Geology of Mount Macedon. 



203 



that the normal dacites contain an excess of alkaline earths over 

 alkalies, and far less alkali than the Macedon rock. Thus 

 Doelter's analysis of the typical Dacite of Transylvania gives 

 66 % of silica, 7'1 % of lime and magnesia, and only 5*5 % of soda 

 and potash. The Macedon rock contains 64-3 % of silica, 

 4"1 % of lime and magnesia, and 9 "8 % of soda and potash. A 

 comparison of the mean molecular composition of the dacites 

 quoted by Loewinson Lessing (11, p. 449) with that of the 

 Macedon rock is shown by the following table, which also gives 

 the nearest analysis to the latter. 



I 



In Rosenbusch's series of analyses of Dacites, no rock is given 

 with so high a percentage of alkalies as we find at Macedon. In 

 his sixteen analyses (18, p. 286) the highest percentage of soda 

 and potash together is 7-95, in the rock to which he gives the 

 precise but lengthy name of " quarzbiotithronzitaugitporphyrit " ; 

 the highest percentage of soda is 4-06 against 6-28 in the rock 

 from Willimigongong. 



The name dacite alone is unsuitable, for tlie dacites are 

 eruptive members of the quartz-diorite group and this Macedon 

 rock is certainly part of a sequence of which the best developed 

 lava is a member of the tinguaite group. The name, moreover, 

 would be locally inconvenient, for normal dacites appear to be 

 extensively developed in Victoria, and it would be inadvisable to 

 include the two rocks under one name. Hence I propose to 

 refer to the fundamental rock of the Macedon range as geburite- 

 dacite, Gebur being the native name of Mount Macedon. 



The geburite-dacites may be defined as intrusive or efiusive 

 dacites, distinguished from the normal dacites by the rarity of 

 quartz and the great excess of alkalies, especially of soda. The 



