106 
NATURE. 
[Marcu 25, 1915 

Without entering into these matters, some of which, 
as I have suggested, call for expert aid, I will take 
for illustration a. single point, the frequent abuse of 
the average. Say that we wish to determine the 
amount of mud annually carried down by the Nile. 
Since there are variations, both seasonal and casual, 
we must take a sufficient number of observations, 
properly distributed in time, and an average, duly 
weighted, will then give us as good a result as the 
nature of the case admits. But now suppose that we 
wish to know the amount of sediment carried by all 
the rivers of the world. We have data for nine rivers, 
data which are likely to differ much in respect of 
probable error. Accepting them, however, as they 
stand, it appears that the water of the Rio Grande 
carries one part in 291 of sediment, that of the 
Uruguay River only one in 10,000, the other seven 
rivers giving intermediate values. The highest figure 
is thus thirty-four times as great as the lowest. Some 
geologists will simply take a mean of the nine figures, 
and proceed contentedly to use this result in the most 
far-reaching conclusions. I do not believe that a mean 
of nine figures so discordant can afford any informa- 
tion of quantitative value. The average must be ex- 
tended over a much wider area, before a result is 
obtained of which we can legitimately make use. 
Where dynamical principles enter into the problem, 
the pitfalls which await the unwary are sometimes 
less evident. I will take as an illustration the case of 
models, such as have been constructed to elucidate the 
mechanism of folding and faulting. In no case, so 
far as I am aware, have geologists had regard to the 
conditions which are necessary in order that a model 
may correctly represent the working of the original. 
The various forces concerned must bear their proper 
ratios. Since the weight (for a given material) is 
reduced proportionally to the cube of the linear dimen- 
sions, the other forces must be reduced in the same 
ratio; and it is, in fact, impossible to make this ad- 
justment as regards the internal forces which resist 
deformation and fracture. Moreover, the velocities of 
the moving parts should be reduced in proportion to 
the square -root of the linear dimensions; and. this 
makes it hopeless to think of imitating the slow pro- 
cesses of mountain-building. Models of this kind may 
afford useful geometrical illustrations, but can throw 
no light on dynamical problems. The same remark 
applies to models of glaciers; but here there is no need 
to go to artificial models to illustrate my point. Some 
geologists still argue from the behaviour of an Alpine 
valley-giacier to that of a continental ice-sheet, with- 
out perceiving how completely the different scale of 
magnitude must modify the mechanical conditions. 
Experiment has undoubtedly afforded valuable help 
in the study of particular questions in the domain of 
physical geology, and this is to be recognised with 
gratitude. As regards the larger and more complex 
problems, however, imitative experiment labours under 
the same disadvantage as mathematical analysis. Any 
concrete problem can be treated only in an arbitrarily 
simplified form, and among the conditions which 
cannot be realised in the laboratory may be some 
which in nature are of vital importance. Especially 
will this be the case where the time element enters. 
There is, however, another department of experi- 
mental geology in which we are justified in expecting 
results of very high value. I allude to the study of 
the conditions of formation and stability of different 
minerals, with the object of elucidating the mode of 
origin of igneous and other rocks. The artificial re- 
production of many of the rock-forming minerals has 
engaged the attention of chemists, especially in 
France, during the last hundred years. Fouqué and 
Michel-Lévy succeeded even in imitating some of the 
NO. 2369, VOL. 95] 

simpler types of igneous rocks. These researches have 
furnished the petrologist with useful information, but 
it is information. mostly of a very general kind. The 
laborious investigations now being carried out, more 
particularly in the Geophysical Laboratory of the Car- 
negie Institution of Washington, are of a different 
order, systematic and precise to the highest degree 
attainable. Their chief object is to apply to the crystal- 
lisation of igneous rock-magmas the methods which 
have proved so fruitful in other branches of physical 
chemistry. This necessitates working over a far wider 
range of temperatures than is usual in laboratory 
operations, and must sometimes include high-pressure 
work also. It involves, too, other practical difficulties, 
arising especially from the slowness with which equili- 
brium is established in many of the transformations 
investigated. Owing perhaps to these obstacles, and 
partly, it may be, to the scarcity of enlightened mil- 
lionaires—for expense is here a weighty consideration 
‘ research on these lines has not yet been widely taken 
up. Meanwhile it is scarcely too much to say that 
Dr. Day and his colleagues at Washington are already 
laying the foundations of an exact science of petro- 
genesis. 
Of all geological questions involving the numerical 
element, none has been more frequently canvassed 
than the problem of geological chronology, and none 
has excited more general interest. Since, moreover, 
it introduces several points germane to my subject, 
a brief glance at its history and present state will not 
be wasted. I suppose it has happened to most of us, 
when relating how in past times the mammoth roamed 
the plains of Holderness, or how coral-reefs once 
flourished where the Craven hills now stand, to be 
met by the inquiry: How long ago was that? The 
answer was perhaps to the effect that geology does 
not deal with the ordinary measures of time, but has 
its own system of chronology, not translatable into 
years and centuries. I must confess, however, to a 
sense of inadequacy in such a reply, and some sym- 
pathy with the lay inquirer who is thus silenced but 
not satisfied. It seems a matter of reasonable regret 
that a science which deals with the history of past 
events should have no definite time-scale, by which 
those events could be ranged in a correct perspective. 
No such reflection, it is safe to say, disturbed the 
minds of the early Uniformitarians, the founders of 
modern geology. Their reaction against the older 
catastrophic school led them constantly to lay great 
stress on the extreme slowness of geological processes, 
and they thus came to assume unlimited time for the 
past changes to which the stratified rocks bear witness. 
To Hutton there was ‘‘no vestige of a beginning, no 
prospect of an end’’; in other words, he regarded 
geological time as infinite, and could no more contem- 
plate reckoning it in centuries than numbering the 
sands on the shore. Later this position was rein- 
forced from another quarter, as Darwin’s doctrines 
gained acceptance; for these were held to push back 
to an immeasurably remote epoch the beginning of 
life on the globe. Geologists and biologists alike saw 
no reason for limiting their prodigal drafts on the 
bank of time. 
From this comfortable attitude they were startled, 
as by a bombshell, some fifty years ago, when William 
Thomson, afterwards Lord Kelvin, published the first 
of his contributions from the mathematical side to this 
and cognate subjects. He pointed out that, apart from 
any changes on the surface of the globe, our planet 
as a whole must be undergoing a change of a secular, 
and so irrevocable, kind; viz., a continual loss of energy 
in the form of heat, as proved by the observed tem- 
perature-gradient. Since the store of energy cannot be 
inexhaustible, we must deduce both a beginning and 
