Palaeozoic Geology of Victoria. 119 



(i.) Fragmeutal Type. — Sections of this typo show all the larger 

 minerals and inclusions as brokenf and angular fragments, with no 

 embayment. The base is siliceous and finely granular, and among 

 the larger fragments felspar is often more abundant than quartz, 

 and tli,e triclinic form distinctly predominates, orthoclase being 

 jare. 



The secondary minerals most abundant are chlorite, and occasion- 

 ally calcite. 



(ii.) Acid Porpht/rites {Xori-F rag mental). — The rocks of this type 

 have perhaps a higher silica percentage than normal porphyrite, 

 and this may in part be due to a certain amount of secondary 

 silicification and alteration. 



The fine grained base of the rock is generally affected to some 

 extent in this way. All the recognisable felspars, however, are domi- 

 nantly triclinic, orthoclase being more or less rare. 



No. 100 is a medium grained porphyritic rock of a red brown 

 colDur. 



In the thin section the phenocrysts have rather irregulaii outline, 

 the quartz is partly rounded, and occasionally embayed. Ihe fels- 

 pars are more abundant than quartz, and occasionally have a 

 regular prismatic outline, but more often are broken and irregular. 

 Twinning after the albite law prevails, but occasional pericline and 

 Carlsbad types are represented. The twin lamellae vary from mode- 

 rately broad to fine bands. Different kinds of felspars appear there- 

 fore to be represented. Only a few were satisfactory for deter- 

 mination by extinction angle. These gave readings up to 20 

 degrees, and are probably therefore andesine. 



The optical features of several examples suggest anorthoclase, 

 and the chemical analysis, further points to some potash felspar, 

 which, however, has not been definitely recognised microscopically. 

 Magnetite is only moderately abundant. The base is micro- to 

 erypto-crystalline, w^ith some evidence of recrystallisation and sili- 

 cification. Minute laths and fragments of felspars and quartz are 

 •scattered through it. 



Chlorite is sparsely distributed through the rock, and calcite is 

 present in moderate amount. 



This rock is regarded as a quartz porphyrite. No. 105 is a green- 

 ish porphyritic roek, but the chemical analysis corresponds closely 

 with that of tlie pi-evious example. 



The thin section is very similai-. Quartz ])lienocrysts are a little 

 more abundant, and some are Ixautifully ein])ay('(l. 'I'lx' felspars 



