Dacite and Granodiorite. 137 



dorite and biotite with rather rare examples of corroded hypersthene. 

 This suggests that differentiation had proceeded rather further, and 

 that the temperature at which crystallisation ceased had been 

 sufficiently low for the reaction between the hypersthene and felspar 

 to be almost complete. 



Dr. N. L. Bowen's5 theory of differentiation by sinking of crystals 

 provides the most reasonable explanation of the relationship of the 

 dacites to the granodiorites. 



It has been shown that in the Macedon area, at the time of the 

 extrusion of the dacite, labradorite and hypersthene had crystallised 

 out, and that the still molten material containing alkali felspar mole- 

 cules was commencing to react with the hypersthene to form biotite. 

 The magma at this time would contain a certain proportion of water 

 and other volatile ingredients, and some of these gases would escape 

 when the lava reached the surface. This loss of water, etc., and the 

 expansion of the lava due to loss of pressure, would serve to convert 

 a magma sufficiently fluid to be able to slope its way upwards into 

 a moderately viscous lava, incapable of extensive flow from the vent. 

 Solidification at the border under these conditions would be rapid, and 

 no further reaction between the hypersthene and felspar molecules 

 would take place. Away from the margin a higher proportion of 

 biotite would be formed, owing to slower cooling, and the lower tem- 

 perature of final consolidation due to less loss of volatile constituents. 



The solidification of the lava would seal up the batholithic cham- 

 ber, and crystallisation of the remaining magma would continue at 

 gradually decreasing temperatures. 



Bowen believes that differentiation is mainly due to the sinking of 

 crystals, so that the magma shows increasing acidity upwards. In 

 the cases under consideration the upper, more acid portion, found its 

 way to the surface, and the upper portion of the material left in the 

 magma reservoir would be less acid than that which had reached 

 the surface. Further crystallisation and sinking of crystals could 

 go on, and this new upper layer would constantly gain in acidity. 

 According to the length of time between the extrusion and the final 

 solidification of the magma in the reservoir, so would the relative 

 silica percentage of the volcanic and plutonic phases vary. 



At Macedon the plutonic rock is the more acid, so that the assump- 

 tion is made that there was a considerable period, relatively between 

 the extrusion of the dacite and the final solidification of the grano- 

 diorite. 



At the Strathbogie Range only two analyses have been made, one 

 of quartz porphyrite from near Violet Town, and the other of the 

 plutonic type from near Trawool. The plutonic type has the lower 

 silica percentage. The temperature of the volcanic phase at Strath- 

 bogie, at the time of extrusion, was lower than that of the Macedon 

 dacite, and differentiation was further advanced. This is shown by the 

 almost complete replacement of hypersthene by biotite, and the greater 

 acidity of the Strathbogie rock. 



5. Bowen, W. L., Jour, of Geology, \'ol. 23, No. 8, Supplement, 1915, 

 and later ^pers. 



