« 이전계속 »
primeval rocks that modern interest and controversy are concentrated, as upon a comparatively quite recent formation. The glacial epoch, of which every one has heard, in geological chronology claims to be modern. It may have
commenced some 200,000 years ago, coming down perhaps to within 50,000 years of the present time. Indeed, 'glaciers may have lingered among the mountains, and
occupied some of the valleys down to a much more recent * period. But the date and duration are of less importance to the scenery of England than the abiding effects attributed to this epoch. Of these Lord Avebury observes that the (most extensive deposits due to ancient glaciers are the • enormous sheets of “Drift,” which, as far south as
the Thames valley, cover a great part of the country, with • the exception of the highest mountain-tops. The evidences of the former glaciation of this island are discussed at considerable length, and the details will be read with great interest. Nevertheless, it is not likely that this part of the volume will pass entirely unchallenged. After adducing various facts of importance, the author says :
" These converging lines of evidence prove that in the period of greatest cold Northern Europe, over an area of from 700,000 to 800,000 square miles, was buried under a vast sheet or mantle of ice, which was thickest in the north and west. Over parts of Scandinavia it was probably not less than 6,000 feet in thickness, in North-West Scotland over 3,000; the tops of the Cheviots and the hill-tops of the West Riding 2,300 feet high are distinctly glaciated, as is also Westdale Crag, near Shap, 1,600 feet, when the ice gradually thinned away to the south and east.' In face of these remarkable figures it is impossible to forget that not so long ago a book was published by Sir Henry Howorth, F.R.S., entitled “The Glacial Nightmare and the • Flood. Neither Sir Henry nor his book receives the slightest notice in the present volume. It seems a pity that a man so well known, so burningly in earnest, so ready to exchange buffets with all comers, should have his opinions and arguments only met indirectly. It is not unknown that he has the saving grace of a genial humour, and that he can treat with candour his own errors as well as those of his opponents. Science, in the mouths of its most nobly endowed masters, has not been in the past so free from gross and obstinate mistakes that it can afford to meet with a conspiracy of silence any disciplined disputant. In dealing with the Glacial Period, Lord Avebury appears to give a general assent to the celebrated theory of Dr. Croll, although
he remarks, 'I ought to add that these views, though supported by Sir R. Ball and other high authorities, are not
üniversally adopted. This will certainly seem a wonderfully mild way of stating the case to any one who has read ' A * Criticism of the Astronomical Theory of the Ice Age,' by E. P. Culverwell, M.A., Fellow of Trinity College, Dublin, in the January and February numbers of the Geological • Magazine' for 1895. Mr. Culverwell claims to show that Croll's argument is absolutely unsound, and that its position is made worse instead of better by the amendment of it that has been attempted. He even says, “It is not unlikely that 'there never was an Ice Age, but that there bave been at
times various local glaciations such as we now see in • Greenland. To the troubled doubt in which the whole subject is still involved Lord Avebury does indeed point the finger by an amusing quotation from the geological expert, Mr. Horace B. Woodward, F.R.S., who says :
• After spending about a year in Norfolk, I began to believe I knew all about the drifts, but during the following seven years of my sojourn in that county, as I moved from place to place, I somehow seemed to know less and less, and I cannot say what would have been the result, but fortunately the geological survey of the county came to an end.
Away from the arena of controversy the general reader, as distinguished from the specialist, will find the present volume rich in things worth knowing, in explanations worth having, and in matters of observation that will put the observing faculty on the alert. There are not a few who may like to learn why it is that Britannia rules the waves, apart from those personal qualities of her sons which our well known and universally eulogised national modesty prevents us from specifying. This marine dominion, then, is in part attributable to the excellence of our harbours. This excellence in turn is due to the character of our coast. For our principal harbours are in the mouths of rivers, just the very places to be blocked and made useless by bars of shifting sand, were it not that round our coasts to a great extent these inconvenient materials are removed by tidal currents. Thereby the sea itself stands our friend, and though its tempestuous billows often bombard our shores, crumble away our cliffs, and in other ways make encroachments on our territory, it is here effectively shown that rain and rivers far exceed the ocean waves as forces of denudation. The courses which our rivers pursue are explained as in many cases due to an involved history. Branches of the
VOL. CXCVI. NO. COUVI.
like to learty on the alepservation that nations wort
and upecifying excellenaracter of rivers, bars
universally e This marineul harbourst. For
,, to be blocare in the character our harbon, then, in
on their malagrant acts of
innocent-seen. They stretc
Thames, for instance, cut through hills which they could never have been tempted to attack in the present configuration of the country. Since we know from our school studies that a single drop continually repeated hollows a stone, it is easy to acknowledge that the perpetual flow of a river may deepen its bed, but the consideration is less obvious that streams are capable of eating their way backwards. Yet, just as the Falls of Niagara are slowly but surely receding by wearing away the hard rocks over which their waters descend, so will the little runnels in which a river begins gradually recede. They cannot be content with the tiny groove which they first carve. As it deepens the power of the current increases, and rain will wash into it materials not only from the sides, but from the rear. Even if a stream starts from the highest part of a watershed, by degrees that summit succumbs to atmospheric agencies, and the stream obtains a continually widening platform for its recession. From this results a singular corollary. By their capacity for moving stealthily backwards rivers are enabled to poach on their neighbours' preserves. Proofs, indeed, are given of the most flagrant acts of piracy. Look on this picture and on that. In the first, two innocent-seeming rivers are seen flowing to the coast equal and parallel. They stretch out affluents each towards the other as if for fraternal handshaking. In the companion picture the sequel is seen. There has not been a friendly embrace, but a wrestling for supremacy. The affluents of one river have succeeded in tapping the head waters of the other and diverting its streams, so that the vanquished competitor has dwindled to a brook, and may die of inanition. Actual examples are cited from our south coast, in that the Lavant, which like • the Adur may be called a beheaded river, has been reduced
to quite a small stream, in dry weather even to a succession of pools ; the Chichester estuary also, which was evidently once the mouth of a large river, is now a comparatively wide
valley without any river at all. It will be a shock to Oxford men to learn that the Ouse is gradually stealing
towards the Cherwell, and if allowed to work its way back for little more than a mile it will carry off the upper half of the Cherwell area, detach it from the Thames, and 'annex it to the basin of the Ouse.' The scandalous treatment to which the Thames has been exposed in another direction is almost too painful to write about. For ages to all appearance the Severn has been marauding upon it so outrageously that not the maiden gentleness of Sabrina, but her enragéd stepdame Gwendolen’ should have been chosen to preside over those brigand waters.
Englishmen are so familiar with the coast cliffs, in cutting which the sea has obviously played a great part, that they readily extend marine action to the formation of inland cliffs. With science enough to tell him that our land has been repeatedly under water, a man has only to use his eyes to be assured that such and such an escarpment represents the margin of an ancient sea. It is disappointing when the teachings of science and the evidence of our senses mischievously combine to put us in the wrong. Only when we drink a little deeper of the Pierian spring, the scales fall from our eyes, and our vision changes its mind because mind has changed our vision. Lord Avebury is at great pains to show convincingly the difference between the chalk cliffs of our coast and our chalk escarpments. He points out, to begin with, what certainly would not occur to every one, that it is not enough for a chalk escarpment to resemble any kind of coast, irrespective of what that kind may be. To establish its claim to be an original sea cliff, it must resemble a coast of one particular kind, namely, a chalk coast. In that case, its frontage should be fairly straight; its base must follow the sea level; the upper edge of its cliffs will be likely to have an undulatory outline; the land is not unlikely to be found rising behind it. But in fact a chalk escarpment is much indented by coombes and valleys ; it is the base line that is given to undulating, while the upper edge remains for long stretches fairly level, and instead of being dominated by any superior elevation, it is nearly always the highest ground in the neighbourhood. To these contrasts is added the general fact that, while sea cliffs . pass from one rock to another, escarpments always keep to
one geological formation. They owe their structure not to the winds and waves of a tidal sea, but to the subterranean forces of upheaval. In the wrinkling of the earth's crust the strata at the convex folds will be stretcbed and strained, while in the concave folds they will be compressed. At the weakest points they will sooner or later give way, and in so doing offer facilities for the work of denudation. A paradoxical result often follows. The original mountain, being formed of loosened materials and by its very eminence exposed to destructive agencies, is by degrees worn down. But the original valley, more solid and more sheltered, bides its time, and is at length converted into a mountain, not so much uplifted as left. Some hills retain in cup-shaped summits a reminiscence of this kind of origin. Thus is explained the local name of the Saddle Back mountain near Keswick, and the saucer-shaped or synclinal arrangement
of the strata' on Snowdon, of which Lord Avebury observes, 'It is indeed a remarkable and interesting fact that “the rocks forming the highest spot in South Britain should
once have been the bottom of a valley.' After pointing out that the so-called “Peak' of Derbyshire is really a tableland, he continues, “It is indeed a “cup” rather than a « « peak," for it forms a flattened basin, the beds on all sides dipping into the hill: the pressure and consequent hardness thus produced bas probably led to the preservation of this portion of the grit, which was no doubt originally continuous with the corresponding beds to the east and west.' A due sense of the slowness with which many results in Nature are produced might tempt a man to think that the loftiest mountains must be the oldest. He will then be surprised to hear that in comparison with the Highlands of Scotland, the Lake district, and the Welsh hills, the Alps and • Himalayas are but of yesterday. It takes longer to lift a mountain to the skies, and then to wear it down again to the sea from whence it rose, than to do the lifting only. When one comes to think of it, that is self-evident. The provoking thing is for most of us that we only come to think of the self-evident when some one else has forced it upon our notice.
That all the statements and arguments should be novel in a book like this, the subject essentially forbids. Acknowledgements to a host of well-reputed authors help us to a general confidence in the conclusions, which in so various a discussion no effort of a single mind could produce. The wide and almost limitless range of topics which the title of the volume might be made to embrace will no doubt inspirit ambitious and critical readers to think of some points overlooked, neglected, or withheld. Of anything really germane to the matter thus omitted they will not easily find examples, or after seeming to find them, on a second and more careful reading of the book, they will perceive by a line here, a paragraph there, or sometimes by a single word, that this or that point has not been forgotten. Of incidental information there is no dearth. It is given, so to say, into the bargain, with a lavish hand out of an overflowing treasury. The reader is told, for example, the difference between the grains of shore sand and desert sand, and how the difference is caused. The golfer learns the origin of his