BY L. A. COTTON. 813 



and directions of flow of the Tertiary streams. These may be 

 seen by a reference to the accompanying geological map, on 

 which they are represented by thick black lines. 



As a preliminary step towards solving the problems connected 

 with the pre-Tertiary stream-development, it will be well to con- 

 sider the present rivers and their origins. The chief of these 

 are the Gwydir River, and its tributary, Cope's Creek. The 

 question arises, has the course of the Gwydir been determined by 

 the re-opening of some Tertiary stream, or has it had its direc- 

 tion controlled by some large structural feature developed sub- 

 sequently to the basalt-period 1 



The accompanying map shows that a pre-basaltic stream flowed 

 from Oakey Creek to Copeton, in a direction approximately 

 parallel to the Gwydir, but in a contrary direction. Moreover, 

 the distance between the two streams is less than two miles, 

 while the difference in their levels is about 100 feet. The great 

 instability of such a system is opposed to the existence of the 

 Gwydir as a pre-basaltic stream. Again, it would be strange 

 indeed if the basalts, which covered the Oakey Creek lead with 

 a thickness of several hundreds of feet of basalt, had not filled 

 the valley of the lower adjoining river. The fa^t that no trace 

 of basalt has been found on the low-lying area to the south-west 

 of the map, is also opposed to this view. It may be concluded, 

 therefore, that the present Gwydir has been developed in a direc- 

 tion quite independent of the pre-basaltic drainage-system. 



If the present course of the Gwydir be regarded as post- 

 basaltic, its position must be due either to haphazard denudation, 

 or to some definite structural feature. The presence of the 

 relatively low-lying block, bordered on its north and east sides 

 by the Gwydir, is the factor which serves to discriminate between 

 these two explanations. It is highly improbable that the pro- 

 cesses of denudation can have exerted such a selective influence 

 on the low area lying to the south and west of the Gwydir, while 

 steep escarpments rise abruptly from the other side of the river. 

 The assumption of a faulted block, on the other hand, accounts 

 both for the low level of the area discussed above, and also for 

 the marked change of direction of the Gwydir at its junction with 



