BY E. C. ANDREWS. 797 



It will be shown hereafter, however, that the great Tertiary 

 uplift consisted of two or three distinct upward series of move- 

 ments. The first one preceded the great basalt period, while 

 another succeeded it, as may be seen by a study of the old river 

 beds. 



The formation of the Lithgow Plain occupied, however, a much 

 longer period of time. The latest cycle, which we shall call the 

 " canon cycle," sufficed for the formation of small valleys only 

 (save in the areas of shales), whereas the former cycle was pro- 

 ductive of wide-spread plains continuous in many directions. 

 The time occupied in the formation of the plateau was doubtless 

 many times that comprised in the canon cycle. Similarly for the 

 formation of the Blue Mountain plain. The earliest formed 

 plain, known as the Jenolan level, had been developed to old 

 age, and indicates a cycle of gradation of such duration in time 

 as to dwarf the times occupied in the succeeding cycles. From 

 these considerations the writer would feel inclined to assign a 

 Lower Cretaceous or Jurassic age for the Jenolan Plain, with an 

 Upper Cretaceous or early Tertiary age for the Lithgow Plain. 



Naturally in the study of levels like these, loss to mesas or 

 "inheritances from previous cycles" of erosion during succeeding 

 plateau cycles must not be overlooked. The remnants of one 

 plateau are also attacked during the succeeding cycle of plateau 

 reduction. 



Let us imagine that, after a pronounced cycle of erosion in 

 which the resultant mesas represent the most obdurate of the 

 rock structures to erosion, a considerable movement of elevation 

 ensues. Let us also suppose that the rock layers are hori- 

 zontally disposed or gently inclined and that a soft layer 

 which formerly lay below base level, and therefore beyond the 

 reach of erosive activities, is now revealed by the elevation. 

 During the new cycle the corrading streams will discover the 

 weak layer, and rapid recession of both the weak and overlying 

 strong structures will occur. Should a thick, hard bench occur 

 beneath the soft layer, a terrace will be formed by the removal 

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