204 PRESIDEiSrT's ADDRESS SECTION C. 



Physiography and Geological Relations. 



The area is one of little relief. The basement of Archsean (?) 

 and Ordovician ( 1) rocks, with glacial conglomerate and Jurassic sand- 

 stones has been planed down, and newer Kainozoic ironstone gravels and 

 other sediments have been deposited in a horizontal belt reaching 

 northwards to the Murray plains. Subsequent uplift and denudation of 

 this plain by rivers has exposed the underlying rocks. The alkali series 

 nowhere rises above the level of this elevated plateau, and is almost 

 certainly older. How much older is uncertain, as no good junctions can 

 be seen with the glacial conglomerate or with the Jurassics. Dennant's 

 find of a block in tuffs containing a Jurassic plant Otozamites suggests 

 the possibility of the tuffs being Jurassic. They cannot be older, but 

 may be younger, as from his wording it is uncertain whether the plant 

 was in a tuff matrix or represented a block of Jurassic sandstone enclosed 

 in the volcanic rock. The latter seems the more probable, and if so 

 the alkali rocks may be of the same age as the Macedon series. Since 

 writing this I have seen Dennant's specimen, and a small piece of the 

 matrix has been sectioned. The lock is a grayish rock, which may 

 represent a Jurassic mudstone altered by contact with a molten 

 magma. It is certainly quite unlike the black basaltic rock of Mount 

 Koroite. It seems safer to regard the Otozamites as occurring in a 

 block of Jurassic mudstone, and to treat the volcanic rock as probably 

 «f Kainozoic age. The alkali rocks include trachyte probably anor- 

 thoclas© bearing, but containing very little of the ferro-magnesian 

 minerals. About 1\ miles N. of Colevaine are two small conical 

 hills, called Adam and Eve, and consisting of a dense basic rock. A 

 trachyte dyke traverses these hills. An oli vine-bearing anorthoclase- 

 basalt occurs at Mount Koroite, and in association with this rock the 

 Otozamites was found. The rock sequence is rather obscure. 



The main trachyte mass niay be older than the basic series, but 

 if so alkali intrusions, represented by a trachyte dyke through Adam 

 and Eve, followed the basic flow. These rocks have been partially 

 described by Hogg (174), and also by Dennant (172, 175). Two 

 analyses by the latter of the trachvtes are available, and are quoted 

 in p. 207. 



Petrological ai>d Chemical Characters. 



Macedon Area. — Professor Gregoiy has described some of the 

 alkali rocks of the Macedon district (176). The following notes of the 

 ■chief types are partly based on his determinations, supplemented by 

 observations by Mr. Summers and myself. 



Solvshergites. — The solvsbergite plugs of the Camel's Hump, the 

 Hanging Rock, and Brock's Monument agree very closely in chemical 

 ■composition, but present some minor points of difference in mineralo- 

 gical content and texture. The Camel's Hump rock may be taken as 

 a type. 



The rock is grayish in colour. The phenocrysts consist of large 

 crystals of anorthoclase, and smaller ones of :egii*ine, and a few brown 

 "pleochroic crystals of cossyrite. The ground mass consists mainly of 

 fluidally-arranged lath-shaped crystals of anorthoclase or soda-sani- 

 dine, and between the felspars are mossy aggregates of a^girine and a 

 blue pleochroic soda-hornl^lende identified by Gregory as riebeckite. 

 Ilmenite, zircon, biotite, and melilite are occasionally noticed. 



