880 THE GEOLOGY OF THE NANDEWAR MOUNTAINS, 



same minerals and a great abundance of glass fragments in the 

 form of minute tubes, and boomerang-shaped and bone-shaped, 

 branched and jagged rods. All this material is of pure volcanic 

 origin, but has evidently been redistributed by the action of 

 water. Subsequent alteration due to regional metamorphism 

 has led to the commencement of secondary crystallisation or 

 regeneration of crystals. Much of the material, originally glassy, 

 is therefore partially devitrified. 



Rocks of this kind, which must be termed Tujfy Ehyolite 

 Conglomerates, prove without doubt that submarine eruptions 

 and land eruptions were in progress near an old shoreline and 

 the materials ejected were redistributed b}'' the waves. 



Interbedded with them are found rhyolites, devitrified porphy- 

 ritic pitchstones and quartz porph3^ries. 



N.17. Loc: laccolite on Dingo Creek, branch of Bullawa 

 Creek. (Plate 1., fig.2). 



Handspecimen a coarse-grained dolerite in some varieties of 

 which large augite phenocrysts occur, but the type here described 

 is rather even-grained. Near the edges of the laccolite this rock 

 graduates into a black aphanitic basalt with occasional amygdules. 

 The intrusion is older than the alkaline rocks. 



Texture holocrystalline, seen under the microscope to be 

 uneven-grained and porphyritic, having crystals of most varying 

 sizes. Fabric hypidiomorphic granular, and ophitic. 



Constituents : basic felspar and titaniferous augite are the two 

 most abundant constituents, occurring in about equal proportions 

 and forming about 60-70 % of the mass. The next constituent 

 in order of abundance is olivine, forming between 10 % and 20 %. 

 Then follow magnetite and ilmenite, forming upwards of 5%. 

 Decomposition-products such as serpentine, chlorite and leucoxene 

 also occur in notable amount. As an accessory minor constituent 

 apatite (in long thin needles penetrating the other minerals) 

 deserves mention. 



Note. — In some varieties of this rock-type analcite forms a 

 constituent mineral. 



