BY E. C. ANDREWS. 807 



From R^^dal to Sydney, as also northwards of the latter place 

 to Gosford and southwards to Illawarra, the upper portion of this 

 great sweep of the Lithgow Plain consists of hard layers of sand- 

 stone, 900 feet thick at Sydney but thinning away rapidly thence 

 to the north, w^est and south. The greater portion of the area 

 between Sydney, Parramatta, Penrith, Glenbrook, Camden and 

 Picton is composed of layers of soft sandstone and shales, thus 

 forming an extremely weak spot in a very hard setting. Again, 

 the hard cap of sandstone overlies other sandstones, sandwiched 

 in with layers of shales, coal seams, etc., the whole forming a 

 very weak structure when once the hard protecting cap has been 

 removed. The rock structures west of Rydal are, in places, 

 excessively strong, consisting of indurated Silurian slates and 

 Devonian quartzites. 



Fig. e of PI. xxxix. represents the original appearance of the 

 Lithgow Plain before the canon cycle, with the flat- topped masses 

 of the Blue Mountain and Jenolan Plains rising above it. The 

 rock structure is also shown approximately, explaining the reason 

 why the soft underlying shales were not attacked during this 

 period. 



Fig. /of PI. xxxix. represents the uplift of this plain for 3000 

 feet above sea-level, and the relation of its structures now shows 

 that at some little distance inland the weak shales and sandstones 

 outcrop high above sea-level, while east of Glenbrook they are 

 still below that level. 



To digress slightly, it will be seen from a glance at the diagrams 

 that the hills to the east and west of the central portions were 

 never so large as those of the centre itself : — 



(1.) Since the elevations were not so pronounced on the 

 marginal or coastal portions as in the central areas. 



(2.) Owing to increased river-action on the marginal areas (the 

 whole of the drainage acting there) the result being that broad 

 valleys are there developed by lateral corrasion, while the canons 

 of the central plateau are in their infancy. 



Thus in all these cycles of gradation we should expect the 

 mountains (unless extremely resistant) away from the central 



