Vol. XXVII, 

 1910 



] Armitage, Country about West Essendon. 95 



by the removal of material from its sides. This will continue 

 until the whole valley has become many miles wide, with almost 

 a fiat floor. 



In concluding, I should like to thank Professor Skeats for 

 facilities afforded me in the geological laboratory ; Mr. Pritchard, 

 for his help with the palaeontology ; and Mr. Grayson, for 

 preparing the photo-micrographs. 



Petrographical Notes. 



No. I. — Older Basalt, from river level, immediately north of the 

 Sand Pits. 



Macroscopically, this is . a very tough, dense, close-grained 

 basalt of a dark colour. With the unaided eye no crystals can 

 be distinguished. Spots of a green amorphous mineral appear 

 on freshly fractured surfaces. These spots become black after 

 a short exposure to the atmosphere, and evidently consist of 

 halloysite. 



Microscopically, the rock consists of small, perfectly un- 

 altered phenocrysts of olivine, some of which have been slightly 

 corroded by the magma. Many show interpenetrative twinning. 

 Plagioclase laths and microlites are scattered promiscuously, 

 with no attempt at fluidal arrangement. Brown augite has 

 begun to crystallize out in small grains from a ground mass of 

 abundant brown glass. Numerous minute octahedra and specks 

 of magnetite are present. No vesicles are present in the rock. 

 No. 2. — Newer Basalt, from surface of outlier east of the Sand 

 Pits. 



Macroscopically, this is a tough rock of .bluish colour, which 

 under the hammer sounds dully, and does not easily break, but 

 tends rather to pound up, when struck, into a grey powder. 

 It is vesicular, some of the vesicles containing minute stalactites 

 of calcite depending from the upper portions into the vesicles. 

 Its texture is very coarse, the crystals being quite easily dis- 

 tinguishable by the unaided eye. 



Microscopically, it shows original phenocrysts of olivine very 

 much altered, sometimes on the outside and sometimes wholly, 

 to a secondary reddish-brown, non-pleochroic mineral, which 

 extinguishes simultaneously with the olivine it surrounds or 

 replaces. This agrees with the mineral iddingsite, which 

 often develops from olivine in basalts on the top of flows. In 

 some cases the alteration of the olivine to iddingsite has been 

 accompanied by the formation of small spherulites of chalcedony. 

 Long lath-shaped felspars, showing twin lamellae, are present in 

 abundance. These felspars, by measurement of their extinction 

 angles, are seen to come well up to the soda end of the soda-lime 

 series of plagioclase felspars, and should be classed as oligoclase. 

 Broad tabulate felspars, well developed on the face 010, are 



