BY H. I. JENSEN. 573 



The late Cretaceous sea being now again dry land, the streams 

 from the mountains deposited much of their silt on the plains, 

 where the velocity decreased on reaching the more level country, 

 thus giving rise to the Black Soil Plains. Valleys were carved 

 in the volcanic conoplain, dissecting the lava-sheets and under- 

 lying sandstones. Thus the Castlereagh at Timor has cut through 

 the phonolitic trachytes into the sandstones below. Likewise 

 Uargon Creek flows between the vertical clifis of JSTaman Ledges 

 (a flow from The Spire crater, probabl}'), and Black Mountain at 

 a level of about 1,850 feet. At the 2,000-feet level on either 

 side the sandstone is capped by lavas which the creek erosion has 

 severed (Fig.5). In such cases the valley widens by the retreat 

 of almost vertical clifls, formed by the sandstone weathering 

 away from under the lava-capping. 



(c) Peneplanation. — As has been shown above, a peneplain, 

 now marked by the 2,000-feet level, was formed at the end of 

 Cretaceous time. Subsequent erosion has not produced another 

 peneplain, yet the Coonabarabran tableland is approaching that 

 end. However, the late Tertiary erosion has tended to reduce 

 or base-level the land, not to sea-level, but to the level of the 

 western plains. Following upon a wet period — probably Pleis- 

 tocene or Pliocene, and contemporaneous with the lake period of 

 parts of Central Australia, when an inland sea covered great 

 areas — there succeeded a dry period, which still persists. This 

 matter I have already touched upon in my preliminary note. 



As evidence proving the existence of an Arid Cyde^ in the 

 area of this Warrumbungle conoplain, the following facts are 

 suflicient : — 



(1) The streams have definite courses in the mountains where 

 they are fed by springs, but dry up and become indefinite on 

 reaching the more level country, especially to the north and west 

 of the Warrumbungles. 



(2) The country is being base-levelled to the level of certain 

 depressions in the western plains, which have become filled with 

 •detritus (black soil) from the mountains. 



Cp. Journal of Geology, Vol. xiii. No. 5, July, August, 1905. 



