[7] The reader who wishes to enter more fully into the geological effects of icebergs, should consult the suggestive section on that subject in De la Beche's Geological Observer; also the Principles and Visit to the United Stales of Sir Charles Lyell, with the various authorities referred to by these writers.
It is pleasant to mark, when once the true solution of a difficulty is obtained, how all the discordant elements fall one by one into order, and how every new fact elicited tends to corroborate the conclusion. In some parts of the glacial beds, there occur regular deposits of shells which must have lived and died in the places where we find them. From ten to fifteen per cent, of them belong to species which are extinct, that is to say, have not been detected living in any sea. Some of them are still inhabitants of the waters around our coasts, but the large majority occur in the northern seas. They are emphatically northern shells, and get smaller in size and fewer in number as they proceed southward, till they disappear altogether. In like manner, the palm, on the other hand, is characteristically a tropical plant. It attains its fullest development in intertropical countries, getting stunted in its progress towards either pole, and ceasing to grow in the open air beyond the thirty-eighth parallel of latitude in the southern hemisphere, and the forty-fifth in the northern. So, too, the ivy, which in our country hangs out its glossy festoons in every woodland, and around the crumbling walls of abbey, and castle, and tower, is nursed in the drawing-rooms of St. Petersburg as a delicate and favourite exotic. In short, the laws which regulate the habitat of a plant or an animal are about as constant as those which determine its form. There are, indeed, exceptions to both. We may sometimes find a stray vulture from the shores of the Mediterranean gorging itself on sheep or lambs among the wolds of England,[8] just as we often see
"A double cherry seeming parted,
But yet an union in partition;"
or as we hear of a sheep with five legs, and a kid with two heads. But these exceptions, from their comparative rarity, only make the laws more evident. When, therefore, we find, in various parts of our country, beds of shells in such a state of preservation as to lead us to believe that the animals must have lived and died where their remains are now to be seen, we justly infer that the districts where they occur must at one period have been submerged. If the shells belong to fresh-water species, it is plain that they occur on the site of an old lake. If they are marine, we conclude that the localities where they are found no matter how high above the sea must formerly have stood greatly lower, so as to form the ocean bed. To proceed one step further. If the shells are of a southern type, that is, if they belong to species[9] which are known to exist only in wanner seas than our own, we pronounce that at a former period the latitudes of Great Britain must have enjoyed a more temperate and genial climate, so as to allow southern shells to have a wider range northwards. If, on the other hand, they are of an arctic or boreal type, we in the same way infer that our latitudes were once marked by a severer temperature than they now possess, so as to permit northern shells to range farther southwards. This reasoning is strictly correct, and the truth involved forms the basis of all inquiries into the former condition of the earth and its inhabitants.
[8] Two of these birds (Neopron pecnopterus) are stated to have been seen near Kilve, in Somersetshire, in October 1825. One was shot, the other escaped.
[9] There is not a little difficulty in reasoning satisfactorily as to climatal conditions, from the distribution of kindred forms. Even in a single genus there may be a wide range of geographical distribution, so that mere generic identity is not always a safe guide. Thus, the elephant now flourishes in tropical countries, but in the glacial period a long-haired species was abundant in the frozen north. I have above restricted myself entirely to species whose habits and geographical distribution are already sufficiently known.
The evidence furnished by the northern shells in the boulder-clay series is, accordingly, of the most unmistakable kind. These organisms tell us that at the time they lived our country lay sunk beneath a sea, such as that of Iceland and the North Cape, over which many an iceberg must have journeyed, and thus they corroborate our conclusions, derived independently from the deep clay and boulder beds and the striated rock-surfaces, as to the glacial origin of the boulder-clay.
CHAPTER III.
How the boulder came to be one "Crag and tail"—Scenery of central Scotland: Edinburgh—"Crag and tail" formerly associated in its origin with the boulder-clay—This explanation erroneous—Denudation an old process—Its results—Illustration from, the Mid-Lothian coal-field—The three Ross-shire hills—The Hebrides relics of an ancient land—Scenery of the western coast—Effects of the breakers—Denudation of the Secondary strata of the Hebrides—Preservative influence of trap-rocks—Lost species of the Hebrides—Illustration—Origin of the general denudation of the country—Illustrative action of streams—Denudation a very slow process—Many old land-surfaces may have been effaced—Varied aspect of the British Islands during a period of submergence—Illustration.