296 
THE TERTIARY VOLCANOES 
BOOK VIII 
Fig. 312. — Volcanic Neck close to that 
shown in Figs. 310 ami 311. 
of the neck at about the same angle as the rudely-stratified agglomerate 
underneath. From the bottom of this basin to the sea-level may be a 
vertical distance of some 30 yards. The basin itself has been filled up by 
three successive flows of basalt, of which the first (c) has merely overflowed 
the bottom, the second (d), entering from the northern rim of the basin, 
extends across to the southern slope, while 
the third ( e ), also flowing from the north, 
has filled up the remainder of the hollow 
and extended completely across it. The 
next succeeding lava (/) stretched over the 
site in such a way as to bury it entirely, 
and to provide a level floor for the piling 
up of the succeeding sheets of basalt. 
The second vent, which is represented 
in Fig 312, exhibits the same features, but 
with some additional points of interest. It 
measures roughly about 20 yards in diameter 
at the sea-level, rises through the same group 
of banded basalt (a a), and is filled with a 
similar agglomerate ( b ). Its more northerly 
wall is now coincident with a line of fault (h) which ascends the cliff, and 
probably marks some subsidence after the eruptions had ceased The 
southern wall shows that a dyke of basalt Qj) has risen between the agglo- 
merate and the banded basalts, and that a second dyke (</) traverses the latter 
at a distance of a few feet. In this instance, also, the upper surface of the 
agglomerate forms a cup-shaped depression which has been filled in by 
two successive streams of lava (c, d). Among the 
succeeding lavas (e) the prominent sill (/) has been 
intruded, to which further allusion is made on p. 323. 
These necks are obviously volcanic vents belong- 
ing to the time of the basaltic eruptions. They 
have been drilled through the basalts of the lower 
part of the cliff, but have been buried under those of 
the central and higher parts. The arrangement of 
their component materials in rude beds dipping 
towards the middle of each vent shows that the 
ejected dust and stones must have fallen back into 
the orifice so as to be rudely stratified towards the 
centre of the chimney, which was finally closed by 
its own last discharges of coarse detritus. The 
saucer-shaped upper limit of the agglomerate seems 
to indicate, as has been suggested above in the 
case of the Portree volcano, that after the eruptions ceased each vent 
remained as a hollow or mmr on the surface of the lava-fields. And the 
manner in which they are filled with successive sheets of basalt shows 
that in course of time other eruptions from neighbouring orifices gave forth 
Fig. 313. — Section of wall of 
another Neck of agglomerate 
in the same group with those 
represented in Figs. 310, 
311, and 312. 
