THURSDAY, FEBRUARY 17, 1870 
THE MEASUREMENT OF GEOLOGICAL TIME 
1G 
MV ODERN geological research has rendered it almost | 
certain, that the same causes which produced the 
various formations with their imbedded fossils, have con- 
tinued to act down to the present day. It has therefore 
become possible that, by means of changes which are known 
to have occurred in a given number of years, some measure- 
ment of the time represented by the whole series of 
geological formations might be obtained. 
changes in the earth’y surface, the records of which con- 
stitute the materials for geological research, occur very 
slowly, yet not so slowly as to be quite imperceptible in 
historical time. Land has risen or sunk beneath the sea, 
rivers have deepened their channels and have brought 
down sediment which has converted water into land, cliffs 
have been eaten away and the surface of the earth has 
been, in many ways, perceptibly and measurably altered 
during an ascertained number of centuries. But it is 
found that these changes are too minute, too limited and 
too uncertain, to afford the basis of even an approximate 
measurement of the time required for those grand mutations 
of sea and land, those contortions of rocky strata many 
thousands of feet thick, those upheavals of mountain- 
chains and that elaborate modelling of the surface into 
countless hills and valleys, with long inland escarpments 
and deep rock-bound gorges, which form the most promi- 
nent and most universal characteristics of the earth’s 
superficial structure. Another deficiency in this mode of 
measurement arises from the fact, now universally ad- 
mitted, that the record of past changes is excessively 
imperfect, so that even if we could estimate with tolerable 
accuracy the time required to deposit and upheave the 
series of strata of which we have any knowledge ; still that 
estimate would only represent an unknown proportion, 
perhaps a minute fraction of the whole time which has 
elapsed since the strata began to be formed. 
But there is another class of geological phenomena 
which enable us to measure those very gaps in the 
record of which we have just spoken, and it is now 
generally admitted that the continual change of the 
forms of animal and vegetable life which each succeed- 
ing formation presents to us, affords the best means of 
estimating the proportionate length of geological epochs. 
Though we have no reason to think that this change 
was at all times effected by a uniform and regular pro- 
cess; yet believing, as we now do, that it was due to the 
action of a vast number and variety of natural causes 
acting and reacting on each other, according to fixed 
general laws, it seems probable that, with much local and 
temporary irregularity, there has been on the whole a 
considerable degree of uniformity in the rate at which 
organic forms have become modified. It may indeed be the 
case that this rate of variation has continually increased 
or diminished from the first appearance of life upon the 
earth until the present day, or has been subject to 
temporary changes ; but so long as we have no proof that 
such was the case, we shall be safer in considering that the 
change has been tolerably uniform. 
To measure geological time, therefore, all we require is 
a trustworthy unit of measurement for the change of 
NATURE 
It is true, that | 
399 
species: but this is exactly what we have not yet been abie 
to get; for the whole length of the historical period has 
| not produced the slightest perceptible change in any living 
thing in a state of nature. Moreover, though, the much 
longer time that has elapsed since the Neolithic or Newer 
Stone age, has been sufficient for some changes of physical 
geography and has, to some extent, altered the distribution 
of animals and plants, it has not effected any alteration in 
their form. It is only when we get back to the Palolithic 
or Older Stone age, when men used chipped flints for 
weapons and Europe was, probably, either just emerg- 
ing from the severity of the glacial epoch, or in some of the 
intercalated milder periods, that we meet with a decided 
change in the forms of life. Elephants and rhinoceroses, 
bears, lions and hyenas then inhabited Europe; but they 
were nearly all of species slightly different from any now 
| existing, while the reindeer, the musk-sheep, the lemming 
and some other animals, were the same as those that still 
live in the Arctic regions: all the mollusca, however, were 
identical with living species. In the newer Pliocene 
Crag, on the other hand, which seems to have been 
deposited just as the glacial epoch was coming on, there 
are II per cent. of extinct species of shells and about 55 
per cent. of extinct mammalia. What we want, therefore, 
is to be able to estimate, by means of the physical changes 
before alluded to, the time since the beginning or the end 
of the glacial epoch. Then we should have the unit we 
require for measuring geological time by the repeated 
changes in the forms of life as we go further and further 
back into the past; but before showing how this may per- 
haps be done, something must be said about physical and 
astronomical determinations of the age of our globe. 
A few years ago, Sir W. Thomson startled geologists by 
placing a limit to the time at their disposal, which they had 
been in the habit of regarding as practically infinite. He 
showed, from the known laws of heat and the conservation 
of energy, that there are determinable limits to the age 
of the sun. Then, applying the same principles to the 
earth, he showed that, from the known increase of hea 
towards its interior and from experiments on the rate of 
cooling of various rocks, it cannot have existed in a 
habitable state for more than about one hundred million 
years. It is within that time, therefore, that the whole series 
of geological changes, the origin and development of all 
forms of life, must be comprised. But, geologists had 
been accustomed to demand a much vaster period than this 
for the production of the series of fossiliferous deposits in 
the crust of the earth; while the researches of Mr. Darwin 
render it almost certain that, however vast the time since 
the Silurian and Cambrian epochs, yet anterior to these, 
at least an equal, and probably a much longer, series of 
ages must have elapsed since life first appeared upon the 
earth, in order to allow for the slow development of the 
varied and highly organised forms which we find in exist- 
ence at those early epochs. Sir Charles Lyell is not dis- 
posed to admit the accuracy of these calculations, and 
Professor Huxley has criticised them in detail, with a view 
of showing that they are, in many respects, unsound ; 
while Mr. Croll as strenuously maintains that they are 
sound in principle and accurate within certain limits. 
We have now to consider the bearing of Astronomy 
upon the problem. In a series of admirable papers in 
the Philosophical Magazine, Mr, Croll has fully discussed 
