^{page 304 note *} Their height is about 4000 feet; that of Cairngorm, one of the highest, is 4060.

^{page 304 note †} See Note A, at the end of the paper.

^{page 304 note ‡} The minute details, which continually occur in the following pages, must, to most readers, appear tedious and useless. It is necessary, therefore, to mention, that Glen Tilt was the first scene, in which Dr Hutton met with what he considered as a confirmation of his views, respecting the relations that subsist between granite and the strata adjacent to it; and, in the controversy that has since prevailed, between his followers and those of Werner, the former have insisted much upon the phenomena in this Glen, while the latter have repelled their arguments by a very different account of the structure of the rocks, and of the substances that compose them. It became of importance, therefore, to ascertain the facts more precisely. With this intention, Mr Playfair and myself passed some days in examining Glen Tilt, in the autumn of 1807; and I returned again, for a short time, in 1808. In my description, I have endeavoured to avoid the language of theory as much as possible. When any thing has been noticed which is trivial, and does not immediately bear upon the main question, it will be remembered, I hope, that the occasion called for a statement of the whole case; and, besides, while the mere outlines of theory are still under discussion, it seems expedient, in describing phenomena which strongly affect any single point of importance, to enter largely into the concomitant circumstances; since it is impossible to foresee what may, or may not, become subservient to a more precise determination of the truth.

^{page 305 note *} This instrument was a Clinometer, in its earliest form, without the brass plate. Though it was by no means so accurate as the improved one, the observations may, in general, be considered as within a few degrees of the truth. Some error may occasionally have arisen, from the surface of the rock being rather uneven, or from its not being quite parallel to the plane of stratification. For a more convenient comparison of the various positions of the strata, the bearing of the line of stretch is, throughout, indicated by the number of degrees east of north. As the line of dip is always at right angles to that of the stretch, it was thought sufficient to mark on which side the depression lay, by mentioning that of the four, or of the eight, principal points of the compass, which was nearest to its bearing.

^{page 306 note *} Dr Hutton mentions, (in a manuscript intended for his third volume, and never published) that he found several dykes, both of grey and red porphyry, in the course of this stream.

^{page 307 note *} The outline of the map is copied from Stobie's Map of Perthshire. An attempt has been made to give a better representation of the mountains near Glen Tilt; some names have been added, and others have been differently spelt, with a view of conveying the Gaelic pronunciation more distinctly.

^{page 307 note †} An observation at the falls of the Merk is included.

^{page 308 note *} This plan was laid down, by taking bearings with the compass along the bank of the river, according to its changes of direction and by pacing the distances. The length of a certain number of paces was determined by measurement.

^{page 309 note *} This stream is called in the country Auld Crochie. Auld signifies brook in Gaelic.

^{page 317 note *} By Compact Dolomite, I mean a magnesian limestone, differing from common dolomite, in having its structure compact, and such as to render the fracture not granular, as in common dolomite, but splintery, and passing to the even. The fracture is thus well described by Mr Jameson, in his account of the dolomite of I-columb-kill, in his Mineralogy of the Scottish Isles; and the name of Compact Dolomite has been used by the Comte de Bournon, in describing some specimens from that island, in the collection of Mr Allan. Compact dolomite is found also in Glenelg, and in Kintail, on the coast opposite to the island of Skye.

^{page 319 note *} This red sienite bears a close resemblance to that of the Malvern Hills in Worcestershire. I do not remember to have seen one like it any where else.

^{page 320 note *} In this plan the rocks were merely sketched in by the eye. So likewise in the plan of D.

^{page 329 note *} In the plan, the bearings of the corresponding substances on the southern bank have in every instance been laid down more to the east of north, than is stated in the description. Perhaps, in sketching the channel of the river, it was made too broad, which would account for the error.

^{page 342 note *} It must be observed, that the fact of the sienite lying under these primary, or primitive, strata, is inconsistent with the geognosy of Werner, who assigns to sienite a place in his Overlying Primitive Formation, that is, in a class of strata, which, according to him, always rest on the primitive strata, when those of the two classes are found in contact.

^{page 344 note *} See the latter part of parag. 135, and note D at the end of the paper.

^{page 345 note *} The least real gradation may in appearance be a considerable one, where the surface of the rock cuts the plane of the junction at a small angle. This is a source of deception to be guarded against in observation.

^{page 348 note *} See note B at the end of the paper.

^{page 351 note *} See Huttonian Theory, art. 81.

^{page 351 note †} This series of gradations, in composition as well as structure, between trap and granite, does not appear to have been duly noticed; and I would particularly recommend it to the attention of those geologists, who, while they admit the igneous origin of trap, cannot bear the idea of violating by any such hypothesis the dignity of a rock, which has been so long venerated as the first foundation of the earth. The specimens, required to exhibit the whole series, would not be numerous, though the shades of difference, in every step, were slight. Perhaps our island does not afford a better field for illustrating the subject than Glen Tilt, and the mountains to the north-west of it.

^{page 354 note *} The large projecting rock below the bridge furnishes a similar argument. The specific gravity of one specimen of hornblende-slate, from the top of this rock, was found to be 2.95, that of another 2.97: the specific gravity of its sienite was not tried, but the mean specific gravity of two specimens of sienite from D is 2.66.

^{page 355 note *} A remarkable example of fragments of a rock penetrated by an extraneous substance is mentioned by M. Daubuisson, as having fallen under his own observation. In a note to § 72. of his translation of Werner's Theory of Veins, he tells us, that near Freyberg a metalliferous vein was found divided into several small veins, containing between them fragments of gneiss, through which galena was disseminated in so large a quantity, that the quintal of gneiss contained thirty pounds of lead. He adds, as something more remarkable, that it was only the gneiss of the vein, which contained galena; for the gneiss of the rock even where adjacent to the vein, contained none at all.

^{page 357 note *} In a rock near the bridge, there was noticed (par. 84.) a shift in one of two veins, which are apparently of posterior formation to the sienite. It may be worth while to observe, that these veins dividing both the sienite, and the imbedded mass of strata, can hardly be ascribed to secretion; for it is highly improbable, that the strata, which are of granular quartz and hornblende-slate, should have agreed with the sienite in secreting the same substance; nor is it likely, that a secretion from the one, should have filled that part of the crevice, which had been formed in the other.

^{page 359 note *} See note C.

^{page 360 note *} Some of these substances bear a considerable resemblance to those which appear on the Malvern Hills, in the cut made by the road to Ledbury. The latter are in a state of decomposition, which makes it difficult to discern their ingredients.

^{page 362 note *} See note D.

^{page 362 note †} In the manuscript, of which some account is given in note C.

^{page 371 note *} A good example of this occurs in the dyke that crosses the Water of Leith, above St Bernard's Well.

^{page 372 note *} See Theory of Veins, § 58, 72, 75.

^{page 372 note †} P. 232. See also p. 239.

^{page 372 note ‡} See Theory of Veins, § 72.

^{page 374 note *} This is one of the exceptions alluded to in parag. 110.