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sandstone beneath formed a portion of that ancient Old Red system which represents, as is now known, the second great period of vertebrate existence on our planet, and which has proved to the palæontologist so fertile a field of wonders : the pale clay above was a deposit of the boulder-clay, resting on a grooved and furrowed surface of rock, and containing in abundance its scratched and polished pebbles. Old Red Sandstone and boulder-clay! a broad bar of each ;--such was the compound problem propounded to me by the Fate that dropped me in a quarry; and I gave to both the patient study of years. But the older deposit soon became frank and communicative, and yielded up its organisms in abundance, which furnished me with many a curious little anecdote of their habits when living, and of the changes which had passed over them when dead; and I was enabled, with little assistance from brother geologists, to give a history of the system to the world more than ten years ago. The boulder-clay, on the contrary, remained for years invincibly silent and sullen. I remember a time when, after passing a day under its barren scaurs, or hid in its precipitous ravines, I used to feel in the evening as if I had been travelling under the cloud of night, and had seen nothing. It was a morose and taciturn companion, and had no speculation in it. I might stand in front of its curved precipices, red, yellow, or grey (according to the prevailing colour of the rocks on which it rested), and might mark their water-rolled boulders of all kinds and sizes sticking out in bold relief from the surface, like the protuberances that roughen the rustic basements of the architect ; but I had no 'Open, Sesame' to form vistas through them into the recesses of the past. And even now, when I have, I think, begun to understand the boulder-clay a little, and it has become sociable enough to indulge me with occasional glimpses of its early history in the old glacial period, glimpses of a half-submerged land, and an icebergmottled sea, turbid with the comminuted débris of the rocks below, you will see how very much I have had to borrow from the labours of others, and that in worming my way into its secret, there are obscure recesses within its precincts into which I have failed to penetrate. Let us now, however, resume its half-told story.
There are appearances which lead us to conclude, that during the formation and deposition of the boulder-clay, what is now Scotland was undergoing a gradual subsidence--gradually foundering amid the waves, if I may so speak, like a slowly-sinking vessel, and presenting, as century succeeded century, hills of lower and yet lower altitude, and an ever lessening area. I was gratified to find, that when reasoning out the matter for myself, and arriving at this conclusion from the examination of one special set of data, Mr. Charles Darwin was arriving at the same conclusion from the consideration of a second and entirely different set; and Sir Charles Lyell,—from whom, on the publication of my views in the Witness newspaper some four years since, I received a kind and interesting note on the subject,—had also arrived at the same conclusion-North America being the scene of his observations—from the consideration of yet a third and equally distinct set. And in the Geological Journal for the present year, I find Mr. Joshua Trimmer and Mr. Austin arriving, from evidence equally independent, at a similar finding. We have all come to infer, in short, that previous to the Drift period the land had stood at a comparatively high level,- perhaps higher than it does now; that ages of depression came on, during which the land sank many hundred feet, and the sea rose high on the hill-sides; and that during these ages of depression the boulder-clay was formed. Let me state briefly some of the considerations on which we found.
The boulder-clay, I thus reasoned with myself, is generally found to overlie more deeply the lower parts of the country than those higher parts which approach its upper limit; and
yet the rocks on which it rests, in some localities to the depth of a hundred feet at even the level of the sea, bear as decidedly their groovings and polishings as those on which, eight hundred feet over the sea level, it reposes to but the depth of a yard or two. Now, had a rising land been subjected piecemeal to the grinding action of the icebergs, this would not have been the case. The higher rocks first subjected to their action would of course bear the groovings and furrowings; but the argillaceous dressings detached from them in the process, mixed with the stones and pebbles which the ice had brought along with it, would necessarily come to be deposited in the form of boulder-clay on the lower rocks; and ere these lower rocks could be brought, by the elevation of the land, within reach of the grinding action of the icebergs, they would be so completely covered up and shielded by the deposit, that the bergs would fail to come in contact with them. They would go sweeping, not over the rocks themselves, but over the clay by which the rocks had been covered up; and so we may safely infer that, had the boulderclay been formed during an elevating period, the lower rocks, where thickly covered by the clay, would not be scratched and grooved as we now find them, or, where scratched and grooved, would not be thickly covered by the clay. The existing phenomena, deep grooves and polished striæ, on rocks overlaid at the present sea-level to a great depth by the boulder-clay, demand for their production the reverse condition of a sinking land, in which the lower rocks are first subjected to the action of the icebergs, and the higher rocks after them. The quarrier, when he has to operate on some stratum of rock on a hill-side, has to commence his labours below, and to throw the rubbish which he forms behind him, leaving ever an open face in front; for, were he to reverse the process, and commence above, the accumulating débris, ever seeking downwards, would at length so choke up the working as to arrest his labours. And such, we infer from
the work done, must have been the course of operations imposed by the conditions of a sinking land on the icebergs of the glacial period : they began their special course of action at the hill-foot, and operated upon its surface upwards as the sea arose. Again, Mr. Darwin's reasonings were mainly founded on the significant fact, that in numerous instances travelled boulders of the ice period may be found on levels considerably higher than those of the rocks from which they were originally torn. And though cases of transport from a lower to a higher level could and would take place during a period of subsidence, when the sea was rising or the land sinking, it is impossible that it could have taken place during an elevating period, when the sea was sinking or the land rising.? A flowing sea, to use a simple illustration, frequently carries shells, pebbles, and sea-weed from the level of ebb to the level of flood ;-it brings them from a low to a high level : whereas an ebbing sea can but reverse the process, by bringing them from a high level to a low.
For the facts and reasonings of Sir Charles Lyell on the subject, I must refer you,-as they are incapable of being abridged without being injured-to that portion of his first work of Travels in America which treats of the Canadian Lake District. But the following are his conclusions: • First,' he says, 'the country acquired its present geographical configuration, so far as relates to the older rocks, under the joint influence of elevating and denuding operations. Secondly, a gradual submergence then took place, bringing down each part of the land successively to the level of the waters, and then to a moderate depth below them. Large islands and bergs of floating ice came from the north, which, as they grounded on the coast and on shoals, pushed along all loose materials of sand and pebbles, broke off all angular and projecting points of rock, and, when fragments of hard
1 See Mr. Trimmer's last paper on Boulder-Clays, Journal of the Geological Society, May 1858, p. 171.- W. S.
stone were frozen into theirlower surfaces, scooped outgrooves in the subjacent solid strata. Thirdly, after the surface of the rocks had been smoothed and grated upon by the passage of innumerable icebergs, the clay, gravel, and sand of the Drift were deposited ; and occasionally fragments of rock, both large and small, which had been frozen into glaciers, or taken up by coast-ice, were dropped here and there at random over the bottom of the ocean, wherever they happened to be detached from the melting ice. Finally, the period of re-elevation arrived, or of that intermittent upward movement in which the old coast lines were excavated and the ancient sand bars or osars laid down.' Such are the conclusions at which Sir Charles Lyell arrived a few years since respecting the Canadian Lake District; and he states, in the note to which I have referred, that he has ever since been applying them to Scotland. Our country, during the chill and dreary period of the boulder-clay, seems to have been settling down into the waves, like the vessel of some hapless Arctic explorer struck by the ice in middle ocean, and sinking by inches amid a wild scene of wintry desolation.
There are a few detached localities in Scotland where the remains of beds of stratified sand and gravel have been detected underlying the boulder-clay; and in some of these in the valley of the Clyde, Mr. Smith of Jordanhill found on a late occasion shells of the same semi-arctic character as those which occur in the clay itself. And with these stratified beds the record in Scotland closes; whereas in England we find it carried interestingly onward from the Pleistocene period, first into the newer, and then into the older, Pliocene ages. I stated incidentally in my former address, that some of the mosses of the sister kingdom, unlike those of our own country, are older than the Drift period; and, from the existence of these under the Drift gravels and brown clay, it has been inferred by Mr. Trimmer, that as the trees which enter into their composition grew