Page:A sketch of the physical structure of Australia.djvu/35

23 near Sydney having a slight dip to the west, which brings in (No. 1 ) the upper shales in the low undulating country occupying the centre of the county of Cumberland. From that district the sandstone rises again towards the west, on to the flank of the Blue Mountains, in the gulleys of which the coal-measures below are again exposed, while towards the north, I believe, it forms a widespread surface of rock and valley, its beds being in a horizontal position till we approach the Hunter River, when they rise to the north, and allow the coal, &c. to appear from beneath them.

Through the whole district under consideration, this Sydney or Hawksbury sandstone is the most striking and conspicuous rock. It is usually very scantily covered with soil, and is almost invariably furrowed by innumerable long narrow winding gulleys and ravines with perpendicular sides, over which the more massive and harder beds often project in the form of overhanging ledges. Wherever this rock acquires any elevated position, as on the flank of the Blue Mountains, this worn and deeply trenched character becomes remarkably pronounced. I once had a good view of the gently sloping plateau of sandstone forming the slope of the Blue Mountains due west of Sydney, from the top of a lofty rock (called Gibraltar rock,) that stands on the east side of the Nepean river, at the first rise of the plateau from the central plain of the Cumberland basin, near Mulgoa. On the opposite or western side of the river, the country rose very gradually for many miles, forming a gently sloping plane, over the farthest edge of which (itself of very considerable altitude), were seen some of the loftier peaks of the central mountain chain. This sloping plateau had no remarkable eminence rising from it, but was everywhere traversed by an infinite ramification of precipitous gulleys and ravines. Its surface indeed seemed about equally divided between these deep valleys and their separating walls of rock. To traverse such a country would be impossible except by either adhering to the top of one of the ridges, and patiently tracing out its labyrinthine foldings, always endeavouring to find its central axis, and never to diverge into one of its branches, inasmuch as each branch would