March 3, 1870] 
period beginning about 240,000 and ending about 80,000 
years ago, is the more likely one. [Even this, however, 
offers difficulties. Denudation in Wales and Scotland 
must probably have gone on as rapidly as the average rate 
of the Rhone valley, especially during the period when the 
old glaciers were disappearing ; and eighty thousand years 
will therefore imply eighty feet of average denudation over 
the whole surface of the country, if less in one part then 
correspondingly more in another ; but how is this consistent 
with the preservation of ice-ground rock-surfaces and 
glacial furrows in so many situations, as well as numberless 
heaps of loose matter, the moraines of ancient glaciers, 
apparently just as they were left by the ice? There are, 
it is true, a few considerations that go to diminish though 
not to remove the difficulty. The amount of denudation 
is now abnormally large, because the large quantities 
of glacial drift left over the surface of the country, supply 
much of the sediment carried down by the streams of 
Alpine countries. Many glacial markings were at first 
covered up and preserved by drift or alluvium, and have 
been since exposed by denudation : those earliest exposed 
are obliterated, but new surfaces are being continually 
uncovered. The amount of denudation of a solid rock- 
surface may not be a tenth part of that which now obtains 
in glaciated districts; a fact which can only be ascertained 
by determining the amount of sediment brought down by 
streams the basins of which are free from drift or gravel, 
and consist almost wholly of compact rock surfaces. We 
still have to deal with the difficulty of the moraines, whose 
form and aspect are often so fresh that we can hardly 
believe them to have been much changed since the ice left 
them, although it is impossible to understand how they 
have escaped the denudation which has lowered the whole 
surface of the country eighty, or even a much smaller 
number of feet. 
It is true Mr. Geikie, in his paper on Modern Denuda- 
tion,* suggests that all the effects of ice-action, now visible, 
are merely the few examples which have been preserved 
owing to a concurrence of favourable conditions, while a 
much larger number have been destroyed ; and I learn 
from him that there are in Scotland moraines in all stages 
of decay. If this be the true explanation of the difficulty, 
it follows that denudation must be extremely unequal, and 
that if one valley or hill-side has remained unaltered 
during 80,000 years, another must have been denuded to 
double the average amount. 
Having thus shown the difficulty there is in accepting 
even the shorter period of 80,000 years for the date of the 
end of the glacial epoch in Europe, let us see what other 
modes of measurement are available. In Sir Charles 
Lyell’s “ Antiquity of Man,” 2nd ed., p. 28, we have three 
different calculations of the age of the bronze and stone 
periods in Switzerland, which would place the latter at 
about from 5,009 to 7,000 years ago. At page 321 of the 
same work, we have an estimate of the age of the upper 
delta of the Tinitre by M. Morlot. The iower delta (by 
the presence of Roman remains in one of the upper strata) 
is calculated with tolerable certainty to be 10,000 years 
old, while the upper delta, 150 ft. above the lake, is ten times 
as large as the lower one, and is therefore supposed to be 
100,000 years old. From its fossil remains it is believed 
to be post-glacial; but it is evident that, during the 
* Transactions of the Geological Society of Glasgow, vol. iii. p. 153. 
MATURE 
453 
melting of the ice, the torrent might have been more 
powerful, and have accumulated a delta much more 
rapidly than now. The peat mosses of Denmark, indi- 
cating that the present beech-tree vegetation of that 
country, which was also characteristic of it in the 
Roman period, was in the Bronze age replaced by oaks, 
and in the still earlier Stone age by fir-trees, imply 
a very long lapse of time; yet this only takes us back 
to the Neolithic age, when all the shells and all the 
mammialia were of existing species. The 8,000 or 10,000 
years of the Swiss Stone age may, however, have sufficed 
for this change. There seems to be no doubt, that the 
time which elapsed from the close of the glacial epoch 
(when man used the rudest flint weapons, and was coeval 
with many extinct animals, when, moreover, the climate 
and physical features of the country were considerably 
different from what they are now) up to the Neolithic age, 
was much greater than from the latter date to the present 
day, but how much greater it is impossible to determine. 
The position of many of the tool-bearing gravels shows 
that rivers then flowed at much higher levels, but 
from the known rate of denudation, a valley might be 
deepened even 50 or 100 ft. in as little as 50,000 years, 
since it is in valleys that the effects of denudation 
would be greatest; and the extinction of the various 
animals might certainly take place in an equal time 
under such conditions as are not unlikely to have 
occurred at a period of great climatic change. 
It does not appear, therefore, that any of the estimates of 
time founded on an actual basis of observed change in a 
known period, require us to assume more than 80,000 years 
since the close of the glacial epoch, while the measure- 
ment of the existing rate of denudation renders it almost 
certain that it was less rather than more. We may fairly 
assume, that even if a large excentricity has been an 
essential condition of a glacial epoch, the ice would main- 
tain itself into a period of less excentricity than would be 
required to bring oneon. Now 74,000 years ago the excen- 
tricity was about double what it is now, and the winter 
of the northern hemisphere then occurred in aphelion, so 
that the glacial epoch would at that time probably have 
been in full force, and we may assume that it might con- 
tinue 3,000 or 4,000 years longer. But when we come to 
65,000 years back, we find the excentricity scarcely more 
than it is at present, and winter nearly in ferihelion,; so 
that we must conclude, if excentricity has anything to do 
with it, that the last glacial period came to an end not less 
than 70,000 years ago. 
Now it is most important to observe that, for the last 
60,000 years, the excentricity has been very small—for 
three-fourths of the time less than it is now. During 
this time the opposite phases of precession, each lasting 
10,500 years, will have produced scarcely any effect on 
climate, which in every part of the earth will have 
been wearly uniform for that long period. But this 
is quite an exceptional state of things; for the curve 
of excentricity shows us that, during almost the whole 
of the last three million years, the excentricity has 
been high—almost always twice, and sometimes three 
and four times as much as it is now. If, therefore, Mr. 
Croll’s theory be correct, there will have been a change 
each 10,500 years during this vast period (in all the extra- 
tropical regions at least) from a very cold to a very mild 
