CHAP. IV 
VESUVIAN TYPE 
41 
Every stage in this process of eflaceinent may he recognized in actual 
progress among the extinct volcanoes of the earth’s surface. Probably 
nowhere may the phenomena be more conveniently and impressively studied 
than among the volcanic districts of Central Prance. On the one hand, we 
meet there with cinder-cones so perfect that it is hard to believe them to 
have been silent ever since the beginnings of history. On the other hand, 
we see solitary cones of agglomerate or of lava, which have been left 
isolated, while their once overlying and encircling sheets of ejected material 
have been so extensively worn away as to remain merely in scattered patches 
capping the neighbouring hills. X^alleys many hundreds of feet in depth 
have been cut by the rivers through the volcanic sheets and the underlying 
Tertiary strata and granite since the older eruptions ceased. And yet these 
eruptions belong to a period which, in a geological sense, is quite recent. 
It is not difficult to contemplate a future time, geologically not very remote, 
when in the valley of the Loire not a trace will remain of the wonderfully 
varied and interesting volcanic chronicle of that district, save the plugs that 
will mark the positions of the former active vents. 
In the British Islands, ancient volcanoes of the Vesuvian type are well 
represented among the Palieozoic systems ot strata. Their preservation has 
been largely due to the fact that they made their appearance in areas that 
were undergoing slow^ subsidence. Their piles of erupted lava and ashes 
were chiefly heaped up over the sea-floor, and w’ere buried under the sand, 
silt and ooze that gathered there. Thus covered up, they were protected 
from denudation. It is only in much later geological ages that, owing to 
upheaval, gradual degradation of the surface, and removal of their overlying 
cover of stratified formations, they have at last been exposed to waste. The 
process of disinterment may be observed in many different stages of progress. 
In some localities, only the tops of the sheets of lava and tuff have begun 
to show themselves ; in others, everything is gone except the indestructible 
lava-plug. 
These inequalities of denudation arise not oidy from variations in the 
durability of volcanic rocks, but still more from the relative position of 
these rocks in the terrestrial crust, and the geological period at which, in 
the course of the general lowering of the surface, they have been laid bare. 
Not only are volcanic rocks of many diflerent ages, and lie, therefore, on 
many successive platforms within the crust of the earth : their places ha^ e 
been still further de])endent upon changes in the arrangement of that crust. 
Fracture, upheaval, depression, curvature, unconforruable deposition of strata, 
have contributed to protect some portions, while leaving others exposed to 
attack. Hence it happens that the volcanic record varies greatly in its 
fulness of detail from one geological system or one district to another. 
Some chapteivs have been recorded with the most surprising minuteness, so 
that the events which they reveal can be realized as vividly as those ot a 
modern volcano. Others, again, are meagi’e and fragmentary, because the 
chronicle is still for the most part buried underground, or because it has been 
so long e.xposed at the surface that only fragments of it now remain there. 
