52 T, W. E. DAVID. 
Both Darwin and the Rev. W. B. Clarke comment upon the highly 
crystalline character of the sand grains in the Hawkesbury Sand- 
stone; the individual grains are distinctly faceted, owing to the 
development of secondary quartz crystals, with brightly reflecting 
faces, around the original sand grains. Several bores for water in 
this sandstone prove it to be, generally, impervious. 
The Hawkesbury Sandstone is traversed by numerous joints. 
These are frequently infilled with hydrated peroxide of iron derived 
from the decomposition of the pyrites crystals, or from the 
alteration of protoxide of iron. The existence of the joints much 
facilitates the work of erosion of these sandstones, and accounts 
for the frequent smooth and vertical faces of rock in the cliff 
sections. Though whitish-grey at a depth, the sandstone weathers 
various shades of yellow and rusty to reddish-brown near the 
surface, the weathering frequently assuming the form of concentri¢ 
shells stained. different colours by iron oxides and producing very 
characteristic features in the superficial structure of the Blue 
Mountain plateau." 
The Hawkesbury Sandstone has yielded the following fossil 
plants, Thinnfeldia, Gleichenites, Phyllotheca, Ottelia(?), Equisetum 
etc. The fossil fauna includes a Palceoniscus, which was found at 
Biloela at a depth of sixteen feet below the sea level ; close to the 
same spot were subsequently discovered the thoracic plate of 4 
Mastodonsaurus, M. platyceps, W. Stephens,? and a Gasteropod, 
Tremanotus Maideni. ‘The latter forms the only example of @ 
marine fosil hitherto discovered in the Hawkesbury Sandstone, 
and is a remarkable instance of a survival in the Southern Hemi- 
sphere, in Triassic time, of a form which had become extinct in 
the Northern Hemisphere, apparently at the close of the Silurian 
1 Mr. W. A. Dixon informs me that he thinks it not improbable that 
the iron in the unweathered portiuns of the Hawkesbury Sandstone is 
present in the form of protoxide in combination with organic matter 
other than the graphite scales already referred to. 
2 Proce. Linn. Soc. N. S. Wales, Vol. 1., (Series “eg 1886, pp. 931, and 
1175 - 1192, pl. 22. 
