808 DIAMOND-DEPOSITS OF GOPETON, N.S.W., 



considerable strength had passed along the channel of the stream, 

 but, at the time of observation, only a small stream was running. 

 The notable feature was the arrangement of the sands and 

 gravels in the creek-bed. These were deposited in wedge-shaped 

 blocks, having the thin edge of the wedge pointing up-stream. A 

 diagrammatic section of these deposits along the length of the 

 stream is given in Text-fig. 1 . This would seem to correspond to 



Andrews' step-like structure in 

 the evolution of stream-develop- 

 ment. The slopes were from 20 



to 30 feet in length, and the 

 Text-fig. 1.— Section of bed of Cope's . o ;. o £ 4. • i • i,x 



^ ^ , ,, w ., . steps irom 2 to 3 feet in height. 



Creek, parallel to the stream. ^ " 



The depressions caused by these 



steps were of the shape of isosceles triangles (Plate xci., 

 fig. 2), having the apex of each triangle pointing down-stream, 

 and the base perpendicular to the direction of flow. The length 

 of the base varied from 6 to 10 feet, and the height of the 

 triangles from 8 to 10 feet. The depressions were in sand 

 having a grain-size from 1 to 3mm. The upward-sloping surfaces 

 of the gravels were covered, to a depth of about 2 inches, with 

 coarse pebbles, from 3 to 7 cm. in diameter. If these were buried 

 under a further load of drift and sand, and then covered by a 

 basalt-flow, they would present the same discontinuity, in the 

 beds of wash, as are met with in the deep leads. The study of 

 such features should be of value in the prospecting of deep leads. 



In a deep lead, there are four well marked zones in the 

 materials deposited, any or all of which may be present. These 

 are — 



(1). The coarse gravels known as the wash. 



(2). The medium-grained sands and gravels constituting the 

 main bulk of the deposits. 



(3). A deposit of fine mud or clay. 



(4). A deposit of vegetable-debris, which has, not infrequently, 

 been converted into lignite. 



Of these, the wash has been partly discussed in relation to its 

 occurrence at the Round Mount. It has already been noted 

 that the leads under consideration are entrenched in granite, 

 save where a few tributaries have intersected the slate-formation. 



