806 DIAMOND-DEPOSITS OF COPETON, N.S.W., 



kaolin. The latter product, on account of the ease with which 

 it is carried in suspension by running water, can be deposited 

 only where the water is comparatively still, as in lagoons. Con- 

 versely, the occurrence of any kaolin-deposits implies compara- 

 tively still-water conditions of deposition. 



A few small tributaries of the main lead have intersected the 

 sedimentary rocks (chiefly clay-slates), and have thus locally 

 introduced a third factor. 



As the processes of river-development do not vary with 

 geological time, the Tertiary leads exhibit features in common 

 with present-day streams. Large boulders, for example, occupy 

 a great portion of the old river-channels. These are often so 

 large as to be mistaken for portions of the walls or bottom of the 

 river-channel, and are a source of continual trouble and per- 

 plexity to miners. In general, of course, there is an arrange- 

 ment of the coarsest material at the base of the drift, and a 

 diminishing coarseness of texture with distance from the channel- 

 bed. This type-arrangement is subject to modification in which 

 a ilumber of coarse bands mark the temporary level of the stream. 



The drifts are almost invariably of a reddish colour, which is 

 due to the staining effect of the iron-oxides leached from the 

 overlying basalts. The thickness of the drifts varies very greatly, 

 even at short distances. This may be attributed to two causes, 

 the first being the irregularity of the bed of the stream, and the 

 second, the differences in grade; a swiftly flowing current, on a 

 steep grade, tends to sweep the channel-bed free from all loose 

 material. 



The greatest thickness of drift observed at Copeton was at the 

 Round Mount. Here a depth of 25 feet was encountered. The 

 " wash," as it is called, differs from the main bulk of the gravel 

 in being coarser in texture and more compact. At the Round 

 Mount, the bulk of the gravels consists of small quartz-grains 

 from 2 to 5 mm. in diameter. The wash, on the other hand, 

 consists chiefly of quartz-pebbles from 5 mm. to 3 cm. in length. 

 The pebbles are of the normal stream-type, being ellipsoidal in 

 form. The slightly predominant tendency of these pebbles to 

 lie with their long axes sloping upwards towards the south-east, 



