sea, westward or southward, producing groovings on the rocks, and the phenomena of crag-and-tail, in directions corresponding to the course of the return waves.

This theory bears the stamp of the acute and original mind of its author, and it offered perhaps the best explanation of the phenomena, which the range of geological information at that time could supply. In the same year we find specimens of striated rocks and crag-and-tail noticed by Colonel Imrie, in his paper on the Campsie Hills, in the second volume of the Wernerian Society's Transactions, and an idea somewhat similar as to their origin thrown out. Sir James Hall's explanation of the phenomena was pretty generally accepted by geologists in this country; and it is still, I believe, adopted, though perhaps in a modified form, by some able men. I shall notice very briefly a few of the leading objections to it.

1st, Striæ must have been produced by a sliding motion, like that of a plane or graving tool, while stones propelled over a firm surface, by a current of water, would have a rolling motion, which might polish the rocks, but could not cut groves in them. 2d, Supposing stones impelled by water to cut grooves, these grooves would not occupy such positions as we find them in, on sloping surfaces like the steep sides of valleys; the force of gravity would render them more or less inclined, while, in such situations, we find them horizontal. 3d, The groovings so cut would be deflected to the right or left, by slight inequalities of surface, and would not possess that wonderful straightness and parallelism which they generally exhibit, and which Mr Lyell has seen extending over a length of 100 yards in the United States. 4th, A great wave or debacle of the magnitude assumed, coming from the west or north-west, would have filled up deep valleys transverse to its course, like that called the Great Glen. Now, that glen, so far from being filled up, has a depth of 770 feet in Loch Ness, measured from the surface of the water, a depth exceeding that of the German Ocean. The fact, that this deep fissure has not been filled up, is presumptive evidence, that no such wave has ever passed over the

island. 5th, The cause assigned does not explain how boulders weighing many tons were carried from the Grampians across the central valley of Scotland, the bottom of which is only 200 feet above the sea, and deposited on the Pentlands, at spots 800 feet higher. A current of water, however powerful, would have dropt them in the low country. 6th, The debacle does not explain other distinct traces of the action of water upon our hills. Mr Chambers, in his recent work, has shewn that satisfactory evidence exists of the presence of the ocean in its proper form of a horizontal sheet of water, up to 1500 feet above its present level. Had this fact been known, and carefully studied, Sir James Hall would have been spared the necessity of resorting to a great hypothetical Atlantic wave.

No agent yet known but ice, or ice conjunctly with water, seems capable of explaining the phenomena for which Sir James Hall called in the aid of a debacle. Those who have read the excellent works of Professor Forbes and Professor Agassiz, are aware that a glacier, during its slow progressive motion, transports vast masses of rock over a distance of many miles; secondly, that it grooves and polishes the bottom and sides of the valley containing it, by means of the stones and gravel which it brings down; and, thirdly, that many of these stones are themselves grooved by the attrition they have undergone in sliding over the fixed rocks. We know also, that as floating ice lifts large stones from the bottom and sides of rivers, or the shores of the sea, and carries them away, it may leave striæ on rocks over which it passes. Mr Lyell found well-marked striæ cut on a rock in the Bay of Fundy, which he attributed, on good grounds, to the packed ice of the preceding season, or of a period very little farther back. The pack ice accumulates there to the depth of fifteen feet.* If this was effected on the shore by so small a mass, it is easy to conceive that our plains might be grooved and abraded by icebergs, armed at bottom with stones or gravel, and floating in a sea 500 or 1000 feet deep. These

*Travels in North America, 1845, vol. ii., p. 173.

moving mountains of ice are known to have reached to a greater depth than this.

If the grooves, scratches, and polishing, seen on our rocks, were produced by ice, those in the deep valleys must be due to the action of glaciers, which are found in the Alps to glide downward at the rate of one or two feet per day, with gravel, stones, and sand adhering to their bottom. In this case the largest grooves should be on that side of prominent rocks which is toward the head of the valley, and this, it will be seen, holds true in Scotland. On the other hand, if the scratches, grooves, and polishing, were caused by an irruption of the ocean from the west, it is evident that the direct wave setting eastward would be vastly more powerful than the indirect or return wave, produced by the supposed recoil of the water from the hills, and setting westward. It follows, that in the west of Scotland, where the effect of both waves must be best seen, the grooving and abrasion should be greatest on the west side, and least on the east side, of exposed rocks. It will be found that the case is just the

reverse.

Glaciers are rivers of ice, which have their source at a higher level in the mountains, in what the Swiss call Mers de Glace, or "Seas of Ice." To account for glaciers in the valleys shortly to be noticed, we must suppose that Scotland, at some former period, had a climate as cold as Labrador or Greenland, and that a permanent envelope of ice and snow covered all the higher region of the Grampians. There is nothing extravagant in the magnitude assigned to this envelope, for Agassiz informs us, that among the numerous mers de glace in the Alps, there are some 20 or 30 leagues* (50 or 75 miles) square. Glaciers or efflux streams of ice from this central mass would glide slowly downward through the openings at the outskirts of the mountains, such as the valleys of Loch Fine, Loch Long, Loch Etive, Loch Earn, and others; and if the hypothesis be correct, the sides and

* Agassiz Etudes Sur les Glaciers, p. 22.