into different parts of Scotland, with a view of comparing certain results of his theory of the earth with actual observation. His account of granite, viz, that it is a substance which, having been reduced into fusion by subterraneous heat, has been forcibly injected among the strata already consolidated, was so different from that of other mineralogists, that it seemed particularly to require farther examination. He concluded, that if this account was just, some confirmation of it must appear at those places where the granite and the strata are in contact, or where the former emerges from beneath the latter. In such situations, one might expect veins of the stone which had been in fusion to penetrate into the stone which had been solid; and some imperfect descriptions of granitic veins gave reason to imagine that this phenomenon was actually to be observed. Dr Hutton was anxious that an instantia crucis might subject his theory to the severest test.

One of the places where he knew that a junction of the kind he wished to examine must be found, was the line where the great body of granite which runs from Aberdeen westward, forming the central chain of the Grampians, comes in contact with the schistus which composes the inferior ridges of the

through a series of very accurate and curious experiments, has produced an instrument which promises to answer all the purposes of photometry as well as hygrometry, and so to make a very important addition to our physical apparatus.

same mountains toward the south.

The nearest

and most accessible point of this line seemed likely to be situated not far to the eastward of Blair in Athol, and could hardly fail to be visible in the beds of some of the most northern streams which run into the Tay. Dr Hutton having mentioned these circumstances to the Duke of Athol, was invited by that nobleman to accompany him in the shooting season into Glentilt, which he did accordingly, together with his friend Mr Clerk of Elden, in summer 1785.

The Tilt is, according to the seasons, a small river, or an impetuous torrent, which runs through a glen of the same name, nearly south-west, and deeply intersects the southern ridges of the Grampian Mountains. The rock through which its bed is cut is in general a hard micaceous schistus; and the glen presents a scene of great boldness and asperity, often embellished, however, with the accompaniments of a softer landscape.

When they had reached the Forest Lodge, about seven miles up the valley, Dr Hutton already found himself in the midst of the objects which he wished to examine. In the bed of the river, many veins of red granite (no less, indeed, than six large veins in the course of a mile) were seen traversing the black micaceous schistus, and producing, by the contrast of colour, an effect that might be striking even to an unskilful observer. The sight of objects which

verified at once so many important conclusions in his system, filled him with delight; and as his feelings, on such occasions, were always strongly expressed, the guides who accompanied him were convinced that it must be nothing less than the discovery of vein of silver or gold, that could call forth such strong marks of joy and exultation.

Dr Hutton has described the appearances at this spot in the third volume of the Edinburgh Transactions, p. 79, and some excellent drawings of them were made by Mr Clerk, whose pencil is not less valuable in the sciences than in the arts. On the whole, it is certain, that of all the junctions of granite and schistus which are yet known, this at Glentilt speaks the most unambiguous language, and most clearly demonstrates the violence with which the granitic veins were injected among the schis

I must take this opportunity of correcting a mistake which I have made in describing the junction in Glentilt, (Illustrations of the Huttonian Theory, p. 310,) where I have said, that the great body of granite from which these veins proceed is not immediately visible. This, however, is not the fact, for the mountains on the north side of the glen are a mass of granite to which the veins can be directly traced. This I have been assured of by Mr Clerk. Dr Hutton has not described it distinctly; and not having seen the union of the veins with the granite on the north side, when I visited the same spot, I concluded too hastily that it had not yet been discovered.

In the year following, Dr Hutton and Mr Clerk also visited Galloway, in search of granitic veins, which they found at two different places, where the granite and schistus come in contact. One of these junctions was afterwards very carefully examined by Sir James Hall and Mr Douglas, now Lord Selkirk, who made the entire circuit of a tract of granite country, which reaches from the banks of Loch Ken, where the junction is best seen, westward to the valley of Palnure, occupying a space of about 11 miles by 7. See Edinburgh Transactions, Vol. IIl. History, p. 8.

In summer 1787, Dr Hutton visited the island of Arran in the mouth of the Clyde, one of those spots in which nature has collected, within a very small compass, all the phenomena most interesting to a geologist. A range of granite mountains, placed in the northern part of the island, have their sides covered with primitive schistus of various kinds, to which, on the sea shore, succeed secondary strata of grit, limestone, and even coal. Here, therefore, Dr Hutton had another opportunity of examining the junction of the granite and schistus, and found abundance of the veins of the former penetrating into the latter. In three different places he met with this phenomenon ; in the torrents that descend from the south side of Goatfield; in Glenrosa, on the west, and in the little river Sannax, on the north-east, of that mountain. From the

first of these he brought a specimen of some hundred-weight, consisting of a block of schistus, which includes a large vein of granite.

At the northern extremity of the island he had likewise a view of the secondary strata lying upon the primary, with their planes at right angles to one another. In the great quantity, also, of pudding-stone, containing rounded quartzy gravel, united by an arenaceous cement; in the multitude of whinstone dikes, which abound in this island; and in the veins of pitchstone, a fossil which he had not before met with in its native place; he found other interesting subjects of observation; so that he returned from this tour highly gratified, and used often to say that he had no where found his expectations so much exceeded, as in the grand and instructive appearances with which nature has adorned this little island.

Mr John Clerk, the son of his friend Mr Clerk of Elden, accompanied him in this excursion, and made several drawings, which, together with a description of the island drawn up afterwards by Dr Hutton, still remain in manuscript.

The least complete of the observations at Arran was that of the junction of the primitive with the secondary strata, which is but indistinctly seen in that island, and only at one place. Indeed, the contact of these two kinds of rock, though it forms a line circumscribing the bases of all primitive