BASATAME Ch AND VAL RD ROCKS, 509 
The columns are from one to four feet in diameter, and from four to 
six sided. Many of them are thirty feet long. The terminal plane 
is flat, as with the columns to the north of Kiama, and it follows the 
same principle in approximating to horizontality whatever the obli- 
quity of the prism. ‘The variation from horizontality is greatest with 
the most oblique columns, being, in some cases, twenty degrees: 
there appears to be a fixed ratio between the amount of variation and 
the obliquity. 
Effects of heat are apparent at the junction of the basalt and con- 
glomerate, though more decided in the basalt. The facts show that 
the fused basalt flowed over a bank of co/d conglomerate. The bluish 
basalt passes gradually to brownish-black, brownish-red and brownish- 
yellow colours, with the last of which it joins the conglomerate; and 
the bank is mottled with purple, green, reddish, and yellowish-gray 
tints, and contains in its clayey base, masses and fragments of basalt. 
Along the line of junction, the conglomerate is much entangled in the 
basalt; yet is scarcely harder here than in other portions, probably 
because the bank was so small as to be heated alike throughout. 
We cannot therefore doubt that the basalt, in fusion, flowed over a 
small ridge of conglomerate, in a stream at least fifty feet thick, 
and derived the position and forms of its columns from the varying 
inclination of the surface. We perceive a tendency to verticality 
between dwvo cooling surfaces, the bank below, and the water above 
(for the eruption was probably submarine); and from the influence 
of the two the curved forms resulted. 
Numerous instances in this region illustrate the fact that columns 
are formed only within certain rates of cooling. If too rapid, there are 
only irregular cracks, with imperfect traces, if any, of a columnar 
structure. ‘This was the condition of the upper part of the basalt at 
the blowhole (figure, p. 497); out of the sixty feet, the upper twenty 
are not at all columnar, though very much fissured. The absence 
of structure in the basalt immediately adjoining the bank of conglo- 
merate above described, may be owing to the same cause. 
A columnar structure is distinct at Prospect and Pennant Hills, 
near Paramatta, and also in the hills adjoining Puenbuen. In the 
latter region perfect columns were observed at one place, but without 
any peculiarities requiring remark. 
A concentric structure, subordinate to the columnar, is observed in 
some of the cliffs, though not common. ‘Towards the north end 
of the second cliff, north of Kiama, there is a fine place for examining 
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