BY H. I. JENSEN. 575 



(2) The streams diverge from a common centre. 



(3) The valleys widen by the retreat of vertical cliffs. 



(4) Alluvial fans are common. Detritus is deposited all round 

 the mountains where the grade diminishes. 



(5) The watercourses frequently change their position; or the 

 waters flow in a sheet when the plains are reached, following no 

 definite course. 



The rainfall in the Warrumbungles comes mostly in heavy 

 showers separated by long dry intervals. This kind of rainfall 

 favours arid erosion and conoplain-formation. 



(d) Vulcanism. — The sequence of the lavas has already been 

 described. Eruptions commenced probably in the Eocene period, 

 and continued for a considerable time. The alkaline trachy- 

 dolerites and basanites may have been as late as Miocene, and 

 the calcic basalts which followed in places may be as late as 

 Pliocene. Owing to the absence of fossils we have only the land 

 forms to enable us to arrive at an approximation in this regard. 

 The eruptions had finished when the very wet cycle commenced. 



Volcanic action was throughout accompanied by elevation. 

 The plugs and cones are not distributed along definite intersect- 

 ing cracks as in the Glass House Mountains. If such cracks 

 ever existed, their traces have been hidden by the enormous 

 amount of lava poured out. Although the igneous mass occupies 

 a somewhat circular area, there is reason to believe that the lava 

 was erupted mainly from a fissure running N.N.E.-S.S.W., 

 through Mount Wheoh, Siding Spring Mountain or Mombara, 

 Berum Buckle, and The Spire (Tonduron). Berum Buckle at 

 Tannabar is apparently the centre of the whole system. Radial 

 cracks were probably developed, originating at this point. One 

 fracture might be imagined running west through Caraghnan, 

 Needle Mountain, The Bluff and Wombalong (Exmouth); another 

 east through Goat Mountain and Bingy Grumble. 



In the country around The Spire the sandstone nearly always 

 reaches a higher level on the western side of a mass than on the 

 eastern. This would seem to indicate that the lava came up 

 diagonally from the west. 



