ice-sheet or such a deposit as is now accumulating on the Great Bank of Newfoundland-must be determined by the teaching of other facts, namely, by what can be learnt from regions the history of which is practically beyond doubt. With this end in view, I have begun by giving a sketch of some such regions, and then have passed on to describe the phenomena which are the subject of more dispute. Here, in order to avoid the risk of misunderstanding the words of others, I have selected, as far as possible, cases which I have personally examined; these, however, so far as I have been able to ascertain from the literature of Glacial Geology, are fair samples of the whole series. To the descriptions-necessarily rather brief-I have appended a short statement of the interpretations which have been proposed, and have pointed out where they appear to me strong and where weak. In other words, I have endeavoured to follow the example of a judge rather than of an advocate; that is, to sum up the evidence on each side of a case, and leave the verdict to the jury. Like any such official, I have my own view as to what that verdict should be, and this doubtless will be disclosed to those who can read between the lines, but I may claim that it has not been formed hastily or without experience. I saw a glacier for the first time in 1856; my earlier geological papers dealt mainly with ice and its work, and though for nearly twenty years I have written mostly on petrological subjects, I have never obeyed the well-known dictum and wholly cast off the old love. In the later part of the book a method is indicated by which, I believe, we can approximate to the temperature at various localities during the Glacial Epoch, and the different explanations of this widespread refrigeration are stated and briefly discussed. To account for it, seems to me, in the present state of our knowledge, the most perplexing of all the problems which this Epoch presents. Glacial geology is a large subject: the volumes of this series are comparatively small. Hence, in order to keep the present one within the prescribed limits, it has been necessary to pass rapidly over some points of considerable interest, and to omit others which have a less direct connection with an ice age. For instance, nothing is said of the coarse gravels on some upland plateaux and in certain river valleys, though many of them almost undoubtedly were formed when the climate was distinctly colder than it is at the present time, and some probably fall within the Glacial Epoch. But as their connection with ice is less obvious and the Glacial Epoch did not begin with a particular year, I have dealt only with those deposits to which it can lay an undisputed claim. For the same reason nothing has been said either as to the physics of ice or as to the relation of Man to the Glacial Epoch; each of these topics having already formed the subject of a volume in this series. A few of the illustrations are from my own rough sketches. For the photograph of the ice-worn rocks near the Grimsel I am indebted to my friend, Mr. J. Eccles, who took it when we were travelling together in Switzerland. I have also to thank other friends for kind help, among them Dr. Du Riche Preller for aiding me in examining the deposits near Zurich, the Rev. E. Hill for co-operation in visiting those of Eastern England, and Miss C. A. Raisin for reading the first proofs of this book. |