BY G. D. OSBORNE. 



175 



to that in -which fine-grained angular fragments are abundant, and the cognate 

 inclusions almost absent. The general body colour of both types is a dark buff. 

 Following this uiiique horizon there are two varieties of tuff differing in texture, 

 the coarser having some features in common with the breccia-portion of the 

 last-mentioned unit. A band of coarse conglomerate is found above these, and 

 after the intervention of a thin flow of decomposed felsite, there is a thick mass 

 of conglomerate. The constituent pebbles of this horizon often attain to a 

 considerable size, a number measuring 2^ft. in diameter, and some rarely 3ft. 

 There is an abundance of pink aplite pebbles, while quartzite and acid por- 

 phjrries rank next in importance. On the surface of this conglomerate has been 

 poured a flow of fine-grained biotite-quartz-keratophyre, in which a little brec- 

 ciation has occurred during crystallisation. This is immediately overlain by a 

 tuffaceous dellenite about forty feet thick at its maximum, the extent beside the 

 line of section being small. Conglomerate is found above this and then an im- 

 portant flow of dellenite containing plenty of free quartz, potash and soda-lime 

 felspar set in a devitrified pumiceous gTOundmass. 



This horizon is succeeded by a variety of massive igneous rocks, the first 

 being a mauve-coloured type with a subconchoidal fracture, very fresh specimens 

 being obtainable in Wattle Creek, the rock outcropping in the bed, and on a 

 steep hillside to the south. The rock is a rhyolite and has been almost wholly 

 glassy except for free quaxtz and a few acieular felspar crystals. Devitrification 

 has been universal, secondary silica and iron-oxides having replaced original 

 glass. The next type is a potash-rhyolite which has been stained locally in such 

 a manner as to give it a blotched appearance. It caps the hill to the south, 

 of Wattle Creek, the next horizon leading down the dip-slope to the south. This 

 rock is either a sodi-potassie rhyolite or a soda-rhyolite, microscopic examination 

 being difiScult. It is not very thick, and ends under the last unit in the Volcanic 

 Stage, which is an andesine-dacite, in which a certain amount of biotite occurs, 

 the cryptocrystalline base being strongly stained in places by iron-oxides. Over- 

 lying this horizon is the basal conglomerate of the Glacial Stage. 

 The Glenoak Section shows the following succession : 



Thickness in Feet 



Hypersthene-andesitic-pitchstone 200 



Tuffs with pebbles lOO 



Biotite-quartz-keratophyre 180 



Volcanic conglomerate, etc 60 



Coarse tuff 100 



Fine tuff eO 



Conglomerate 180 



Felsite 60 



Conglomerate . . . 450 



Fine-grained quartz-keratophyre 90 



Tuffaceous dellenite 40 



Conglomerate 45 



Dellenite .. 80 



Rhyolite 250 



Potash-rhyolite 130 



Sodi-potassic rhyolite 45 



Dacite 120 



Total Thickness 2190 Feet 



The Oakendale Section. (G-H on Map). Text-fig. 4. 



The chief interest in the Oakendale Section lies in the fact, that while 

 close to the Glenoak Line, the number of units developed is much smaller, the 

 massive igneous rocks being very poorly represented. 



