SANDSTONE BELOW THE COAL. 487 
similar to what has been explained. ‘The preceding figure was taken 
at Wollongong, and represents on a reduced scale, the usual appear- 
ance of the layers where they have been worn by the sea. In general 
the enclosed area is very low convex. ‘The centre has a compact 
structure, and is enveloped by layers which diminish in thickness to- 
wards the sides. In some examples the whole is composed of layers, 
while others have the compact central mass more 
than half the whole diameter. The latter, when 
worn by the sea, have a flattened convexity as the 
centre of the concretion; and this convex surface is 
in some places singularly eroded, as in the annexed 
figure. 
The connexion of the concretionary structure with the fissurings 
or cracks of the rock, (which is apparent from the fact that each area 
is a separate concretion,) is well shown along certain large fissures 
which follow a single course, independent of any areas. The follow- 
ing sketch represents a part of the seashore layers, about thirty feet in 
length, just north of Wollongong Harbour. Along the centre runs a 
narrow fissure, which enlarges irregularly in two places, and then con- 
tracts again to a narrow crack, that nearly disappears and branches 
in different directions. On each side of this fissure there is the same 
schistose structure as in the regular concretions: six to twelve layers, 
about half an inch thick, run parallel with the fissure, and are brought 
out in relief by the wearing away of the intermediate portions, which 
appear, as elsewhere, to be softer than the centre of the layer. Around 
the enlargements the schistose structure is interrupted; and instead of 
it, the rock is collected into rough shapeless masses, cracked in every 
direction, which, on account of their hardness, have resisted abrasion, 
and stand out as a prominent boss on the surface of the rock. On 
each side of the fissure, for many yards, there are irregular cracks, 
some of which are continuous with those of the boss, and appear to be 
of the same origin. The fissure is nearly lost to the right, where it 
