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Tins volume contains twelve essays written at various times

during recent years. Many of them are studies contributed to

Scientific Reviews or delivered as popular lectures. Some are

expositions of views the scientific basis of which may be

regarded as established. Others—the greater number—may be

described as attempting the solution of problems which cannot be

approached by direct observation.

The essay on The Birth-time of the World is based on a lecture

delivered before the Royal Dublin Society. The subject has

attracted much attention within recent years. The age of the

Earth is, indeed, of primary importance in our conception of the

longevity of planetary systems. The essay deals with the

evidence, derived from the investigation of purely terrestrial

phenomena, as to the period which has elapsed since the ocean

condensed upon the Earth's surface. Dr. Decker's recent addition

to the subject appeared too late for inclusion in it. He finds

that the movements (termed isostatic) which geologists recognise

as taking place deep in the Earth's crust, indicate an age of the

same order of magnitude

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as that which is inferred from the statistics of denudative

history.[1]

The subject of _Denudation_ naturally arises from the first essay.

In thinking over the method of finding the age of the ocean by

the accumulation of sodium therein, I perceived so long ago as

1899, when my first paper was published, that this method

afforded a means of ascertaining the grand total of denudative

work effected on the Earth's surface since the beginning of

geological time; the resulting knowledge in no way involving any

assumption as to the duration of the period comprising the

denudative actions. This idea has been elaborated in various

publications since then, both by myself and by others.

"Denudation," while including a survey of the subject generally,

is mainly a popular account of this method and its results. It

closes with a reference to the fascinating problems presented by

the inner nature of sedimentation: a branch of science to which I

endeavoured to contribute some years ago.

_Mountain Genesis_ first brings in the subject of the geological

intervention of radioactivity. There can, I believe, be no doubt

as to the influence of transforming elements upon the

developments of the surface features of the Earth; and, if I am

right, this source of thermal energy is mainly responsible for

that local accumulation of wrinkling which we term mountain

chains. The

[1] Bull. Geol. Soc. America, vol. xxvi, March 1915.

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paper on _Alpine Structure_ is a reprint from "Radioactivity and

Geology," which for the sake of completeness is here included. It

is directed to the elucidation of a detail of mountain genesis: a

detail which enters into recent theories of Alpine development.

The weakness of the theory of the "horst" is manifest, however,

in many of its other applications; if not, indeed, in all.

The foregoing essays on the physical influences affecting the

surface features of the Earth are accompanied by one entitled _The

Abundance of Life._ This originated amidst the overwhelming

presentation of life which confronts us in the Swiss Alps. The

subject is sufficiently inspiring. Can no fundamental reason be

given for the urgency and aggressiveness of life? Vitality is an

ever-extending phenomenon. It is plain that the great principles

which have been enunciated in explanation of the origin of

species do not really touch the problem. In the essay—which is an

early one (1890)—the explanation of the whole great matter is

sought—and as I believe found—in the attitude of the organism

towards energy external to it; an attitude which results in its

evasion of the retardative and dissipatory effects which prevail

in lifeless dynamic systems of all kinds.

_Other Minds than Ours_? attempts a solution of the vexed question

of the origin of the Martian "canals." The essay is an abridgment

of two popular lectures on the subject. I had previously written

an account of my views which carried the enquiry as far as it was

in

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my power to go. This paper appeared in the "Transactions of the

Royal Dublin Society, 1897." The theory put forward is a purely

physical one, and, if justified, the view that intelligent beings

exist in Mars derives no support from his visible surface

features; but is, in fact, confronted with fresh difficulties.

_Pleochroic Haloes_ is a popular exposition of an inconspicuous but

very beautiful phenomenon of the rocks. Minute darkened spheres—a

microscopic detail—appear everywhere in certain of the rock

minerals. What are they? The discoveries of recent radioactive

research—chiefly due to Rutherford—give the answer. The

measurements applied to the little objects render the explanation

beyond question. They turn out to be a quite extraordinary record

of radioactive energy; a record accumulated since remote

geological times, and assuring us, indirectly, of the stability

of the chemical elements in general since the beginning of the

world. This assurance is, without proof, often assumed in our

views on the geological history of the Globe.

Skating is a discourse, with a recent addition supporting the

original thesis. It is an illustration of a common experience—the

explanation of an unimportant action involving principles the

most influential considered as a part of Nature's resources.

The address on _The Latent Image_ deals with a subject which had

been approached by various writers before the time of my essay;

but, so far as I know, an explanation

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based on the facts of photo-electricity had not been attempted.

Students of this subject will notice that the views expressed are

similar to those subsequently put forward by Lenard and Saeland

in explanation of phosphorescence. The whole matter is of more

practical importance than appears at first sight, for the

photoelectric nature of the effects involved in the radiative

treatment of many cruel diseases seems to be beyond doubt.

It was in connection with photo-electric science that I was led

to take an interest in the application of radioactivity in

medicine. The lecture on _The Use of Radium in Medicine_ deals with

this subject. Towards the conclusion of this essay reference will

be found to a practical outcome of such studies which, by

improving on the methods, and facilitating the application, of

radioactive treatment, has, in the hands of skilled medical men,

already resulted in the alleviation of suffering.

Leaving out much which might well appear in a prefatory notice, a

word should yet be added respecting the illustrations of scenery.

They are a small selection from a considerable number of

photographs taken during my summer wanderings in the Alps in

company with Henry H. Dixon. An exception is Plate X, which is by

the late Dr. Edward Stapleton. From what has been said above, it

will be gathered that these illustrations are fitly included

among pages which owe so much to Alpine inspiration. They

illustrate the

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subjects dealt with, and, it is to be hoped, they will in some

cases recall to the reader scenes which have in past times

influenced his thoughts in the same manner; scenes which in their

endless perspective seem to reduce to their proper insignificance

the lesser things of life.

My thanks are due to Mr. John Murray for kindly consenting to the

reissue of the essay on _The Birth-time of the World_ from the

pages of _Science Progress_; to Messrs. Constable & Co. for leave

to reprint _Pleochroic Haloes_ from _Bedrock_, and also to make some

extracts from _Radioactivity and Geology_; and to the Council of

the Royal Dublin Society for permission to republish certain

papers from the Proceedings of the Society.

_Iveagh Geological Laboratory, Trinity College, Dublin._

July, 1915.

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