BY G. D. OSBORNE. 173 



scantily developed, the gTOund-mass being pumieeous and devitrifled. This is 

 overlain by a conglomerate composed of small pebbles, which is followed by a 

 thin horizon of a tuffaceous sodie rock, the fragmentation apparently having 

 occurred during the solidification of the rock. Quartz, biotite and albite are 





Text-fig. 3- The Langlands Section. (Line C-D on map.) 



present, the rock being a keratophyre. This is immediately succeeded by a dai-k 

 purple rock which shows an abundance of free quartz in large crystals, and 

 subordinate felspar set in a fine-grained base. The rock, which is a potash-, 

 rhyolite, breaks with a very uneven fracture. Following this horizon is the 

 equivalent of the thick mass of dellenite in the Mt. Gilmore Section. Here the 

 composition of the rock is just about on the border-line between dellenite and 

 toscanite, the plagioclase being basic oligoclase and in about the same proportion 

 as the orthoclase. The lava forms a distinct ridge which can be followed from 

 the Williams Eiver to the Maitland Road. 



Succeeding this is one of the most interesting groups of rocks in the area. 

 They may for the present be referred to as tuffaceous volcanic conglomerates 

 and flow-breccias. They present a variety of characters, but the general features 

 consist of the occurrence of rounded and partially rounded inclusions of soda- 

 felsite with phenoerysts of quartz, together with numerous angular chips of 

 glassy and pumieeous rocks and an odd piece of felspar, all compacted together 

 by felsitic material of similar composition to the corroded inclusions which 

 has been very much altered by secondai-y silica replacement, so that the appear- 

 ance in hand-specimen is that of a number of rounded inclusions and angular 

 fragments set in a subordinate matrix of interstitial strings of quartz. In places 

 the rounded inclusions decrease or even disappear, and the rock becomes an 

 even-grained tufl: or breccia. A considerable amount of investigation will have 

 to be made upon these rocks before their sig-nificance is fully appreciated^ but 

 it seems probable that they have originated in the following- manner. The 

 rounded and partially rounded pieces of soda-felsite have resulted from the 

 breaking up of portions of the crust of an acid alkaline lava during cooling, 

 subsequent corrosion of the fragments being efl'ected by the unconsolidated and 

 still fairly mobile magma. Simultaneously, tuft'aceous material was being 

 showered in varying amount over an area more extensive than that occupied 

 by the lava, the latter incorporating the tufi: in many places, and the residuum 

 of magma consolidating as interstitial felsitic material. Post-dating these pro- 

 cesses, siliceous solutions have altered portions of the rocks, especially the ground- 

 mass, effecting replacement. The association of rounded fragments of sodic 



