CHAP. XLI 
VENTS OF THE BASALT-PLATEAUX 
295 
columnar and jointed or prismatic sheets are more common, hut the most 
prominent band is the great sill, to which further reference will be made in 
the next Chapter. 
I 11 the course of the gradual retreat of the cliff, as the waves tunnel its 
base, and slice after slice is detached from its vertical front, a group of at 
least five small vents has been uncovered lying along a nearly north and 
south line. Of two of these a segment remains still on the cliff-wall and 
passes under the basalts ; the others have been dissected and half cut away 
from the cliff, while groups of stacks and rocky islets of agglomerate may 
mark the position of others almost effaced. The horizontal distance within 
which the vents are crowded is probably les6» than half a mile, but the lofty 
proportions of the precipice tend to lead the eye to underestimate both 
heights and distances. 
The agglomerate is a thoroughly volcanic rock, consisting of large and small 
/ 
a 
a Jj a 
Fig. 311. — Section of the same Neck as that shown in Fig. 310. 
blocks of various basalts, among which large slags are specially conspicuous, 
the whole being wrapped in a granular matrix of comminuted volcanic 
detritus. The arrangement of this material is best seen in the fourth vent 
(Figs. 310 and 311). In this characteristic volcanic neck ( l in Fig. 311) 
the boundary walls, as laid bare on the lace of the precipice, are vertical, and 
are formed of the truncated ends of the banded lavas (a a) which have been 
blown out at the time of the formation of the orifice. The visible diameter 
of the vent was roughly estimated by me to be about 100 yards. No 
appreciable alteration was observed in the ends of the lavas next the 
vent. 
The agglomerate is coarsest in the centre, where huge blocks of slaggy 
lava lie imbedded in the amorphous mass of compacted debris. On either 
side of this structureless central portion the agglomerate is distinctly strati- 
fied from the walls towards the middle, at angles of 30 to 35°. Even from 
a distance it can be observed that the upper limit of the agglomerate is 
saucer-shaped, the sloping sides of the depression dipping towards the centre 
