56 T. W. E. DAVID. 
few inches up to over a foot in diameter, and is composed of very 
much the same kind of rocks as those in the recent gravels of the 
Nepean. The sandy matrix in which they lie is compacted in 
places into a fairly coherent rock ; and I should think it probable 
that this river channel dates back at least to the Miocene or 
Eocene period. The determination of its exact geological age 
would be of great importance ; there can be little doubt that the 
river which formed it was an ancestor of the Nepean, and probably 
therefore the chief drainage channel of the Blue Mountains in 
that age. 
e. Pleistocene (?). A formation, presumably of this age, is 
developed chiefly in the valley of the Nepean, between Mulgoa 
and Richmond ; and consists of a terrace of red sandy soil, over- 
lying gravels, the surface of which is about twenty feet above the 
level of the highest modern floods. (See diagram 2, Plate 2.) No 
determinable fossils have, so far as I am aware, been found in it. 
f. Recent Alluvial. Formations of this age are developed 
chiefly in the Nepean and Hawkesbury valleys, in the estuaries 
of the Hawkesbury, the Parramatta River, George’s River, etc. 
An observer contemplating the vast sheets of alluvial gravels, 
- forming the plains of Mulgoa, Penrith, Windsor and Richmond, 
cannot fail to appreciate the vast erosive force, that must have 
been exercised by the Nepean River and its tributaries, to trans- 
port such a bulk of rock material through their narrow gorges, 
some of which has been carried for a distance of perhaps over fifty 
miles. From Mulgoa to Richmond, the alluvial gravels vary from 
one to two miles in width, and extend in an unbroken sheet about 
twenty miles long. Their thickness is at least forty-seven feet. 
The iron piers for the Penrith Railway Bridge over the Nepean 
are sunk a few feet below the bed of the river in gravel, the base 
of this gravel is forty feet above sea level. If the bulk of the 
Pleistocene gravel be added to that of the recent alluvial gravel 
it will amount approximately to about thirty square miles ; and 
those who depreciate the erosive power of the rivers of the Blue 
Mountains, should not forget that almost the whole of this grave 
