the comagmatic origin 

 of the rocks, the striking similarity in their mineralogical com- 

 position and their intimate association in the field may be put 

 forward in support of this view. Especially significant is the 

 occurrence of zone plagioclase and microperthitic orthoclase in 

 the granodioi'ite, and exactly the same types of felspar in the 

 dacites and rhyolites. Garnet is also present in all of these rocks. 



6, — Conclusions. 



1. Folded Upper Silurian shallow water marine sediments 

 form the basal bods of the Healesville area. Fossils are rare. 

 Haliserites Dechenianus, Goppert, was found in the north of the 



