PREFACE.

DURING the ten years which have elapsed since the publication of Climate and Time, great progress has been made in Physical and Geological science; but, so far as I am aware, not one of the main conclusions of this work has been, materially affected thereby. On the contrary, many of these, some of which were received at first with considerable hesitation, have been confirmed in a very remarkable manner. For example, it has now been proved that the Glacial Epoch was not one long period of continuous cold, but consisted of a series of alternate cold and warm periods; and it has also been proved that during several of those cold or glacial periods the whole of North-Western Europe, including the German Ocean and the Baltic, was covered with one continuous sheet of ice several thousands of feet in thickness; and that, on the other hand, during the warm or interglacial periods the ice disappeared and the country became covered with a rich and luxurious vegetation. It is now also generally admitted that the views advocated in this volume as to the cause of Ocean Currents and to the amount of heat transferred by them from equatorial to temperate and polar regions, and the important influence which this has on climate, are in the main correct.

As the original addition approached exhaustion it became necessary for me to determine the manner in which I should treat a new edition. Guided principally by the fact above stated, that the progress of investigation has been entirely confirmatory of my conclusions, and being moreover conscious that the theory as advanced and supported in this work is still an object of enquiry and discussion, I have deemed it proper to re-issue the text without alteration or comment. The volume has accordingly been reprinted from the American Edition issued some time after the original appearance of the book in London; and this issue is, in effect, the third unaltered edition.

While I have nothing to withdraw, and little to modify in my position, a great mass of material has accumulated in my hands further elucidating the subject, and all tending to confirm its main propositions. These materials I have incorporated in a separate work entitled Discussions on Climate and Cosmology, which is issued simultaneously with this volume.

EDINBURGH, October, 1885.

PREFACE.

IN the following pages I have endeavoured to give a full and concise statement of the facts and arguments adduced in support of the theory of Secular Changes of the Earth's Climate. Considerable portions of the volume have already appeared in substance as separate papers in the Philosophical Magazine and other journals during the past ten or twelve years. The theory, especially in as far as it relates to the cause of the glacial epoch, appears to be gradually gaining acceptance with geologists. This, doubtless, is owing to the greatly increased and constantly increasing knowledge of the drift phenomena, which has induced the almost general conviction that a climate such as that of the glacial epoch could only have resulted from cosmical causes.

Considerable attention has been devoted to objections, and to the removal of slight misapprehensions, which have naturally arisen in regard to a subject comparatively new and, in many respects, complex, and beset with formidable difficulties.

I have studiously avoided introducing anything of a hypothetical character. All the conclusions are based either on known facts or admitted physical principles. In short, the aim of the work, as will be shown in the introductory chapter, is to

prove that secular changes of climate follow, as a necessary effect, from admitted physical agencies, and that these changes, in as far as the past climatic condition of the globe is concerned, fully meet the demand of the geologist.

The volume, though not intended as a popular treatise, will be found, I trust, to be perfectly plain and intelligible even to readers not familiar with physical science.

I avail myself of this opportunity of expressing my obligations to my colleagues, Mr. James Geikie, Mr. Robert L. Jack, Mr. Robert Etheridge, jun., and also to Mr. James Paton, of the Edinburgh Museum of Science and Art, for their valuable assistance rendered while these pages were passing through the press. To the kindness of Mr. James Bennie I am indebted for the copious index at the end of the volume, as well as for many of the facts relating to the glacial deposits of the West of Scotland.

JAMES CROLL.

EDINBURGH, March, 1875.