CHAl'. I. 
EFFECTS OF DENUDATION 
461 
effected within a limited and relatively recent period of time. This change 
has resulted from the operation of the same agents by which it is still being 
carried on. No break in the history can be detected. There is not the 
least reason to suppose that the agents of denudation and sedimentation 
have, during the period in question, differed in their rate ol working. 
Their activity at the present time is probably neither greater nor less than 
it was then. If, therefore, during so recent an interval such a stupendous 
amount of material has been worn away from the surface of the land and 
deposited on the sea-floor as the Tertiary volcanic rocks demonstrate, the 
geologist may surely contemplate without misgiving the lapse of time 
required for the completion of older geological revolutions. He may oppose 
to the arguments of the physicist the measurements and computations 
which he himself makes from data which are at least as reliable as the 
postulates whereon these arguments are based. The rate at which denuda- 
tion and sedimentation are now taking place has been measured with tolerable 
accuracy, and a fair average for it has been obtained. M hatever may be 
maintained as to this rate in early geological ages, there can be no serious 
opposition to its being taken as fairly constant since older lertiary time. 
We are thus provided with data for estimating the minimum amount of 
time that can have elapsed since the volcanic plateaux began to be denuded. 
But as no relic remains of the original upper surface of those plateaux, 
and as we are consequently ignorant of how much rock has been removed 
from their highest surviving outliers, it is obvious that such estimates are 
more likely to err in understating than overstating the amount of time 
required. 
It would be beyond the scope of the present volume to enter fully into 
the measurements and calculations required for the adequate treatment of 
this subject. I will merely illustrate my argument by again taking a few 
data from the plateau of Mull. The original height of this plateau is 
shown by the outlier of Ben More to have been at least o200 feet. II to 
this figure we add the portion of the basalt-group submerged under the sea 
the height will probably be increased by several hundred feet. But let us 
take 3000 feet as a moderate computation for the average thickness of the 
volcanic series here at the close of the plateau-period. Until a number ot 
sections have been carefully plotted from the Ordnance Maps, in oidei to 
ascertain with approximate accuracy the average height of the present suiface 
of the Mull basaltic plateau, making due allowance for the vast erosion of the 
Sound of Mull and the numerous glens and sea-lochs that traverse the 
island, any estimate which may be offered as to this average must be merely 
provisional. If, in the meantime, we suppose the present mean level of the 
plateau to be 1000 feet above the sea, the difference between this amount 
and the assumed original height will be 2000 feet. If, further, we take 
the present average rate of degradation of the Mull plateau to be -fRfTTTT 
a foot in a year, which has been shown to be probably a fair estimate, then 
the time required for the lowering of the Mull plateau from its original to 
its present average level amounts to twelve millions of years. Yet this 
