460 
THE TERTIARY VOLCANOES 
BOOK VIII 
l ujs of Auvergne, they may still retain, in their conical forms, indications 
ot the original shapes which their component material assumed at the 
time of its protrusion, we may be confident that their existing surfaces 
ha've been reached after the removal of much rock which once lay above 
them. This inference is confirmed by the fact that these eruptive bosses 
have been invaded by a younger system of dykes. The black ribs of basalt 
which may he traced along their pale declivities, which cross the glens that 
ha\e been eroded in them and which mount up to their very crests, prove that 
since the latest manifestations of volcanic energy in the West of Scotland, 
extensive changes in the topography of the land have been effected by the 
operation of the suhterial agents of degradation. 
So much lor what can be demonstrated. But how much more may, 
with the highest probability, he inferred! The original limits of the 
plateaux are unknown. The waves of the wide Atlantic now roll over 
many a square league of the old lava-plains, and wide tracts of the islands 
and the mainland from which the basalt has been entirely stripped, or where 
it remains only in scattered outliers, were once deeply buried under piles of 
lava-sheets. It would probably be no exaggeration to affirm that over the 
British area, as well as over the Faroe Isles, the amount of Tertiary volcanic 
rock that now remains, large as it is, falls short in amount of what has 
been removed. The geologist who has made himself familiar with the 
effects of^ denudation in other Tertiary volcanic districts, such as Central 
I ranee, Saxony and Bohemia, will be prepared for almost any conceivable 
amount of erosion among the far older volcanic series of the north-west of 
Europe. 
To the student of the origin of the existing topography of the land 
there is a profound interest in the demonstration which these volcanic rocks 
supply of the vast changes which the terrestrial surface has undergone 
within a period geologically so recent as older Tertiary time. When, 
on the one hand, he finds himself more and more restricted in his de- 
mands fur time by the confident assertions of the physicist that all the 
phenomena of geological history must have been comprised within a few 
millions ot years, and when, 011 the other hand, he watches the seemingly 
feeble and tardy operations ot the forces of denudation and sedimentation 
which have played the chief parts in that history, he may well be excused 
it sometimes he is apt to despair of ever reconciling the facts which he 
observes with the physical deductions that are somewhat dogmatically 
brought forward in opposition to his interpretation of them. He may feel 
sure that his facts cannot be gainsaid, and he may he unable to find any 
othei wa\ ot comprehending them save by the admission that they necessi- 
tate a liberal allowance of time. Yet he may not feel himself to" he in a 
position to offer any valid objections to the arguments from physical con- 
siderations that would so seriously abridge the length of time which geology 
requires. 
i In fchese circumstances it is some satisfaction to he provided with 
definite measurements of the amount of geological change which has been 
