Geology of Broadmcadoivs. 169 



average of the 161 analyses of typical basalts collected by R. 

 A. Daly (8). This finds expression in the petrographical des- 

 cription. The rather higher percentage of CO2 does not mean 

 that the rock is not fresh, but is explained by the presence of 

 vesicles filled with arragonite. 



Petrographical Characters. 



Specimen No. 35 (B), Section VI., Will Will Rook. Mega^ 

 scopically it is a blue, even-grained rock in which crystals of 

 green olivine, black augite, and felspar may be seen with the 

 aid of a lens. A few vesicles may be seen lined with arragonite. 



Microscopically it consists of phenocrysts of olivine, augite and 

 felspar laths set in a ground-mass, partly felspar, partly augite, 

 but mostly glassy. 



The olivine crystals are often much altered in that along the 

 edges and cracks the crystals have the appearance of iron 

 staining. This iron-staining sometimes extends right across the 

 olivine crystal, which then assumes the characters of iddingsite, 

 dark-brown colour, slight pleochroism, and straight extinction. 



The augite is developed in irregular plates or phenocrysts to a 

 much less degree than olivine. It is occasionally twinned, and is 

 of a very pale yellowish-green variety. The bulk of the augite 

 is distributed as grains and prisms throughout the ground-mass. 

 It is probably a titaniferous variety. 



The felspar laths show lamellar twinning, and the measure- 

 ment of the extinction angles of a large number of sections gives 

 a maximum of 35 deg., indicating a labradorite of approximately 

 the composition of Al>2Ang. In addition there is another felspar 

 with the same characters as the second type of felspar found in 

 the older basalt, and therefore to be described as soda-orthoclase 

 or anorthoclase. 



Grains of magnetite or ilmenite occur scattered through the 

 ground-mass, and minute prisms of apatite occur in the felspar 

 laths. 



The rock may be called an olivine anorthoclase (1) basalt. 



Distribution and General Characters of Limestone. 



The limestone occurs in a more or less irregular way along the 

 valley of the left branch of the Moonee Ponds Creek. It occurs 



