7 2 Sedimentary Formations 
But the cementing matter is not always ferruginous ; afelspathic 
cement holds them together with used mica evidently derivative, 
and sometimes with graphite. 
Another variation in character of the liawkesbury rocks is in 
their cohesion. In 18-30 I was Chairman of the Artesian Well 
Board, and remember tlie difficulty we had in procuring tools 
hard enough to pierce the quartzosc sandstone at the gaol in 
Sydney ; the boring after a small depth was abandoned — one of 
the workmen precipitating the conclusion by blocking the bore¬ 
hole. But in parts of the Bailway Lines, there have been instances, 
as stated to me by the Bngilicer-in-Chief, when the largest blocks 
have been shivered to atoms by a not very heavy fall over an 
embankment. 
I his group of liawkesbury rocks has been by some persons 
denominated “Sydney Sandstone.” The designation was derived 
from the early settlers, who had not gone far into the country ; 
but it is a misnomer, for it neither represents the whole of the 
series nor the whole of the material of the rocks, besides making 
confusion with the Si Sydney Sandstone ” of the Cape Breton 
Coal-field of British America. The latter has a clearer rmht 
perhaps, to the title of “ Carboniferous,” as it is of the age of 
the very lowest of our Australian Coal Measures. Yet with its 
Lcpidodcndra, &e., it has fishes of the one genus which occurs in 
our liawkesbury and Wianamatta beds, over our Upper Coal- 
JVictncun(i1tu Beds.- The Hawkosburv rocks are succeeded by 
another group or series of strata named by me from the Wiana- 
matta, or South Creek, which runs longitudinally through the 
basin which tills in the area between a surrounding enclosure of 
the former series which must have been broken up in part and 
denuded, either completely before or during the deposit of the 
sandstones over-lying the Coal Measures. The deep ravines which 
mark the. liawkesbury rocks give place to rounded smooth undu¬ 
lating softer argillaceous strata, in the bottom of the creeks of 
which and in the beds of the river Nepean or Hawkesbury and of 
George’s Biver are marks of old erosion in the harder rocks 
below the argillaceous shales. Pot-holes are very common in the 
liawkesbury beds under the Wianamatta strata where exposed 
at the points of junction at some distance from the present 
creeks and drainage channels. Such may be traced at Myrtle 
Creek, near Pieton, and on the Windsor fioad near Parramatta, 
these certainly prove a partial or general erosion before the 
whole series of the Wianamatta strata were laid down. The 
nearest beds of the latter to the underlying liawkesbury rocks, 
are shales which have occasionally filled in hollows previously 
existing, or contributed patches forming considerable masses as 
well as thin layers to the uppermost liawkesbury rocks. In this 
