which have been washed away from all their other sides. Every larger stone in a water-course, after the torrent fed by a thunder-shower has just subsided, shows, on the same principle, its trail of sand and shingle piled up behind it,— sand and shingle which it kept from being swept away; and the simple effect, when it occurs on the large scale, is known to the geologist as the phenomenon of 'Crag and Tail.' The rock upon which Edinburgh Castle stands, existing as the 'crag,' and the sloping ridge which extends from the castle's outer moat to Holyrood, existing as the tail, may be cited as a familiar instance. We find the same phenomenon repeated in the Calton Hill, and in various other eminences in the neighbourhood; as also in the Castle Hill of Stirling. And in all these, and many other cases, the tail which the crag protected is turned towards the east, indicating that the current which in the lapse of ages scooped out the valleys at the sides of the protecting crags, and in many instances formed, by its eddies, hollows in advance of them, just as we find hollows in advance of the larger stones of the water-course of my illustration, was a current which flowed from the west. The testimony of the ice-grooved rocks, and of the eminences composed of crag and tail, bear, we see, in the same line.

Now, this westerly direction of the current seems to be exactly that which, reasoning from the permanent phenomena of nature, might be premised. There must have been trade winds in every period of the world's history, in which the earth revolved from west to east on its axis; and with trade winds the accompanying drift current. And, of consequence, ever since the existence of a great western continent, stretching far from south to north, there must have been also a gulf stream. The waters heaped up against the coasts of this western continent at the equator by the drift current ever flowing westwards, must have been always, as now, returning eastwards in the temperate zone,

to preserve the general level of the ocean's surface. Ever, too, since winter took its place among the seasons, there must have been an arctic current. The ice and snows of the higher latitudes, that accumulated during the winter, must have again melted in spring and early summer; and a current must in consequence have set in as the seasons of these came on, just as we now see such a current setting in in these seasons in both hemispheres, which bears the ice of the antarctic circle far towards the north, and the ice of the arctic circle far towards the south. The point at which, in the existing state of things, the gulf stream and the arctic current come in contact is that occupied by the great bank of Newfoundland; and by some the very existence of the bank has been attributed to their junction, and to the vast accumulation of gravel and stone cast down year after year from the drift ice to the bottom, where these two great tides meet and jostle. Be this as it may, the number of boulders and the quantity of pebbles and gravel strewed over the bottom of the western portions of the Atlantic, in the line of the arctic current, from the confines of Baffin's Bay up to the forty-fifth degree of north latitude, must be altogether enormous. Captain Scoresby counted no fewer than five hundred icebergs setting out on their southern voyage on the arctic current at one time. And wherever there are shallows on which these vast masses catch the bottom, or grate over it, shallows of from thirty to a hundred fathoms water, -we may safely premise that at the present time there is a boulder-clay in the course of formation, with a scratched and polished surface of rock lying beneath it, and containing numerous pebbles and boulders striated longitudinally. That the point where the gulf and arctic currents come in contact should now lie so far to the west, is a consequence of the present disposition of the arctic and western continents,— perhaps also of the present position of the magnetic pole. A different arrangement and position would give a different

point of meeting; and it is as little improbable that they should have met in the remote past some two or three hundred miles to the west of what is now Scotland, as that in the existing period they should meet some two or three hundred miles to the east of what is now Newfoundland. The northern current would be deflected by the more powerful gulf stream into an easterly course, and would go sweeping over the submerged land in the direction indicated by the grooves and scratches, bearing with it, every spring, its many thousand gigantic icebergs, and its fields of sheet-ice many hundred square miles in extent. And these, armed beneath with great pebbles and boulders, or finding many such resting at the bottom, by grinding heavily along the buried surface,-like the rafts of my illustration along the bed of the river,—would gradually wear down the upper strata of the softer formations, leaving the clay which they had thus formed to be deposited over, and a little to the east of, the rocks that had produced it. It is further in accordance with this theory, that in Scotland generally, the deeper deposits of the boulder-clay occur on the eastern line of coast. The cutler, in whetting a tool with water on a flat Turkey stone, drives the grey milky dressings detached by the friction of the steel from the solid mass, to the end of the stone furthest from himself, and there they accumulate thick in the direction of the stroke. And so it is here. The rubbings of the great Scotch whetstone, acted upon by the innumerable gravers and chisels whetted upon it, and held down or steadied by the icebergs, have been carried in the easterly direction of the stroke, and deposited at the further, that is to say, the eastern, end of the stone.

But fearing I have already too much trespassed on your time and patience, I shall leave half told for the present the story of the Pleistocene period in Scotland. If, instead of presenting it to you as a piece of clear, condensed narrative, I have led you darkly to grope your way through it by a

series of fatiguing inductions, you will, I trust, sustain my apology, when I remind you that this dreary ice-epoch in the history of our country still forms as debatable a terra incognita to the geologist as the dreary ice-tracts which surround the pole do to the geographer. We have been threading our twilight way through a difficult North-West Passage; and if our progress has been in some degree one of weariness and fatigue, we must remember that without weariness and fatigue no voyager ever yet explored

'The ice-locked secrets of that hoary deep
Where fettered streams and frozen continents
Lie dark and wild, beat with perpetual storm
Of whirlwind and dire hail.'

'We might expect,' says Professor Sedgwick, 'that as we come close upon living nature, the characters of our old records would grow legible and clear. But just where we begin to enter on the history of the physical changes going on before our eyes, and in which we ourselves bear a part, our chronicle seems to fail us; a leaf has been torn out from Nature's book, and the succession of events is almost hidden from our eyes.' Now it is to this age of the drift-gravels and the boulder-clay that the accomplished Professor here refers as represented in the geologic record by a torn page ; and though we may be disposed to view it rather as a darkened one, much soiled, but certainly not wanting,-we must be content to bestow on its dim, half-obliterated characters, more time and care than suffice for the perusal of whole chapters in the earlier books of our history. And so, casting myself on your forbearance, I shall take up the unfinished story of the Pleistocene period in Scotland in my next address.

LECTURE SECOND.

Problem first propounded to the Author in a Quarry-The Quarry's Two Deposits, Old Red Sandstone and Boulder-Clay-The Boulder-Clay formed while the Land was subsiding-The Groovings and Polishings of the Rocks in the Lower Parts of the Country evidences of the fact--Sir Charles Lyell's Observations on the Canadian Lake District-Close of the Boulder-Clay Record in Scotland-Its Continuance in England into the Pliocene Ages-The Trees and Animals of the PreGlacial Periods-Elephants' Tusks found in Scotland and England regarded as the Remains of Giants-Legends concerning them-Marine Deposits beneath the Pre-Glacial Forests of England-Objections of Theologians to the Geological Theory of the Antiquity of the Earth and of the Human Race considered-Extent of the Glacial Period in Scotland-Evidences of Glacial Action in Glencoe, Gareloch, and the Highlands of Sutherland-Scenery of Scotland owes its Charac teristics to Glacial Action-The Period of Elevation which succeeded the Period of Subsidence-Its Indications in Raised Beaches and Subsoils-How the Subsoils and Brick Clays were formed-Their Economic Importance-BoulderStones interesting Features in the Landscape--Their prevalence in ScotlandThe more remarkable Ice-travelled Boulders described-Anecdotes of the 'Travelled Stone of Petty' and the Standing-Stone of Torboll-Elevation of the Land during the Post-Tertiary Period which succeeded the Period of the Boulder-Clay-The Alpine Plants of Scotland the Vegetable Aborigines of the Country-Panoramic View of the Pleistocene and Post-Tertiary PeriodsModern Science not adverse to the Development of the Imaginative Faculty.

I REMEMBER, as distinctly as if I had quitted it but yesterday, the quarry in which, some two-and-thirty years ago, I made my first acquaintance with a life of toil and restraint, and at the same time first broke ground as a geologist. It formed a section about thirty feet in height by eighty or a hundred in length, in the front of a furze-covered bank, a portion of the old coast line; and presented an under bar of a deep-red sandstone arranged in nearly horizontal strata, and an upper bar of a pale-red clay roughened by projecting pebbles and boulders. Both deposits at the time were almost equally unknown to the geologist. The deep-red

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