CHAP. V 
AGGLOMERATE NECKS 
6i 
that the slice looks like a pattern of delicate laced In necks of earlier 
date, such as those of older Tertiary, and still more of Palaeozoic, time, the 
glass has generally been altered into some palagonitie material. 
This finely puniiceous substance appears to be peculiar to the vents and 
to the deposits of tuff immediately derived from them. It is not found, so 
far as 1 know, among any of the superficial lavas, and, of course, would not be 
looked for among intrusive rocks. It was evidently a special product of the 
volcanic chimney, as distinguished from the mass of the magma below. We 
may perhaps regards it as in some way due to a process of quiet simmering 
within the vent, when the continual passage of ascending vapours kept 
the molten lava there in ebullition, and gave it its special frothy or finely 
pumiceous character. 
Tlie compacted dust, sand or gravelly detritus found in necks, and 
comprised under the general name of Tuff, consists partly of the finer particles 
produced during the violent disruption of already solidified rocks, partly of 
the detritus arising from the friction and impact of stones ascending and 
descending above an active vent during times of eruption, and partly of 
the extremely light dust or ash into which molten lava may be blown by 
violent volcanic explosions. In old volcanic necks, where the rocks have 
long been subjected to the influence of percolating meteoric water, it is not 
perhaps possible to discriminate, except in a rough way, the products from 
these three sources. The more minutely comminuted material has generally 
undergone considerable alteration, so that under the microscope it seldom 
reveals any distinctive structures. Here and there in a slide, traces may 
occasionally be detected of loose volcanic microlites, though more usually 
these can only be found in lapilli of altered glass or finely pumiceous 
lava. 
The composition of the detritus in a neclc of agglomerate or tuff has 
almost always a close relation to that of any lavas which may have been 
emitted from tliat vent. If the lavas have been of an acid character, such 
as rhyolites, felsites or obsidians, the pyroclastic materials will almost 
always be foiuid to be also acid. Where, on the other hand, the lavas 
have been intermediate or basic, so also will be the tuffs and agglomerates. 
Occasionally, however, as has already been pointed out, from the same or 
closely adjoining vents lavas of very different chemical composition have been 
successively erupted. Felsites or rhyolites have alternated with diabases, 
basalts or andesites. In such cases, a commingling of acid and basic 
detritus may be ob.served, as, for example, among the volcanoes of the Old 
Hed Sandstone. It has even happened sometimes that such a mixture of 
material has taken place when only one class of lavas has been poured out 
at the surface, as in the agglomerates that fill vents among the basalts of 
the Inner Hebrides. But we may be sure that, though not discharged at 
the surface, the lavas of which pieces are found in the tuffs must have 
risen high enough in the vents to be actually blown out in a fragmentary 
form. The occurrence of felsitic fragments among the otherwise basic 
1 M. Houle, Bull. Cart. Geol. Frame, No. 28, tome iv. (1892) p. 193. 
