PRESIDE^'^T■S ADDRESS SECTION C. 205 



Anorthoclase-trachijte. — This type is described by Professor 

 Gregory as trachyphonolite. He was probably influenced by the 

 analysis of one of these rocks which is published in his paper, which 

 records over 13 per cent, of total alkalies and over 8 per cent, of soda. 

 More recent analyses by Lewis and Hall (266) show that the alkali 

 percentage is about 10 equally divided between soda and potash. 

 This together with the absence of nepheline and the very sparing 

 occurrence of microscopic nosean has led Mr. Summers and myself to 

 the conclusion that the rock is better described as a soda-trachyte, 

 and as anorthoclase is the dominant felspar we call it an Anorthodase- 

 trachyte. The rock covers a large area as a number of separate 

 flows. The more acid types present close chemical and petrological 

 relationships, and the rock fi-om the Turritable Falls at Upper 

 Macedon may be taken as the type. It is a dark-greenish rock. 

 Large phenocrysts of anorthoclase are numerous. The ground mass 

 has sometimes a fluidal arrangement of laths of anorthoclase, in other 

 cases the ciystals are stouter and the structure orthophyric. Small 

 crystals of legirine are scattered through the rock, a little green glass, 

 a few sections of nosean, ilmenite, and occasionally melilite are also 

 pi-esent. 



Anortliodase-olivine-trachyte. — In the field several flows appear 

 to be more basic than the rock just described, and confirmation of 

 this is given by the microscopic evidence, which shows that while in 

 other respects resembling the more acid type it contains, in addition, 

 smaller or larger quantities of olivine. The upper part of Sugarloaf 

 Hill, north .of Wood End, consists of this type. 



Olivine-anortlioclase-hasalt. — A still less acid type occurs at the 

 base of Sugarloaf Hill, and at other localities near it. It differs 

 mainly from the last t}^e in the greater abundance of olivine and 

 less frequent anorthoclase. 



Macedonite. — A peculiar and interesting lock occurs at Spring 

 Mound, north-east of Hesket and west of Lancefield, on the plains 

 north of Mount Macedon. It is a dark, dense, rather basaltic looking 

 rock. No perfectly fresh samples have been obtained. It is not 

 porphyritic and consists largely of minute felspars, a colourless to 

 green interstitial mineral, either glass or chlorite, a little nosean, 

 serpentine or chlorite pseudomorphs after olivine, some light-brown 

 biotite and apatite prisms. Melilite is not uncommon, and abundant 

 cctahedra of perofskite occur, some of which are opaque, others a 

 dark-grayish green colour. The exact relations of this rock are 

 difficult to determine. Chemically it is in some respects intermediate 

 between the tephrites and the orthoclase basalts but mineralogically 

 it is quite distinct. It appears to be a new rock tj'pe for which we 

 suggest the name of Macedonite. 



Olivine-anorthoclase-andesite. — Several still more basic flows have 

 characters which ally them to som.e extent to the newer basalts of the 

 plains, but are distingiiished by the presence of occasional anortho- 

 clase phenocrysts which are often corroded. Some of the later flows 

 from a hill called the Jim Jim, north of Hanging Eock, are of this 

 tyjDe. The general structure of the rock is that of a basic andesite. 

 Lath-shaped plagioclas© and granular, or ophitic augite, magnetite 



