614 NEW SOUTH WALES. 
are widely unlike in tendency to decomposition. It is impossible to 
procure a specimen of unaltered rock from some dikes, without exca- 
vating to a depth of several feet. There is a large dike on the first 
point south of Kiama, which appears externally like a bed of brownish- 
yellow earth, filling a fissure in the rocks; and upon close examina- 
tion only faint traces of the original structure could be detected. The 
dike of Nobby is another illustration of this rapid decomposition. 
Along the beach, however, where washed by the sea, it is still un- 
changed. It is a general and important fact that a rock which alters 
rapidly when exposed to the united action of air and water, is wholly 
unchanged when immersed in water or exposed to constant wetting 
by the surf. 
The process of decomposition is finely exhibited on the second cliff 
north of Kiama, towards the north end. At first sight, a distinct argil- 
laceous deposit was supposed to overlie the columnar basalt; for it 
was twenty feet thick, and of a whitish colour, resembling a soft 
crumbling marl, thus wholly unlike the basalt, and the common re- 
sults of basaltic decomposition. Still it had proceeded from the alter- 
ation of a regular columnar variety having a dull grayish-blue colour. 
The original rock is exceedingly compact, showing no trace of crys- 
tallization, excepting an occasional minute crystal of feldspar; and 
within the reach of the swell, it was still compact and solid. 
The rock has a concentric structure, and to this it owes in part its 
rapid decomposition. The alteration commences between the con- 
centric layers, rendering them apparent, although not so before. At 
first a thin ochreous line appears, arising from iron; either magnetic 
iron disseminated in the rock, or from that of the constituent mineral 
augite. ‘This ochreous colour afterwards mostly disappears, and the 
concentric coats become separated by thin clayey layers of a white 
colour, more or less striped with ochreous lines. In a more advanced 
stage of the process large ovoidal masses of basalt, (but little changed 
in appearance excepting the development of a slaty concentric struc- 
ture,) le in the cliff separated by a considerable thickness of the 
whitish clayey layers, which are stained by irregular ochreous lines. 
At last the centres of the spheroidal masses yield, and finally the 
change is so complete that the concentric arrangement is entirely lost, 
and a soft whitish or yellowish-white argillaceous deposit, with few 
ochreous spots or lines, takes the place of the compact basalt. 
In basalts of more compact structure these changes take place more 
slowly. The grayish-blue basalt in the Illawarra range, near 
Broughton’s Head, when long exposed, is discoloured exteriorly to a 
