BY G. D. OSBORNE. 189 



ridge. 'J'he rock in almost every case is a dellenite, but one specimen, under the 

 microscope, proved to be a toscanite. At the northern end of the ridge the 

 maximum development occurs, the field relations not being very clear; one is 

 almost inclined to regard the mass as intrusive on account of its irregular ex- 

 tent, but the extrusive nature shows up in thin section, and the westerly con- 

 tinuations of the unit can only be viewed as volcanic occuiTences. On account 

 of the plunging anticline between Mt. Gilmore and Glenoak, this flow swings 

 round to the Williams River, gi-adually decreasing in thickness, and on the 

 right bank has a somewhat different appearance in hand-specimen, the rock 

 proving under the microscope to be a phase on the border-line between dellenite 

 and toscanite. From here it is traced to the Maitland Road, where this crosses 

 Tumbledown Creek, and the outcrop is only continued for a few chains to the 

 north-west when faulting occurs, large horizontal displacement placing the rock 

 south of Glenoak, where it is developed on the timbered lulls in portion 29, 

 Parish of Seaham. In this outcrop the rock is more potassic than elsewhere, 

 being more of a potash-rhyolite than a dellenite. In order to follow it we start 

 again in portion 189, Parish of Uffington, where it runs north-north-west to a 

 small creek in that portion, suffers a small displacement by faulting, and then 

 continues through the Oakendale paddocks along Tumbledown Creek to the 

 large fault near portion 99, Parish of Barford. Along the line of outcrop just 

 mentioned it has a marked effect on the drainage and, near the fault plane, is 

 tilted up, dipping at an angle of 40° to the S.S.W. The fault in question has 

 the effect of sending this horizon to the south, where it outcrops in portion 142, 

 Parish of Barford. From here it forms a pronounced ridge which runs west, 

 reaching the railway line by a gradual decrease in height, which is due to the 

 pitching nature of a broad earth-fold, of which structure the lava forms a part. 

 From this ridge the rock spreads out in a dip-surface to the south, forming 

 much of the area drained by the headwaters of Tucker's Creek. The most 

 westerly outcrop of this portion of the flow is in the railway cutting, just south 

 of the first overhead bridge south of Martin's Creek, the rock being a toscanite. 

 About here it is intersected by a large fault, and is then found appearing at 

 intervals through the alluvium and recent wash in portions 27 and 28, Parish of 

 Houghton, striking N. 15° W., finally crossing the Paterson River and out- 

 cropping in the southern end of portion 132. In these occurrences along the 

 right bank of the Paterson River the rock shows a decrease in the content of 

 phenocrystic orthoelase, and in portion 27 it is a dacite, unless the potash mole- 

 cule be retained in the cryptocrystalline base. 



Of the upper members of the Volcanic Stage, the da«ites are perhaps the 

 most important. These in general have a glassy base and in some occurrences 

 contain fragments of foreign felsitic rocks. They are poorly developed on the 

 plain to the east of Mt. Johnstone, and outcrop also in the valley of Tucker's 

 Creek. Near Glenoak and Langlands Estate their extent is limited, but in the 

 Mt. Gilmore area they attain a considerable aggregate thickness. In one part 

 of the Gilmore ridge the main dacite, due to faulting, forms the ridge-capping, 

 which elsewhere is composed of the Mt. Gilmore dellenite. From the slopes to 

 the south-east of the main Gilmore ridge in portions 51 and 52, Parish of Wil- 

 mot, to near the Williams River in portion No. 2, the outcrops of the dacites 

 are quite distinct. 



There still remain the more tuifaceous types of rock and the intercalated 

 conglomerates, of which some mention must be made. The soda-rhyolite tuffs 

 and tuffaceous soda-rhyolites appear to be confined to the western portion of the 



