BOOKS. - WHEN a great theory has been built up upon
observations made by many different people at many different times and in many different places, and has grown into favour owing to the vigour of its advocates, the convenience of referring all diffi- culties to some one potent cause, and the impossibility of bringing it, as a whole, to the test of experiment, it is natural to find that the details of the evidence are often long left us.- -challenged. Every new observation pointing towards a similar interpretation takes its place in the long chain; while facts which seem to require a modification of the theory, or of its application to particular cases, are laid aside as having some- thing queer about them to be looked into by-and-by, but as inadmissible in the form in which they are stated, seeing that they are contradictory to the views founded on such a mass of evidence as has been collected in support of the theory.
When any competent person will take the trouble to formu- late each proposition which has been assumed to have been proved, and will fairly criticise the evidence in each case, in the light of all the new facts which may have been established, The Glacial Nightmare and the Flood a Second Appeal to Gammon-Sense from the Reratnigamoe of some Recent Geology. By Sir Henry H. Howorth, , M.P., P.R.S., Ste. London: Sampson, Low, and Co. 1893, it not unfrequently happens that the application of the theory to particular cases, if not the theory itself, has to be seriously modified.
This task has been undertaken and well performed by our author for that great group of phenomena to account for which the Glacial Theory has been evolved. This expression is, however, not very useful, as it, like the words" orthodoxy" and "heterodoxy," conveys quite a different idea to different persons. After an exhaustive criticism of the views of previous writers, our author has put forward his own conclusions on some of the points raised. But it would be an unfair estimate of the book to suppose that its value would be destroyed if some of these positive expressions of opinion should prove to be not well supported ; for the scope of the work is much wider than the mere exposition of the author's own personal convictions. He holds that the advocates of the extreme glaciation of the Northern Hemisphere in comparatively recent times have pushed their theories to extravagant lengths, and that the action of ice must be greatly limited, whereas an examination of the various facts has led him to attach much more importance to the evidence of a great submergence during the glacial epoch ; and further, to consider that it was of such a sudden nature as to produce floods to which he would attribute many of the phenomena of the drift. He examines most of the collateral theories which have grown up round the central idea of circumpolar glaciation ; and, in working out his plan, has produced a small encyclopedia of glacial literature, which must be in the bands of any one who ever again writes on the subject.
The facts on which theories are founded are slowly gathered. Sedgwiek in the first quarter of the century wrote :—" As we are unacquainted with the forces which put the diluvial waters in. motion, we are also, with very limited exceptions, unable to determine the direction in which the currents have moved over the earth's surface. Many parts of the North of Europe seem to have been swept over by a great current . from the north. In some parts of Scotland there has been a great rush of water from the north-west It may, perhaps, be laid down that the diluvial gravel has been drifted down all the great inclined planes which the earth's surface pre- sented to the retiring waters,' Here was the diluvial theory. Years afterwards Sedgwick said, in one of his lectures, "I met an old gentleman at Keswick, and he said there was no way of accounting for these boulders except by icebergs from Norway. I didn't believe a word of it then ; it was a marvel- lous fact, but I was a young man." Hero was the iceberg theory. He went on to say, I recollect going with the poet Wordsworth one day, and, looking down over the valley above Rydal, we saw the traces of the ancient glaciers. I said, 'You can see the SCrabbings and markings of the ancient glaciers.' Wordsworth called the rounded bosses sons of the earth," skulls of giants.'" Sedgwick had now accepted the view that ice had once filled the valleys of the North of England. He illustrated its formation from the snow of the nenTO with his usual felicity. "When I was at school," said he in the same lecture, "many of my schoolfellows had wooden shoes, with a piece of iron underneath like a horse-shoe. In this the snow gathered and hardened. When we pelted one another with snowballs we reckoned it unfair to use this hard snow. I have seen transparent ice formed in this way."
Our author takes us more fully and slowly through this development of theory, and first quotes the opinions of those who saw in the boulders and the drift evidence of the passage of great bodies of water across the land. The line of reason- ing they followed was, as far as it went, quite sound. Immense boulders were, as a matter of fact, transported by floods, and, as a matter of calculation, the deeper the water and the greater its velocity, the greater its transporting power. Such floods had occurred. Rapidly melting snow, waters pent up by landslips and suddenly let loose on the valley, or rushes of water such as earthquake-waves, were all invoked either in explanation or illustration. Here, it was argued, scattered over the surface of the country, was the evidence that similar floods had swept the land. The material was distributed just as it was found to be after such torrents as there had been opportunity of observing. The mistake was that the different kinds of drift had not been discriminated, and it had not been appreciated that one explanation was insufficient to account for all the phenomena observed in connection with them.
A great deal of the early work of establishing a scientific theory is done by men who collect evidence in support of some working hypothesis ; and therefore, when we are criticising the various tentative explanations put forward daring the progress of research among any group of phenomena for which there were not yet sufficient data either in respect of the facts to be explained, or the modus operandi of Nature in the production of similar results, we must take account of the bias of thought and the state of science generally at the time. Did giants hurl those blocks at one another in anger or in play P—everybody knew that there were giants in those days. Did a vast body of water suddenly sweep the face of Nature P— there was the highest authority that such a flood had occurred in recent times.
Among those, however, who thought that such deluges had occurred, there were soon found some whose acquaintance with Arctic conditions led them to suggest that it would be easier to explain the positions in which some of the blocks were perched if we supposed that ice had floated though the flood had carried them :— " When it began to be seen, therefore, that it is impossible to account for the transport and distribution of the boulders, and to explain their associated phenomena by a reference to water acting in its usual way, and that, especially in North Germany and in other countries remote from mountains and impetuous rivers, we must, if we appeal to water at all, have recourse to its action in a quite abnormal way, men began to turn elsewhere to find some explanation which should not involve other than forces actually at work and working in a normal way. It was natural that those living near the Baltic and in North America, who had opportunities of noticing the tremendous force exercised by shore-ice and by floating icebergs, should turn to these causes and supplement them by theories of widespread submergence, which seemed also warranted by the finding of marine shells at a considerable eleva- tion in many places."
Various theories were started to account fer the floods to float the ice-rafts. It is interesting to note that the greater obliquity of the ecliptic was suggested in 1784 by St. Pierre, who thought that the consequent melting of the snow at the pole caused the flood which produced the phenomena of the drift and the mixture of northern and southern forms of life found in some of the Pleistocene deposits. Others shifted the axis of the earth to account for the supposed violent movement in the watery envelope that surrounds it. But facts, founded on accurate observations, were accumulating fast. Darwin explained the transport of the boulders which he observed in South America by reference to icebergs, men- tioning, as one of the probable modes of transport, the descent of glaciers with boulders on them down to the sea, where they break off as icebergs. Our author gives the result of his own observations also, in South America, in confirmation of the wide distribution of drift and boulders. This, then, was certainly a vera causa, for icebergs had been observed large enough to transport any of the materiai of the drift. They travelled straight on, or were deflected by winds and currents. On grounding, they would contort the previously deposited drifts, or grind the rocls s below them. On melting, they would drop boulders and smaller debris where they stuck.
But it had long been noticed that the ice of glaciers advanced and retreated with the seasons. What if it could be shown that the glaciers of Northern Europe once advanced much further, so as to debouch on the level ground at the foot of the valley, or even so as to coalesce with others, and advance far over the plains Playfair in 1802 saw in the glacier an agent which could transport masses of rook over intervening hills and valleys, and drop them at its terminus on any prominence, or in any hollow, or even, as Venetz held, might carry them across the great Swiss valley, and perch the boulders of Mont Blanc upon the Jura. So the various canoes of the drifts and the shire were being made out, and Agassiz came to England. Its Boulder Clay differed considerably from the glacial deposits with which he was most familiar on the Continent, but the general drift. phenomena of Scotland seemed to him to point to the former existence of glaciers in the mountains of Great Britain, though he thought the drift of East Anglia should be referred to iceberg action. From this time, it was generally received that the glaciers of Western Europe were once much more exten- sive than they now are, and that the erratic blocks and debris are largely the wreckage of former moraines modified, it might be in many cases, by the action of glacier-streams, or of the sea. In America, similar phenomena were similarly explained. Buckland, the author of the ReliquiFe Diluvianw, was won over to the glacier idea ; but Whewell saw the flaws in the reasoning as then presented, and thought the theory incom- plete, because it did not explain the wide areas covered with drift in countries differing in physical features, climate, Sea., nor the occurrence of the remains of marine animals found asso- ciated with the drift, because, in short, he realised that the drifts were of various age and origin, and could not be all explained, by any single hypothesis such as that put forward.
So our author carries us through the development of these various theories. He then goes on to show how similar explanations were pushed to unwarrantable lengths, in such hypotheses as that of circumpolar ice-sheets, and how other extreme views were grafted on to the original theory. To these we shall return in a future notice.
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