Geology oj the Uiamond ('reek Area. 335 



(1)) Basalts. 



1. Fine grained Ijusalt, ([uarry near the northern limit of the basalt 

 at Greensboroiigli. Macrcscopically this rork is dark grey in colour, 

 and is compact, and aphanitic with the exception of occasional 

 felspar phenocrysts. A sample was collected from the quarry above 

 mentioned and analysed by the writer in tlie Univei'sity geological 

 laboratory. The weathering of the basalt in this quarry has led to 

 the solution of lime and magnesia, and these have been redeposited 

 as a niagnesian limestone. For the purpose of comparison an 

 analysis by F. L. Stillwell of Uie older l)asalt from near Broad- 

 meadows is appended. 



A. Fine grained ba.salt quarry 

 off main road, near northern 

 limit of the basalt, Greens- 

 borough— S.G. 2.94. 



B. Older ba.salt (A) quarry, 

 Section 15, TuUniarine, county 

 of Bourke. 



Mici'oscopically the texture is aphanitic with occasional por- 

 phyritic felspars. The mineral composition of the rock is thus : — 

 Plagioclase, olivine, augite, magnetite, ilmenite. zeolites and 

 apatite. Plagioclase occurs in two generations, firstly as long 

 iaiomorphic clear and gla.ssy laths showing lamellar twinning 

 generally, but occasionally only simply twinned. Pressure effects, 

 probably of a local origin, are noticed in the twisting and fractur- 

 ing of the laths, by wedging of the twin lamellae, and also by cross 

 fractures almost at right angles to one another, and generally 

 obli(|ue to the planes of twinning. Olivine is almost indeterminate 

 in ordinary light, due to its invasion by the ground mass. Concen- 

 tration of black iron oxide occurs frequently in the centre of the 

 crystals, and radiating linearly arranged rods of the same material 



