108 
NATURE 
[Marcu 25, 1915 


measure of time in geology? I do not draw this con- 
clusion, but rather that we must search outside strictly 
geological phenomena for some physical process of 
which the rate is not affected by any disturbing con- 
ditions. There are, I think, only two classes of 
changes for which so much can be claimed—the trans- 
formations of the radio-active group of elements 
and the astronomical movements. It seems not 
improbable that in one or other of these two 
directions the solution of the problem may eventually 
be found. 
The chemists have taught us that radium is derived 
from the spontaneous breaking up of uranium, the 
change taking place apparently in two stages and 
involving the liberation of three atoms of helium. 
But radium itself disintegrates spontaneously, giving 
the radium-emanation named niton and liberating 
another atom of helium. Niton in its turn undergoes 
disintegration, and so on through a succession of 
changes. The final product is lead, and in the gradual 
conversion of uranium to lead eight atoms of helium 
in all are set free. Of these various spontaneous 
changes some proceed with extreme slowness, others 
with comparative rapidity; but in each case there is 
a constant rate which, so far as experiment has tested 
it, is independent of temperature or pressure. 
Prof. Strutt has shown that this gradual liberation 
of helium can be made the basis of a method of 
estimating the absolute ages of minerals and rocks. 
For example, phosphates and some iron-ores are rich 
in radium, derived frcm uranium. ‘They also contain 
helium, and the ratio of helium to uranium is found 
to be higher in the older deposits. Estimates of age 
calculated from these data give high figures: e.g. the 
age of the hematite overlying the Carboniferous 
Limestone in Cumberland is given as 140 millions of 
years, and even that of the Eocene iron-ores of Antrim 
thirty millions. The results show some irregularities, 
and it is, of course, admitted that the method has 
its own difficulties. If, however, the chief source of 
error is, aS appears probable, the loss of helium by 
leakage, the figures found will be under-estimates. 
Helium comes from the thorium series of derivatives 
as well as from the uranium series, and this is to be 
taken into account where thorium is found. Zircons 
from various igneous rocks have also been examined 
by Strutt, and found to give consistent results as re- 
gards the helium-ratio. Mr. A. Holmes has ap- 
proached the question in a different way, by consider- 
ing the ratio of lead to uranium in various minerals 
rich in the latter element. The igneous rocks of the 
Christiania district, of Devonian age, are in this way 
calculated to be about 370 million years old. For the 
Archean rocks of different countries the estimates 
range from 1000 to 1600 millions of years. Holmes’s 
results are in general nearly twice as high as those of 
Strutt; but, if we bear in mind the error due to the 
escape of helium, which is proved to take place, a 
discrepancy to this extent is no more than should be 
expected at this early stage of the inquiry. 
The other method which has been suggested for 
obtaining an absolute measure of geological time is 
of a more speculative kind, although the principle of 
it is sufficiently simple. It consists in detecting some 
clearly marked rhythm or cycle in the geological re- 
cord, and correlating it with one of the known periodic 
movements of the earth. It was on these lines that Croll 
attempted to explain the recurrent glacial epochs; but 
more to our present purpose is the theory which Blytt 
has based upon a study of the alternations observed ina 
succession of sedimentary strata. The most important 
astronomical cycle of long period is doubtless that 
which depends upon the precessional movement, by 
which the relation of summer and winter to peri- 
NO. 2369, VOL. 95! 


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helion and aphelion is gradually changed. This in- 
volves a change in the relative lengths of summer and 
winter, and must undoubtedly exercise a marked effect 
upon climatic conditions, though there is much differ- 
ence of opinion concerning the exact nature of this 
effect. Changes of climate may in their turn cause 
differences in the nature of the sediments deposited 
successively at a given place, differences which wiil 
repeat themselves in a cycle corresponding with that 
of the precession. Probably the most noticeable effect 
will be a recurrence of limestones and chemical 
deposits alternating with detrital sediments. 
If the matter were no more complex than this, it 
would be sufficient, where such alternations can be 
detected, to count them, as we count the rings of 
growth of a tree, and reckon 21,000 years for each 
sedimentary cycle, that being the period of the pre- 
cession corrected for the movement of the perihelion. 
lf the alternations can be distinguished only in some 
parts of the succession, some hypothesis must be 
devised to take account of the intervals. Gilbert has 
discussed in this way a succession of beds, 3900 ft. 
thick, forming part of the Cretaceous system in Colo- 
rado. Alternations of calcareous beds with shales 
come in four times, being separated by unbroken 
thicknesses of shale. Gilbert calculates for the part 
of Cretaceous time represented a duration of about 
twenty million years, with an uncertainty indicated 
by the number 2 as a “factor of safety.” 
We have to remember, however, that sedimentation 
is controlled by other conditions besides climate, and 
climate depends upon other causes besides the pre- 
cession of the equinoxes; and, further, that most of 
these contributing causes cannot be described as 
periodic in any intelligible sense. There is, it is true, 
a second astronomical movement to which both Croll 
and Blytt have made appeal, viz., the variation in 
the eccentricity of the earth’s orbit. This goes through 
a period of about 90,000 years; but there are consider- 
able irregularities which repeat themselves in the 
course of 1,450,000 years, giving a larger cycle which 
embraces sixteen of the smaller cycles. The change 
of eccentricity must modify the effect of the preces- 
sional movement; but Blytt argues that it will also 
react on the ellipsoidal shape of the globe itself, and 
so give rise to a displacement of shore-lines. He 
claims to have traced this effect, as well as the 
climatic cycle, in such cases as the succession of the 
Tertiaries in the Paris basin and the Isle of Wight. 
His conclusion is that Tertiary time comprises two of 
the larger cycles, i.e. about three million years. 
It has usually been assumed that the year is too 
short a period to leave any recognisable mark on the 
geological record. This is probably true in general, 
but in certain favourable circumstances it may perhaps 
be possible to count annual layers of sediment. De 
Geer has recently attempted this in the case of certain 
finely laminated clays of late Glacial and post-Glacial 
age in Sweden. The material was brought down by 
sub-glacial streams at a time when the ice had re- 
treated to the higher ground. Consequently the 
seasonal variations were strongly marked, and the 
accumulation of sediment was rapid enough to yield 
an appreciable thickness in each year. From such 
data De Geer has estimated that the recession of the 
last ice-sheet occupied a duration of about 5000 years; 
and he further gives 7000 years as the lapse of time 
since the recession of the ice. 
As regards the longer astronomical cycles, it is clear 
that the argument involves a large element of hypo- 
thesis, and its application, as Blytt allows, is beset 
with practical difficulties. It possesses a special in- 
terest as lending a new significance to the details of 
stratigraphy, but as a means to the establishment of a 
