184 . president's address — section c. 



The rocks of Mount Leinster lise above, and in part rest upon a con- 

 siderable area of quartz-diorites. In Howitt's view they are genetically 

 related to the diorites, and form the latest and most acid of the series 

 of igneous rocks intruded into the district probably in Devonian 

 times. They are the youngest rocks in the district, and show chemical 

 and petrological resemblances to the Mid-Kainozoic alkaline rocks of 

 the Macedon District, and further evidence as to their age is needed. 



Petrological and Chemical Characters. 



Howitt describes several allied types among the rocks of Mount 

 Leinster, but only names one rock which he analysed. This consists 

 essentially of orthoclase and augite with subordinate ferric hydrate 

 and plagioclase, and is described by Howitt as a syenite-porphyry. 

 The porphyries of Mount Leinster, according to him, consist essen- 

 tially of orthoclase and augite, with accessory triclinic felspars and 

 occasional quartz ciystals, and a small amount of ores of iron. They 

 are, therefore, to be classed as " Augite-syenite-porphyries." 



The following is the analysis of the syenite-porphyry described 

 above : — 



SiO., 60-57 K2O 6-29 



AlaO^ 19-05 Na,0 5-55 



Fe O3 5-2!) H„0 TGO 



CaO 0-92 



MgO 1-5(5 Total =100-83 



In connection with this rock, Howitt remarks that, though the 

 felspar appeared to be mainly orthoclase under the microscope, yet 

 the analysis suggests the presence of a large percentage of a triclinic 

 felspar near to, if not in fact, albite. 



I have examined Howitt's rock sections from this locality, and 

 find that a large amount of the felspar identified by Howitt as ortho- 

 clase has the optical character of anorthoclase. The augite also is 

 green and slightly pleochroic, and is a variety of segirine-augite, while 

 a little of a blue soda-hornblende occurs interstitially. These observa- 

 tions serve to bring the microscopical characters into agreement with 

 the results of the chemical analysis, and to show that, while 

 structurally the rock i-esembles a syenite-porphyry, mineralogically 

 and chemically it is allied to the alkaline intrusive rocks of inter- 

 mediate composition In a short visit made to the district in company 

 with Mr. Dunn (30) we noticed that near Mount Leinster homestead 

 a hill is composed largely of pyroclastic rocks of a truchytic character, 

 through which a number of dykes penetrate. I hope to. publish a 

 petrological account of these rocks shortly. At present it may suffice 

 to say that solvsbergites are present among the Mount Leinster rocks, 

 while others are allied to the bostonites, and amon<r the pyroclastic 

 rocks the most abundant fragments are those of an alkaline rock with 

 a trachytic texture, probably an alkaline trachyte. 



Howitt's analysis shows a close agi'eement with those of the 

 alkaline trachytes from the Western District, Macedon and Omeo, to 

 b3 referred to later. 



