232 GEOLOGY OF THE WARRUMBUNGLE MOUNTAINS, 



on the north in the Pilliga country, and may be due to their 

 crossing the intake beds of the artesian system. 



The country has been base-levelled to the level of certain 

 depressions in the western plains, which probably were lakes in 

 the wet period, and now form black soil plains. The wet period 

 probably was coincident with the generally accepted Pleistocene 

 pluvial period of Australian geology and glaciation on Mt. 

 Kosciusko. At that time the detritus was deposited on the 

 plains, alluvial fans (now black soil flats) were formed in the 

 mountains, and gorges were carved. In the subsequent arid 

 period the integrated drainage system, brought into existence at 

 the end of the pluvial period, has been destroyed; the mountain 

 valleys have been enabled to retain their youthful appearance; 

 scorched plains devoid of soil have developed in the mountains 

 through the aridity of the climate, and the "bad land" topography 

 of the Coonabarabran tableland has been shaped. 



In a recent paper* Ida H. Ogilvie describes a conical lacco- 

 lithic mass under the new term of conoplain. The district 

 studied by her has been dissected at its present altitude, with 

 the result that the mountain peaks diminish in height from the 

 centre outwards, alluvial fans flank the mountains, and valleys 

 widen by the retreat of nearly vertical cliffs. 



A study of the paper referred to, in conjunction with my 

 observations in the Warrumbungles, shows a striking resemblance 

 between the latter district and that of the Ortiz Mountains, 

 which are taken as a type of the conoplain. There is the same 

 group of high peaks surrounded by smaller and smaller heights, 

 showing that all are but the remnants of a conical pile of igneous 

 rock. We have a surrounding plain sloping gently away from 

 the mountains, and covered with vast deposits of rock waste from 

 them; the want of permanency of watercourses in the plain 

 ■country; the deep gorges in the mountains, with steep cliffs of a 

 youthful appearance, pointing to occasional heavy rainfalls with 



* " The High Altitude Conoplain " (The American Geologist, vol. xxxvi., 

 No. 1, July, 1905). 



