490 NEW SOUTH WALES. 
At Kiama, a fissure runs for one hundred yards north-by-east ; in 
the second cliff to the northward, the general direction of the fissures 
is east-by-north and northeast ; in the second cliff to the southward, 
north-by-east and northwest. 
At Black Head, near the extremity, there are fissures running north- 
by-east and northivest. They are more numerous than usual, and 
divide the layers into large blocks. 
Decomposition.—The sandstone of this formation undergoes rapid 
alteration when exposed to the influence of air and water. Both at 
Harper’s Hill and Wollongong, the upper portion of the cliff for 
eight feet is changed from its usual grayish-blue to a rusty yellowish 
colour, and is very fragile. ‘This layer might be mistaken for a sub- 
sequent deposit of different composition from that below, especially 
as its limits are distinctly marked; but the fossils and obvious transi- 
tions prove its identity with them. 
Along the road, half a mile west of Wollongong, where the rock is 
exposed in a quarry, the outer surface facing the road is altered in the 
manner just described, and is quite soft and crumbling; but, when 
quarried out, the layers present the usual characters. 
The upper layer at the Wollongong Cliff appears to be altered 
throughout; for the quarryings in it, which have already been exten- 
sive, do not open to any unaltered rock. The fossils, moreover, are 
mere casts; the shell which, before the alteration, appears from those 
below to have been siliceous, has been dissolved out, and removed 
during the progress of decomposition. 
Decomposition in some places makes rapid progress. At Harper’s 
Till, the rock during the two years past has been altered to a depth 
of a foot, and become a black granular material, too tender to bear 
handling. It resembles the soil from greenstone rocks, and is actually 
of the same nature, as the sandstone is made up largely of basaltic 
material. 
In much of the rock at Wollongong, exposure to drying developes 
a concentric structure, not before apparent, and the masses fall to 
pieces by peeling off in layers. Large blocks, quarried out for use, 
thus undergo a natural destruction. If kept under water, however, 
the rock is durable; and it has been successfully used for building a 
basin at that place. We might suppose that the salt of the sea-water, 
or of the spray from the surf, has much to do with this peeling pro- 
cess, and probably it does promote it. Yet the same effect takes place 
in the interior of the country, where the sandstone is not exposed to 
the sea. 
