ORIGIN OF LATERITE IN NEW ENGLAND DISTRICT, N.S.W. 233 



4.— ORIGIN OF THE LATERITE IN THE NEW 

 ENGLAND DISTRICT OF NEW SOUTH WALES. 



By T. W. Edgeworth David, B.A., F.G.S., Geological Survey 

 of New South Wales. 



introduction. 



The term Laterite in the present paper is applied by me to a rock 

 of very variable character associated with basalt. The colour is 

 of a prevailing dull brick red or ochreous hue, and the rock shews 

 eveiy gradation of texture from that of a soft earthy rock, to 

 that of a hard pisolitic ironstone. The characteristics of laterite 

 have been studied by me chiefly at Vegetable Creek, and also to a 

 limited extent in the Tingha District. I have come to the con- 

 clusion that the laterites are altered basalt tuffs and scoriae. 



The bulk of the rocks at Vegetable Creek, which are partly 

 sedimentary and partly igneous, are of Palaeozoic age. Mesozoic 

 rocks are not i-epresented, unless, perhaps, some of the intrusive 

 dykes belong to that group. A great unconformity consequently 

 exists between the Paljeozoic and the Tertiaiy formations. The 

 latter ai'e represented by sheets of volcanic rocks, chiefly basaltic, 

 from ten to sixteen miles in length, and from half a mile to two 

 miles wide, with small sheets of felsite. Associated with these 

 volcanic rocks are gravel, sand and clay, chiefly of fluviatile origin. 

 These volcanic rocks occupy the sites of old valleys belonging to 

 various epochs of the Tertiary Era. The Tertiary volcanic rocks 

 are characterized by their general horizontality and their association 

 with deposits of stream tin. 



A vast period of time must have elapsed between the flow of 

 the eai'liest and the latest basaltic lavas, as in some cases there is 

 evidence of a lava stream, after it had flooded an old valley and 

 usurped its river channel, liaving been excavated to a considerable 

 depth by I'enewed fluviatile forces, and partially covered by 

 sedimentary deposits ; and there is evidence then of the newer 

 river gravels being buried under a second massive sheet of lava. 



Some idea may be formed of the vastness of the time which 

 has elapsed since the eruption of the first basalts, from the fact 

 that they frequently form the main lines of water-parting at points 

 where they immediately overlie the old river channels, and the 

 gravels in these channels are in some cases over 550 feet above the 

 level of the nearest modern rivers capable of forming such gravels. 



The tertiary volcanic rocks at Vegetable Creek may be divided 

 petrologically into : — 



1. Felsites. 



2. Basalts. 



3. Laterite. 



4. Tachylite tuffs 



