BASALTIC DIKES INTERSECTING COAL. 511 
thus afforded by sandstone that are not given by basalt, which is both 
harder and is of the same nature as the erupted matter. We may 
comprehend the origin of the harder longitudinal lines in some dikes, 
(causing the small parallel ridges or the riband structure,) if we con- 
sider that such ejections may often take place by successive efforts or 
pulsations; each alternation of rest and motion would naturally pro- 
duce a separate and parallel effect on the rising fluid rock. 
The action of the heat of dikes on the adjoining walls is scarcely at 
all apparent when they intersect basalt. When intersecting sand- 
stone, the result is various. At times nothing can be detected; but 
generally there is a slight hardening and discoloration. The red 
colour is sometimes rendered a deeper red, or brownish-red, as near a 
dike in the second cliff south of Kiama; in another place, it is altered 
to gray. The grayish-blue sandstone adjoining the large dike of 
‘ Rocky Cove is hardened and altered to a dirt-brown rock. Adjoin- 
ing a dike at Black Head, the sandstone is much fissured for a foot, 
and exhibits some approach to a columnar structure at right angles 
with the lateral surface of the dike. 
The most surprising change we met with occurs in the island of 
Nobby. This island, excluding its low and wide beach, is about two 
hundred yards long and thirty-four high. Towards the northern end, 
the dike cuts vertically through the layers of rock and the two coal 
| = 
Il wy 
TO 
SS 
TU 
(|| 
an 
beds. Owing to the heat of this dike, the rocks of the whole island, 
from one end to the other, have been literally baked. ‘The same layers 
