CHAI'. V 
AGGLOMERATE NECKS 
63 
the agglomerate, just as they fell back into the chimney and came to rest there. 
The larger masses are placed at all angles, or stand on end, and are sometimes 
especially conspicuous in the centre of a neck, though more usually dispersed 
through the whole. Sucli a thoroughly tumultuous accumulation is precisely 
what might be expected where explosions liave taken place in still liquid 
and ill already consolidated lavas, and where the materials, violently discharged 
to the surface, have fallen back and come finally to rest in the chimney of 
the volcano. 
Nevertheless, this absence of arrangement sometimes gives place to a 
stratification which becomes more distinct in proportion as the material of 
the vent passes from coarse agglomerate into fine tuff. It is possible that 
the existence and development of this structure depend on the depth at 
whieh the materials accumulate in the funnel. We may conceive, for 
instance, that in the lower parts of the chimney, the stones and dust, 
tumultuously falling and rebounding from projections of the rugged walls, 
will hardly be likely to show much trace of arrangement, though even there, 
if the explosions continue to keep an open though diminishing passage in 
the vent, alternations of coarser and finer layers, marking varying phases of 
eruptivity, may be formed in the gradually heightening pile of agglomerate. 
Itude indicatiojis of some such alternations may sometimes be detected in 
what are otherwise quite unstratitied necks. 
In the upper part of a volcanic funnel, how'ever, close to and even 
within the crater, the conditions are not so unfavourable to the production 
of a stratified arrangement. As the pipe is filled up, and the activity of 
eruption lessens, explosions may occur only from the very middle of the 
orifice. The debris that falls back into the vent will gather most thickly 
round the walls, whence it will slide down to tlie central, still ernptive hole. 
It will thus assume a stratified arrangement, the successive layers lying at 
the steepest angles of repose, or from .30° to 35°, and dipping dowm in an 
inverted conical disposition towards the centre. If tlie process should 
continue long enough, the crater itself may be partially or completely filled 
up with detritus (Fig. 25). 
Of this gradual infilling of a volcanic chimney with stratified agglomerate 
and tuff, examples belonging to different geological periods will be cited in 
subsequent chapters. I may here especially allude to one of the most recently 
observed and best marked illustrations, which occurs on the west side of 
Stromo, in the Faroe Islands (see Figs. 310, 311, 312). A neck has there been 
filled up with coarse agglomerate, which is rudely stratified, the layers dip- 
ping steeply into the centre, where the tumultuous assemblage of large blocks 
no doubt points to the final choking up of the diminished oilfice of explosion. 
The walls of the neck are nearly vertical, and consist of the liedded basaltic 
lavas through wdiich the vent has been opened. They terminate upward 
in a conical expansion, evidently the old crater, which lias subsequently 
been filled up by the inroads of several lava-streams from adjacent vents. 
It is here manifest that the bedded agglomerate belongs to the uppermost 
part of the volcanic funnel. 
