CHAP. V 
AGGLOMERATE NECKS 
59 
and crests. If submerged in the sea or a lake, the cones have been washed 
down, and tlieir materials have been strewn over the bottom of the water. 
If standing on the land, they have been gradually levelled, until perhaps 
only the projecting knob or neck of solidified rubbish in each funnel has 
remained to mark its site. The buried column of compacted fragmentary 
material will survive as the only memorial of the eruptions (Tig. 24. For 
views of necks formed of agglomerate or tuff see Figs. 23, 82, 102, 123, 
144, 178, 192, 203, 204, 209, 210, 212, 216). 
The volcanic agglomerates of such vents sometimes include, among 
their non-volcanic materials, pieces of rock which bear evidence of having 
been subjected to considerable heat (see vol. ii. p. 7 8). Carbonaceous shales, 
for instance, have had their volatile constituents driven off, limestones have 
been converted into marble, and a general induration or “ baking ” may be 
perceptible. In other cases, however, the fragments exhibit no sensible 
alteration. Fossiliferous limestones and shales often retain their organic 
remains so unchanged that specimens taken out of tlie agglomerate cannot 
be distinguished from those gathered from the strata lying in sitw outside. 
Some stones have evidently been derived from a deeper part of the chimney, 
where they have been exposed to a higher temperature than others, or they 
may have been lain longer within the influence of hot ascending vapours. 
The volcanic materials in agglomerate range in size from the finest dust 
to blocks several yards in length, with occasionally even much larger 
masses. The proportions of dust to stones vary indefinitely, the finer 
material sometimes merely filling in the interstices between the stones, at 
other times forming a considerable part of the whole mass. 
The stones of an agglomerate may be angular or subangular, but are 
more usually somewhat rounded. Many of them are obviously pieces that 
have been broken from already solid rock and have had their edges rounded 
by attrition, probably by knocking against each other and the walls of the 
chimney as they were hurled up and fell back again. Their frequently 
angular shapes negative the supposition that they could have been produced 
l)y the discharge of spurts of still liquid lava. As already stated, they 
have probably been in large measure derived from the violent disruption of 
the solidified cake or crust on the top of the column of lava in the pipe. 
Jlany of them may have been broken ott' from the layer of congealed lava 
that partially coated the rough walls of the funuel after successive uprises 
of the molten material. Among them may be observed many large and 
small blocks that appear to have been derived from the disruption of true 
lava-streams, as if beds of lava had been pierced in the formation of the 
vent, or as if those that congealed on the slopes of the cone had been broken 
up by subsequent explo.sions. These fragments of lava are sometimes strongly 
amygdaloidal. A characteristic feature, indeed, of the blocks of volcanic 
material in the agglomerates is their frequent cellular structure. Many 
of them may be described as rough slags or scoriae. These have generally 
come from the spongy crust or upper part of the lava where the imprisoned 
steam, relieved from pressui'c, is able to expand and gather into vesicles. 
