however, the prevalent substance. In several places, as near Futabrugh, much mica is diffused through it; and in the vicinity of Aithsness, it contains small sparing portions of clayslate, by which it becomes the grauwacke of some authors. At Papa, Little, Aithsness, Nunsburgh, Clousetter Voe, and Burrafiord Voe, the rock is in several places varied by the presence of small angular portions of red felspar, and passes into felspar-porphyry. At Aithsness it contains greenstone as well as limestone, in the form of thin beds or veins. At Dale, on the west coast, it includes compact felspar and felspar-porphyry; and on the banks of the hill of Aithsness it is impregnated with much ferruginous matter in the shape of an oxide. I also noticed in some places veins of sparry iron-ore.

The quartz-rock generally consists of minute grains firmly united together, and possessing semi-crystalline forms. In this respect it differs from a particular variety of sandstone described on the east of the island, where the texture is much looser, and where the grains shew numerous marks of attrition. South of the Hill of Aithsness, and at a few other places, the particles of the quartz diminish so much in size, that the rock appears of a compact structure. On the south of Dale, I detected in one or two places small concretions blended with granular particles of the size of pebbles, that shewed all the appearance of attrition.

The stratification of the quartz-rock seems remarkably connected with the partial occurrence of mica, whilst its line of direction does not follow that of other mountain masses of Shetland, that have been represented as stretching from S. by W. to N. by E. On the contrary, it extends from S. 60° E., to N. 60° W., from E. to W., and from S. 70° W. to N. 70° E. The dip is at various opposite points of the compass, while the angle of inclination is, as it appears in a horizontal section, from 40° to 45°. In several places the quartz is decidedly unstratified, and yields to blows of the hammer equally in various directions; but whether stratified or unstratified, it is resolved by other seams into polyedrous masses of various magnitudes.

At the North Voe of Clousetter, there occurs, resting on the quartz, a small roundish conglomerated mass of granite, felspar, and quartz, scarcely, perhaps, more than 150 yards in diameter; a gradual transition of one rock into the other being observable at the junction. A few of the conglomerate strata next to those of the quartz range at a common angle of 45°. But crossing the strata for a distance of a few yards only, they gradually acquire an inclination with the horizon of only 10°, and, maintaining this position, they are terminated. Now, there is every reason to suppose, that the lower edges of the strata of the conglomerate rock are in contact with the surface formed by the upper edges of the strata of quartz, in the manner of certain conglomerate strata I formerly described. This change of inclination, therefore, from 45° to 10°, can be satisfactorily accounted for, on the supposition, that the strata had acquired an addition of new matter in their descent, which may either consist in

an increased thickness of the strata, or in an accession of new strata. It is thus that the increased accumulation of matter, which the strata may possibly have received in proportion to their depth, would produce an elevation of the uppermost strata.

From the head of Bigsetter Voe to Valay Island, the quartz rock is joined to the granite of Sandsting (described page 341.) The former rock passes into the latter, by gradually losing its homogeneous appearance, and by its ingredients being interspersed in a distinct form of larger grains or concretions, until the rock is at length wholly composed of semi-crystalline portions of quartz and felspar. Occasionally, however, an alternation takes place of the two rocks. To the north of Bigsetter Voe, the quartz-rock is contiguous to gneiss, but the junction is, for the most part, much concealed by a deep moss; at Braganess, the line of demarcation between the two rocks is perfectly complete, no transition taking place. The junction of the quartz on the north, with another mass of the same description of a reddish hue, and with gneiss, will be described when these rocks are treated of separately.

The quartz-rock is elevated into numerous irregular ridges, not above 600 or 700 feet high, running from east to west, and intersected by valleys, which form receptacles for deep pools of water. On the east of Walls, it has yielded much to the sea, forming high and precipitous banks.

Much of the surface of the quartz-rock is concealed by a deep peat-moss; but every where the moor presents a great display of bog-iron, particularly on the brow of the Hill of Aithsness.

2. Quartz-Rock of the Hills of Walls.-The bluish-grey quartz last described is connected in its northerly bounding line with another, but smaller, mass of red quartz ; the form of the latter, as presented by an horizontal section, being nearly similar to that of the former, with this difference, that, in the red quartz, the apex, from which its bounding lines diverge, is bent outwards, in the form of a horn of land, so as to be separated for a mile from the bluish-grey quartz. This horn of land is geographically represented by Snaraness, being situated on the east of Kilista Voe. The northerly bounding line, extending in a direction of S. 65° W. to East Gio, on the west of Walls, is five miles in length; the southerly bounding line, extending S. 55° W. near the village of Dale, is of the same length; and the greatest breadth of the mass, which gradually diminishes towards Snaraness, is two miles.

The colour of this mass of quartz is, according to WERNER'S nomenclature, cochineal red. It contains much felspar in its composition; it is of a granular structure, and it is generally unstratified. There is no necessity for entering much farther into the particulars of this rock, since, in nearly every character, except colour, it resembles the quartz last described.

The quartz-rock constitutes a hill about 930 feet high; on the west side it has yielded considerably to the sea, so as to form the high cliff of Deepdale. Its rela:

tion with a mass of a different colour presents nothing remarkable. Alternations of beds belonging to the two rocks may be seen near Dale.

3. Strata of Hornblendic Gneiss to the North of the Quartz Rocks.-These strata extend along the north coast of the parishes of Walls and Aithsting, reaching from Braganess on the Mainland, and Papa Little, due west, to Norbay, a distance of ten miles, and being from half a mile to two miles in breadth. They consist of gneiss, so associated, however, with hornblende-slate, and intimately combined with particles of hornblende, that I have given to the rock the name of Hornblendic Gneiss. In most specimens which we examine, the hornblende is either openly manifested, or is in such an intimate state of union with the felspar, as to impart to this ingredient of the rock a greenish tinge. The other ingredients of quartz, felspar, and mica, are, as in all other varieties of gneiss described, to be found in very different proportions. At Norbay, the gneiss is very quartzose; at Braganess, it is porphyritic; and at Swarbuck's Head, quartz and hornblende-slate alternate.

There is no gneiss in Shetland that is traversed with more veins of granite than that of this district. A vein at the promontory named the Neing is of very considerable size, being not less than 45 feet broad, and running from SW. by S. to NE. by N. It is of the same kind of granite as that which has been described as occurring in Northmavine, and may probably be a prolongation from that immense mass. At Vementry, veins of granite are particularly tortuous, and ramify in the most remarkable manner. See the Plate, Page 490.

Besides granite, we often see veins of quartz, compact felspar, felspar-porphyry, greenstone, or sienitic-greenstone.

At Braganess, Sonsoness, Burraford, Busta, the Loch of Collista, and other places, there are thin interstrata of very pure limestone: the granitic veins which traverse them are often diverted from their course in the most remarkable manner, shewing great signs of distortion.-(See Plate, Page 496.)

I have before had occasion to state, that the presence of beds of hornblende-slate, or of intermixed particles of hornblende, has a strong tendency to disturb the strata among which they appear. Nowhere is this remark better exemplified than in the rocks of the south-west district of Shetland. On this account, any general statement of the most prevalent line of direction would be impossible. I certainly often found the direction to be from SW. by S. to NE. by N. which may possibly be the most prevalent. The dip was mostly to the west, at angles from 50° to 70°.

The relations of the strata are much concealed by the sea. At Vementry and Brathe line of direction stretches from the bluish-grey quartz rock, near Aithsness, ganess, to the granite of Vementry; consequently, the strata are opposed to the bounding lines of each of these masses at various angles. More westerly, they only meet, under similar circumstances, the quartz rocks of this district.

At Vementry, the invasions which take place, from the contiguous mountain of granite, in the form of dikes or veins, are very large, being particularly seen near the junction of the gneiss and granite, not far from the house of Mr DICKSON. (See Plate below.)

The irregularity of the form, and the unevenness of the surface of the hills, is so great as to almost bid defiance to description. Every where they are broken into rough hollow crags, that afford deep lodgments for water. On the coast, they have yielded much to the ocean,-Swarbuck's Head, the Neing, and the Noup of Norbie, being among the highest cliffs of the Mainland.

4. Sandstone and Claystone (Secondary Porphyry of the French) situated on the Mainland (west of the parish of Sandness), and the Island of Papa Stour.-That part of the parish of Sandness which is occupied by strata of sandstone, is two miles in length, and one in breadth, being bounded on the east by a line drawn from East Gio to the Bay of Norbay. Papa Stour, an irregular-shaped island, is about three miles in length, and the same in breadth.

The sandstone of this district may be generally described as of a red, reddishwhite, or bluish-grey colour. It contains in its composition a considerable quantity of felspar, which is very evident in the decomposition of the rock. There are few characters, with the exception of colour, by which it is distinguished from the sandstone described on the east coast of Shetland. If possible, it has a less firm texture, and does not approach so much to the character of quartz rock.

Another rock of this district is claystone,-described by the French under the name of Secondary Porphyry. It so closely resembles in its characters the mass of the same name described on the south-west of Northmavine, that it would be perfectly su perfluous to repeat the description of claystone that has been already given; all that is necessary, therefore, is occasionally to allude to the variation it presents in colour. Our object, then, at present is confined to the explanation of the various relations that may subsist between the claystone and the sandstone.

At East Gio, west of the coast of Sandness, may be observed a mass of sandstone, the easterly bounding line of which is to be traced from its junction with the red quartz-rock of the Hill of Walls, along the coast as far as the House of Melby, and the Holm of Sandness. It differs from the contiguous mass of red quartz, by containing less felspar,-by possessing a structure less firm, with grains that are for the most part devoid of a crystalline form, being rather of an arenaceous character,—and by the whiteness of its colour. The direction of the strata is generally from S. 55° W. to N. 55° E. the dip being to the east under an angle of 45°; frequently, however, a much less inclination is observable. Near East Gio, the sandstone is associated with strata of claystone nearly horizontal, and near Norbie with a conglomerate rock composed of fragments of porphyritic claystone.

The claystone of Papa Stour generally contains imperfect crystals of felspar of a different colour from their base, by which the rock acquires the name of Porphyritic ClayIt is then found presenting three varieties of colour. The first occurs chiefly on

stone.