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In a glacial bed near Greenock, Mr. A. Bell found the fry of living Mediterranean forms, viz., Conus Mediterraneus and Cardita trapezia.

Although deposits containing shells of a temperate or of a southern type in glacial beds have not been often recorded, it by no means follows that such deposits are actually of rare occurrence. That glacial beds should contain deposits indicating a temperate or a warm condition of climate is a thing so contrary to all preconceived opinions regarding the sequence of events during the glacial epoch, that most geologists, were they to meet with a shell of a southern type in one of those beds, would instantly come to the conclusion that its occurrence there was purely accidental, and would pay no special attention to the matter.

Evidence derived from "Borings."-With the view of ascertaining if additional light would be cast on the sequence of events, during the formation of the boulder clay, by an examination of the journals of bores made through a great depth of surface deposits, I collected, during the summer of 1867, about two hundred and fifty such records, put down in all parts of the mining districts of Scotland. An examination of these bores shows most conclusively that the opinion that the boulder clay, or lower till, is one great undivided formation, is wholly erroneous.

These two hundred and fifty bores represent a total thickness of 21,348 feet, giving 86 feet as the mean thickness of the deposits passed through. Twenty of these have one boulder clay, with beds of stratified sand or gravel beneath the clay; twentyfive have two boulder clays, with stratified beds of sand and gravel between; ten have three boulder clays; one has four boulder clays; two have five boulder clays; and no one has fewer than six separate masses of boulder clay, with stratified beds of sand and gravel between; sixteen have two or three separate boulder clays, differing altogether in colour and hardness, without any stratified beds between. We have, therefore, out of two hundred and fifty bores, seventy-five of them representing a con

dition of things wholly different from that exhibited to the geologist in ordinary sections.

The full details of the character of the deposits passed through by these bores, and their bearing on the history of the glacial epoch, have been given by Mr. James Bennie, in an interesting paper read before the Glasgow Geological Society, to which I would refer all those interested in the subject of surface geology.

*

The evidence afforded by these bores of the existence of warm inter-glacial periods will, however, fall to be considered in a subsequent chapter.†

Another important and unexpected result obtained from these bores to which we shall have occasion to refer, was the evidence which they afforded of a Continental Period.

Striated Pavements.-It has been sometimes observed that in horizontal sections of the boulder clay, the stones and boulders are all striated in one uniform direction, and this has been effected over the original markings on the boulders. It has been inferred from this that a pause of long duration must have taken place in the formation of the boulder clay, during which the ice disappeared and the clay became hardened into a solid mass. After which the old condition of things returned, glaciers again appeared, passed over the surface of the hardened clay with its imbedded boulders, and ground it down in the same way as they had formerly done the solid rocks underneath the clay.

An instance of striated pavements in the boulder clay was observed by Mr. Robert Chambers in a cliff between Portobello and Fisherrow. At several places a narrow train of blocks was observed crossing the line of the beach, somewhat like a quay or mole, but not more than a foot above the general level. All the blocks had flat sides uppermost, and all the flat sides were striated in the same direction as that of the rocky surface through

* Trans. of the Geol. Soc. of Glasgow, vol. iii., p. 133. See also "Great Ice Age," chaps. xii. and xiii.

+ Chap. XXIX.

out the country. A similar instance was also observed between Leith and Portobello. "There is, in short," says Mr. Chambers, "a surface of the boulder clay, deep down in the entire bed, which, to appearance, has been in precisely the same circumstances as the fast rock surface below had previously been. It has had in its turn to sustain the weight and abrading force of the glacial agent, in whatever form it was applied; and the additional deposits of the boulder clay left over this surface may be presumed to have been formed by the agent on that occasion."*

Several cases of a similar character were observed by Mr. James Smith, of Jordanhill, on the beach at Row, and on the shore of the Gareloch.† Between Dunbar and Cockburnspath, Professor Geikie found along the beach, for a space of 30 or 40 square yards, numbers of large blocks of limestone with flattened upper sides, imbedded in a stiff red clay, and all striated in one direction. On the shores of the Solway he

found another example.+

The cases of striated pavements recorded are, however, not very numerous. But this by no means shows that they are of rare occurrence in the boulder clay. These pavements, of course, are to be found only in the interior of the mass, and even there they can only be seen along a horizontal section. But sections of this kind are rarely to be met with, for river channels, quarries, railway cuttings, and other excavations of a similar character which usually lay open the boulder clay, exhibit vertical sections only. It is therefore only along the sea-shore, as Professor Geikie remarks, where the surface of the clay has been worn away by the action of the waves, that opportunities have hitherto been presented to the geologist for observing them.

There can be little doubt that during the warm periods of the glacial epoch our island would be clothed with a luxuriant

*Edin. New Phil. Journ., vol. liv., p. 272.

"Newer Pliocene Geology," p. 129. John Gray & Co., Clasgow.
"Glacial Drift of Scotland," p. 67.

flora. At the end of a cold period, when the ice had disappeared, the whole face of the country would be covered over to a considerable depth with a confused mass of stones and boulder clay. A surface thus wholly destitute of every seed and germ would probably remain for years without vegetation. But through course of time life would begin to appear, and during the thousands of years of perpetual summer which would follow, the soil, uncongenial as it no doubt must have been, would be forced to sustain a luxuriant vegetation. But although this was the case, we need not wonder that now scarcely a single vestige of it remains; for when the ice sheet again crept over the island everything animate and inanimate would be ground down to powder. We are certain that prior to the glacial epoch our island must have been covered with life and vegetation. But not a single vestige of these are now to be found; no, not even of the very soil on which the vegetation grew. The solid rock itself upon which the soil lay has been ground down to mud by the ice sheet, and, to a large extent, as Professor Geikie remarks, swept away into the adjoining seas.* It is now even more difficult to find a trace of the ancient soil under the boulder clay than it is to find remains of the soil of the warm periods in that clay. As regards Scotland, cases of old land surfaces under the boulder clay are as seldom recorded as cases of old land surfaces in it. In so far as geology is concerned, there is as much evidence to show that our island was clothed with vegetation during the glacial epoch as there is that it was so clothed prior to that epoch.

* "Glacial Drift of Scotland," p. 12.

CHAPTER XVI.

WARM INTER-GLACIAL PERIODS IN ARCTIC REGIONS.

Cold Periods best marked in Temperate, and Warm Periods in Arctic, Regions. -State of Arctic Regions during Glacial Period.-Effects of Removal of Ice from Arctic Regions.-Ocean-Currents; Influence on Arctic Climate.Reason why Remains of Inter-glacial Period are rare in Arctic Regions. -Remains of Ancient Forests in Banks's Land, Prince Patrick's Island, &c. -Opinions of Sir R. Murchison, Captain Osborn, and Professor Haughton. -Tree dug up by Sir E. Belcher in lat. 75° N.

In the temperate regions the cold periods of the glacial epoch would be far more marked than the warm inter-glacial periods. The condition of things which prevailed during the cold periods would differ far more widely from that which now prevails than would the condition of things during the warm periods. But as regards the polar regions the reverse would be the case; there the warm inter-glacial periods would be far more marked than the cold periods. The condition of things prevailing in those regions during the warm periods would be in strongest contrast to what now obtains, but this would not hold true in reference to the cold periods; for during the latter, matters there would be pretty much the same as at present, only a good deal more severe. The reason of this may be seen from what has already been stated in Chapter IV.; but as it is a point of considerable importance in order to a proper understanding of the physical state of things prevailing in polar regions during the glacial epoch, I shall consider this part of the subject more fully.

During the cold periods, our island, and nearly all places in the northern temperate regions down to about the same latitude, would be covered with snow and ice, and all animal and vegetable life within the glaciated area would to a great

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