510 NEW SOUTH WALES. 
it, though it is scarcely distinguishable before incipient decomposition. 
As the process of change advances, the structure becomes very appa- 
rent, and the columns are resolved into a pile of spheres made up of 
concentric coats which are peeling off. This subject will be farther 
remarked upon when treating of the decomposition of these rocks. 
In the dekes of basalt, a transverse columnar structure is very com- 
mon; but it is mostly restricted to the central portion. Some of the 
dikes are naturally divided into three parts, of which the outer or two 
lateral are similarly non-columnar. This is the fact with the dike 
represented in figure 1, page 506. The 
annexed figure of another dike shows 
the structure referred to; in this case 
the central portion is nearly twice the 
width of either lateral portion. The lat- 
ter instead of having transverse fissures, 
are at times divided longitudinally; in some cases by fissures filled 
with calc spar or quartz, but more commonly the longitudinal struc- 
ture is produced by harder seams of basalt, which, when the surface 
is worn, stand out in small ridges. This is exhibited in the above 
figure. In some instances the longitudinal structure prevails through 
the whole width of a dike, either in the form of fissures or ridgelets, 
or both. ‘This is the fact with a dike a foot wide intersecting imper- 
fectly columnar basalt on the second point north of Kiama. The cal- 
careous seams are from an eighth to half an inch wide, as in a dike at 
Rocky Cove. 
The colour of a dike is seldom uniform throughout. Though dark 
or nearly black at centre, they are on either side generally grayish- 
white, greenish or yellowish, from mixture with the powder of the 
rock it intersects. The dike sketched in the last figure has a light 
grayish-green colour for a depth of three inches; the aspect of this 
portion is dull clayey, the texture rather soft and quite decomposable. 
A thin and soft ochre-yellow seam lies exterior to all. The dike on 
Nobby is argillaceous for eight inches on each side. ‘The dike, three 
feet wide, just south of Newcastle Point, is similarly impure for two 
and a half inches either side. ‘The breadth of the impure basalt 
varies with the size of the dike. More or less of it characterizes all the 
dikes that intersect the sandstone formation, while it is very seldom 
met with in those that cut through the basalt. It proceeds beyond 
doubt, as suggested above, from the inclosing rock, which is more or 
less scraped and worn by the ascending fluid basalt. Impurities are 
