BASALRIC“AND ALLIED ROCKS. 497 
nean passage, and its columnar rocks are shown in the sketch here 
given. ‘he channel is about twenty feet broad and eighteen high. The 
i 
t 
l 
tv 
ENTRANCE TO THE KIAMA BLOW-HOLE. 
advancing sea enters and swells onward with only a gentle murmur- 
ing for a distance of about two hundred feet; there a wall of prismatic 
basalt stands before it, against which it throws itself with a deafening 
roar, and then dashes upward through an opening forty feet deep, 
rising in a column to a height at times of a hundred feet, and falling 
around in a thousand changing arches. 
The basaltic scenery of Illawarra is said to be exceeded by that 
of Van Diemen’s Land, where these rocks occur under very similar 
circumstances. 
Inthological Characters.—It is yet doubtful whether the igneous 
rocks under consideration are wholly basalt, or are in part trap, that 
is, whether they always contain augite with the feldspar, or sometimes 
hornblende. Some ambiguous rocks might be referred to either 
variety. But on tracing out their transitions, I have often found them 
graduating into coarser rocks in which the augite could be readily 
distinguished ; and no instance was met with affording satisfactory 
evidence of the occurrence of hornblende. The fact that these mine- 
rals, augite and hornblende, are so nearly related in composition and 
differ principally in the temperature of crystallization, or rather, in rate 
of cooling, renders it more important that they should be distinguished 
by the geologist. The following varieties of rock were observed. 
A. The basalt is in some places a tough compact black rock with 
125 
