382 E. W. Skeats: 



tiallj similar. This implies that the fiagmental rock from the main 

 shaft had originally the composition of an olivine-monchiquite. 

 The mici-oscopic evidence quoted above entirely supports this con- 

 clusion. However, the subsequent alteration of the two rocks has 

 apparently been different.' Whereas the content of lime, magnesia, 

 jind the alkalies in B, the monchiquite dyke, is quite normal for 

 such a rock, the proportions in the fragmental rock are absolutely 

 abnormal, the alkaline earths totalling only three per cent., while 

 the alkalies total thirteen and a-half per cent. This implies the 

 removal of the bulk of the alkaline earths present in the fresh rock, 

 and the introduction of about 10 per cent, of alkalies, principally 

 soda. At first I refused to credit the results of the analysis. Mr. 

 Plante, however, determined the alkalies twice, and obtained a total 

 of 12 to 14 per cent, for the determinations. At a later date Mr. 

 Thiele re-determined the alkalies, and his results are includeil 

 above. The microscopic examination fails to indicate in what 

 mineral form the alkalies are present in the rock. There are nu 

 felspars, and while analcite may be present there is nothing in the 

 appearance of the section to suggest its presence in large quantity. 

 I am not aware of any similar type of alteration in an igneous 

 rock having been previously recorded. There seems to be no doubt 

 of the original nature of the rock, and of its present composition ; 

 the alteration has occurred, but I am quite unprepared to suggest 

 the chemical and mineralogical changes by which the alteration has 

 l>een effected. 



The Origin and Age of the Deposit. 



The evidence of tlie mode of occui icnre of the deposit shows a 

 probably vertical contact with the Ordovician. while the microsroijic 

 and chemical evidence shows that the bulk of the material, especially 

 below the surface, consists essentially of fragments of an ultra- 

 basis volcanic rock, a monchiquite agglomerate. The relations 

 are suggested in the sketch section accompanying the geological 

 map. (Figure in text.) Further, the chemical and miscrocopical 

 evidence shows the closest relations between the agglomerate and 

 the monchiquite dyke penetrating it. Elsewhere^ Mr. Stiliwell has 

 suggested a correlation between the iiionclii(iuite dyke of Bendigo 

 and elsewhere in Victoria, and some of tlic limburgites of the 

 Macedon district, 2 which are not older tlian mid-Kainozoic. and 



1 Stilluell op. cit. 



2 Skeats and Sumniei'.s. Geolo<,'.v and Petrolojiry of the Macedon District, Bulletin '.'4. (Jeol. 

 .Survey of Victoria, 1912. 



