Geology of Lilyrlole. 349' 



In direction these l)asif dykes diverge a few degrees west of noitli. 

 the divergente being greatest in the southernmost of them. 



Regarding their (n/c. it is dithcult to decide whether they are 

 rehtted to the Devonian cyck' or to the Tertiary basaltic eruptions. 

 But the folloAving considerations favour the foinu-r view : — 



1. The neighbouiing basahs contain no augite except in the 

 ground mass, whereas the dykes are full of piicnocrysts. 



'2. Tlie close association of the dykes in eai-h case with the 

 Devonian igneous rocks. 



.'3. The directions of the dykes are consistent with the hypothesis 

 tliat ttiey are part of a radiating system from some centre in tlie 

 Dandenongs. 



If this view l)e right, they nuist be regarded as dirt'erentiated 

 dykes, representing possibly the complementary products of the 

 acid lavas which appeared in the earlier part of the volcanic jdiase. 



What appears to be a pair of complementary dykes is marked at 

 the foot of the north slope of the Dandenongs in the Lower Dacite 

 Series, adjacent to Olinda Creek in Allot 7.^. The two dykes out- 

 crop within 10 yards of each other, but the outcrop is too limited 

 to determine their direction. 



The Basic Complementary dyke is a Doleiite. It is a l)luish-black. 

 tough, fine-grained rock, showing abundant pyrites. It contains 

 laths and idiomorphic phenocrysts of plagioclase (mainly lal)ra- 

 dorite). and pseudomorphs of two other minerals, one apparently 

 after a pyroxene, and showing ophitic structure, the other aftei- 

 olivine. Iron oxide is common, pyrites abundant, and calcite 

 occurs in the vesicles. The ground mass is cry})tocrystalline. and 

 shows no flow structure. 



The Acid Complement;iry ilyke is a light gi-ay i)oi])liyritie iock. 

 crowded with phenocrysts of (piartz, and streaked with small, elon- 

 gated, white aieas. due to the occupation of vesicles by secondai-y 

 minei-als. It contains phenoeiysts of quartz, idiomorphic, but 

 often eorroded ; ortlioelase. with S(|uare or rliond)ic outlines and 

 diagonal extinction; plagioclase (andes.-lal>rad.) ; biotite in flakes- 

 flexed in conformity with the flow lines, and often containing the- 

 sagenite wel) of rutite needles. Apatite and ziiTon in good pro- 

 portions. Besides the orthoelase phenocrysts. there are alsc, 

 numerous orthoclases of similar shape, but of progressively smallei- 

 size, till those of the ground mass are reached, which is ])acked ftdl 

 of them. 



Between Lilydale and Kvelyn two dykes are visilile in sections of 

 the railway eiitting. one (Allot. 26) about 20 feet wide, the other 



