58 
SUBTERRANEAN VOLCANIC ACTION 
irrespective of the lithological nature of the materials. For further and 
more precise designation, when an agglomerate is mainly made up of 
fragments of one particular rock, the name of that rock may be prefixed as 
sandstone -agglomerate, granite -agglomerate, basalt -agglomerate, trachyte- 
agglomerate. Volcanic agglomerate is a useful general term that may 
include all tlie coarser detritus ejected by volcanic action. 
Where volcanic explosions have been of sufficient violence or long 
continuance, the upper part of the funnel may be left empty, and on the 
cessation of volcanic activity, may be filled with water and become a lake. 
The ejected detritus left round the edge of the orifice sometimes hardly 
forms any wall, the crater-bottom being but little below the level of the 
surrounding ground. Explosion-lakes are not infrequent in Central France 
and tire Eifel (Maare). A more gigantic illustration is afforded by the 
perfectly circular crater of Coon Butte in Arizona, about 4000 feet in 
diameter and 600 feet deep. It has been blown out in limestone, the 
debris of which forms a rampart 200 feet high around it. Examples will 
afterwards be cited from the Tertiary volcanic plateaux of North-Western 
Europe. Vents may also he formed by an engulphment or subsidence of 
the material, hke that which has taken place at the great lava caiddron of 
Hawaii, still an active volcano. The picturesque Crater Lake of Oregon is 
an admirable instance of this structure. 
(2) Nechn of Agglomurate or Tuff . — In the vast majority of cases, the ex- 
plosions that clear out a funnel through the rocks of the upper part of the 
crust do not end by merely blowing out these rocks in fragments. The 
elastic vapours that escape from the molten lava underneath are uAially 
followed by an uprise of the lava within the pipe. Eelieved from the 
enormous pressure under which it had before lain, the lava as it ascends is 
kept in ebullition, or may be torn into bombs which are sent whirling up 
into the air, or may even be blown into the finest dust by the sudden 
e.xpansion of the imprisoned steam. If its ascent is arrested within the 
vent, and a crust is formed on the upper surface of the lava- column, 
this congealed crust may be disrupted and thrown out in scattered pieces 
by successive explosions, but may re-foran again and again. 
In many vents, both in recent and in ancient times, volcanic progress 
has never advanced beyond this early stage of the ejection of stones and 
dust. The column of lava, though rising near enough to the surface to 
supply by its ebullition al)undant pyroclastic 
detritus, coarse and fine, has not flowed out 
above ground, nor even ascended to the top 
of the funnel. It may have formed, at the 
surface, cones of stones and cinders with 
enclosed craters. But thereafter the erup- 
tions have ceased. The vents, filled up with 
the fragmentary ejected material, have given 
passage only to hot vapours and gases. As these gradually ceased, the 
volcanoes have become finally extinct. Denudation has attacked their sides 
Fig. 24. — Section of neck of agglomerate, 
rising tlirougli sandstones aiul sliales. 
