GEOLOGY OF ERUPTIVE AND ASSOCIATED ROCKS, POKOLBIN. 381 



as far as Mount Bright, it once more turns E.S.E. and 

 follows an irregular course. The range forms part of the 

 boundary of the old Hunter Valley, which extends for many 

 miles to the east as a generally level plain. This range 

 probably represents the remnant of a ge-anticline of Permo- 

 Carboniferous rocks which had a roughly meridional axis. 

 It is composed of Permo-Carboniferous sediments dipping 

 to the west, capped by gently dipping Triassic sediments — 

 the Narrabeen Series and Hawkesbury Sandstone. 



At Matthews' Gap, where the range turns eastwards, 

 the eruptive rocks are encountered, and of these the range 

 is composed as far as the southern boundary of the district 

 we have to deal with. Professor David considers that, 

 subsequent to the eruption of the lavas of this district, 

 marine conditions obtained, and the eruptive masses existed 

 as islands or at all events as submarine elevations in the 

 Permo-Oarboniferous sea. Elevation and denudation have 

 laid them bare again, above sea level. At present the 

 physiography is a mixture of maturity and youth. It is 

 evident on the one hand that a considerable amount of 

 dissection and denudation took place before the deposition 

 of the Permo-Oarboniferous sediments, as conglomerates 

 can often be traced filling in what were Permo-Oarbonifer- 

 ous valleys and containing pebbles of the Carboniferous 

 rocks. Furthermore some of the hills of to-day must be 

 substantially as they were at the close of Carboniferous 

 times, as we find the conglomerate dipping off their flanks 

 with comparatively little evidence of further denudation. 

 Drake's Hill is a case in point. 



On the other hand, Post-Triassic erosion has in many 

 places cut into the eruptive rocks, and the work of dissec- 

 tion is still proceeding. In the Matthews' Gap area the 

 youthful physiography of the country is particularly evident, 

 V-shaped valleys being formed with eruptive rocks on 



