98 On tlie Occurrence of the Diamond near Mudgee. 



structure like tliat of the older portions of its upper basin, except 

 that limestone bands are wanting. No members of the Carboni- 

 ferous series occur in this portion. The whole course of the 

 river lies through a rugged and mountainous country. 



In the neighbourhood of the junction of Wialdra Creek outliers 

 of Carboniferous rocks are frequent. They consist of sandstones, 

 conglomerates, with shales containing G-lossopteris and other 

 plants. These outliers form links at trifling intervals, connecting 

 the Carboniferous formation of the Dividing Range with the coal 

 of the Talbragar. A few miles to the north of the junction the 

 Carboniferous beds form horizontal cappings on hills of slate or 

 granite, whilst at Gruntawang they are met with in the river 

 valley, and near the junction they occur at a similar low level, 

 and have been covered up by the Older Pliocene basalt without 

 the intervention of drift. The great difi'erences in level which 

 the Carboniferous beds occupy deserve consideration. For our 

 present purpose, however, it is enough to show that vast masses 

 of Carboniferous strata have sufi'ered denudation, and along the 

 main stream we find relics of these rocks not only in the present 

 bed but also in the older drifts. 



The rocks immediately surrounding the diamond localities are 

 nearly vertical beds, with a general strike N.N.W., consisting of 

 red and yellow, coarse and fine-grained, indurated sandstones ; 

 thin white laminated argillaceous shales ; pink and brown fine- 

 grained sandstone, beautifully banded, by purple stripes, in 

 concretionary layers ; slates and hard metamorphic schists ; flinty 

 shales ; hard brecciated conglomerate, containing nodules of 

 limestone, flint, and red felspar, in a greenish siliceous base. The 

 last is not unlike the trappean ash-beds of Ireland, described by 

 Jukes. "With these there also occur dykes and outbursts of 

 intrusive greenstone, which follow the strike of the beds 

 irregularly, and indurate the rocks with which they are in contact. 

 The rocks in general are devoid of mica. 



Having thus sketched the geology of the Cudgegong drainage 

 area, and seeing that the diamonds occur in an ancient river-drift, 

 we are led to enquire whether the diamond is of drift origin, like 

 the materials with which it occurs. If so, which of the formations 

 aflford the diamond? The Carboniferous, Devonian, or Upper 

 Silurian ? Or have the diamonds grown in the drifts in which 

 they are now found? Before dealing with such theoretical 

 questions we will enumerate the materials which compose the 

 drift, and give a more minute description of its structure. 



The Older Pliocene diamond-bearing drift is a coarse and 

 heavy deposit, for the most part loose, but portions of it are 

 united into compact conglomerate by a white siliceous cement 

 sometimes coloured light-green by silicate of iron ; in other cases 



