512 NEW SOUTH WALES. 
on Newcastle Point consist of argillaceous sandstones and soft clay, or 
clayey shales, as shown in the section on page 474. But on Nobby, 
the clay layers are a fine compact chert, looking like flint, and of 
flinty hardness and conchoidal fracture. The argillaceous sandstone, 
excepting a coarse variety, has experienced the same metamorphosis, 
and has become a chert, scarcely distinguishable from that made from 
clay. The coarser sandstone, though baked, shows still its granular 
structure, and the white clayey particles intermingled. Nodules 
originally of clay in this coarse rock are now as fine a chert as any 
on the island, breaking with a smooth conchoidal fracture, and afford- 
ing sharp-edged fragments. 
These alterations, although extending through the island, are less 
distinct towards its southwestern end, which is one hundred and fifty 
yards from the dike. The finer argillaceous layers are a perfect chert 
even here; but the sandstones are only a little hardened; they 
crumble readily on exposure, and undergo more rapid disaggregation 
than the same rock on the main land, where not operated on by the 
high heat. 
The chert has mostly a uniform grayish-blue colour, like the clay 
before the baking. Some varieties, however, have a brown, light 
orayish-green, or light blue tint; and others are striped with these 
colours so as to resemble riband jasper. The layers still contain vege- 
table remains, and in general they retain their black colour; but very 
near the dike many of the specimens have lost this colour through 
the combustion of the carbon, and only impressions of leaves remain. 
The alterations in the coal are equally interesting, and they extend 
to a distance of six or eight feet each side of the dike. Within this 
distance the bitumen has been expelled, and the coal where not clayey 
approaches charcoal. ‘The impure layers, or those more or less argil- 
laceous, have been baked to a black coaly rock almost as compact as 
the chert. The surf, which has torn away the rocks at the foot of the 
cliff, and probably at some former period removed many a yard from 
its face, has spent its force less effectually against this indurated coal 
layer; a black wall of it, a few feet high, extends across the beach for 
two hundred yards, although the basalt alongside has been washed 
away to the level of the beach. The preceding figure shows its position : 
it averages four feet in height, and is from five to six feet wide. At 
a distance it looked much like an immense knotted log that had been 
charred. In texture it is extremely tough, and it contains imbedded 
masses of compact charcoal. Adjoining the dike, fragments of the 
