346 ELEMENTARY GEOLOGY 
tary strata. This is the case in some of the trap hills 
of the Connecticut valley. Not only is material added 
to the earth’s surface in the form of lava flows, but also 
in the condition of volcanic ash; and since much of 
this falls into the sea, this source becomes an important 
one for the sediments forming in the ocean. Another 
effect. of geological importance is the building of the 
cone, which forms a 
distinct and impressive 
feature of the land. 
Since the material 
that comes out at the 
crater must reach the 
surface through a con- 
duit or tube, and since 
this must be filled when 
Pompeii with Vesuvius in the background. the volcano last erupts, 
Tus ve vas baled by te eunion ot in. the contre OF every 
extinct volcano there is 
a solidified plug of lava. This is harder than the ash 
and the lava flows that have built the cone, and when 
it is reached by denudation, remains as an elevated 
portion, mainly because of its greater resistance. 
These steeply rising hills are called volcanic necks or 
plugs (Fig. 215). 
When a volcanic eruption breaks a fissure through a 
cone, as is so frequently done in the volcanoes of the 
