Notes on MoncJilqihlte Dijkes. 9 



east of the main lines of reef. In the hand specimen it is a hard, 

 greenish rock with shiny tlakes of mica. Microscopically, the rock is 

 much altered and of porphyritic character. The outstanding pheno- 

 crysts are those of an originally well crystallised mineral, which is now 

 completely altered to brownish unrecognisable material, though here 

 and there small patches have only gone as far as chlorite and calcite. 

 Some of these well bounded sections are eight-sided, and indicate 

 original augite. Others are six-sided, and by their prism angle indicate 

 original hornblende. No unaltered augite or hornblende is present. 

 Plagioclase felspar occurs sparingly as crystals which are not much 

 altered. Phenocrysits of biotite with ragged edges are common, with 

 pleochroism varying from a deep brown to a very pale straw colour. 

 It has undergone considerable alteration to chlorite. Some crystals of 

 biotite have undergone internal bleaching, the iron having been leached 

 from the centre, and concentrated in a ring which now gives a dark 

 border to an interior of chlorite and calcite. Quartz is abundant, and 

 the chlorite and calcite extremely so. Ilmenite occurs scattered in very 

 small crystals, and apatite is also accessory in fine needles. 



The rock is a typical mica, lamprophyre, and, if placed in Rosen- 

 busch's classification, it would be termed kersantite. 



The specific gravities of these rocks were determined from small 

 fragments, and found to be : — 



Central Red, White and Blue sample - - - 2.95 

 Forbes Carshalton sample ----- 2.99 

 Goldfields No. I. sample ----- 3.05 



One Tree Hill sample - - - - - - 2.78 



Age of the Bendigo Dykes. 



The dykes include ordovician sediments, and are certainly post-ordo- 

 vician. South of the Big Hill tunnel, a monchiquite dyke cuts a granitic 

 dyke. The granitic dyke is probably associated with the Harcourt 

 granite, and the monchiquites are probably post-devonian. At Kan- 

 garoo Flat a dyke is found cutting through a glacial conglomerate. If 

 this conglomerate is part of the derrinal series, and of permo-car- 

 boniferous age, the dykes must be later than permo-carboniferous. 

 Further than this the stratigraphy of the Bendigo district cannot help 

 one, for no later geological period has left its record except in .some 

 tertiary and recent river gravels. 



No structural evidence is forthcoming, though there is a possibility 

 that the relations of the dykes to the series of earth movements that 

 have affected the Bendigo field may be found and co-ordinated in time 

 with general movements throughout Victoria. 



