206 



PRESIDENT S ADDRESS SECTION C. 



uiid olivine are the normal constituents, but corroded phenocrysts of 

 anorthoclase wliich are present serve to link this type with the alkali 

 series. 



Liinhurgite. — Very basic rocks occur in a few localities which 

 show no anorthoclase, and yet are distinct from the normal basalts. 

 Chemically they are related to the orthoclase basalts, but structurally 

 they are limburgites. Such a rock is represented in a few small 

 plugs of dense, black prismatic basalt just south of Wood End. 

 A section of one of these rocks shows phenocrysts of fresh olivine set 

 in a dark, dense, fine-g-rained ground mass, consisting- of lath-shaped 

 minute augites, magnetite, and small felspars with a residuum of 

 dark glass. 



Newer Basalt. — The last eruptions appear to be those of the 

 newer basalt series occurring on the plains surrounding tlie Macedon 

 district. They include a number of types langing- from a basic 

 andesite to normal basalt. 



The following analyses of a few of the Macedon rocks made by 

 Bailey, Lewis and Hall, will serve to indicate the composition of a 

 few of the types (266). 



1. Solvsbergite, Camel's Hump. 



2. Anorthoclase-trachyte, Turritable Falls. 



3. Maeedonite, Spring Mound. 



4. Auorthoclase-olivine-andesite, the Jim Jim. 



5. Limbixrgite, Quarry S. of Wood End. 



One noticeable peculiarity of the alkali rocks of Mount Macedon 

 is the occurrence of melilite in rocks containing abundant felspar. 

 This is especially noticeable in the Spring- Mound Rock where it is 

 associated with much perofskite. 



The relative ages of tlie solvsbergite and anorthoclase-trachyte 

 are difficult to determine. Probably the former is the older. Tlie 

 sequence of the rocks is rather peculiar, and perhaps may be explained 

 on the assumption that a magma differentiated into two parts, one 

 calcic, the other alkalic, and that eruptions of the differentiated parts 



