Page 31.) With regard to the gneiss and mica-slate of Garthsness, they separate into polyedrous fragments, which are somewhat larger than those of clay-slate.

MASSES INTERPOSED AMONG THE STRATA.-At Garthsness, not far from the junction of the clay-slate and mica-slate, are interposed among the strata thin alternating beds or INTERSTRATA of hornblende.

Near the same place is a bed of iron-pyrites, observing in its course the direction of the strata among which it occurs. Its proper appellation, therefore, agreeably to the phraseology I have expressed my intention of using, is an INTERSTRATUM. This mass was unsuccessfully wrought a few years ago by a Mining Company, for the purpose of finding copper-ore, whilst many hundred tons of iron-pyrites were thrown iuto the sea. The site of the ore is so much concealed by the fragments thrown up from the mine, that its breadth is not very distinct. I observed four and a half feet in width, of what the Cornish miners, who worked here, called Gossan, which consisted of pulverulent siliceous matter, mixed with iron-ore, and eight and a half feet of what was named by the same men the Rider, consisting of iron-pyrites, occurring in a highly indurated blue quartz. In this last substance was contained the sparing quantity of copper-ore, which was the sole object of the mining operations. Dr Fleming, who had a much better opportunity than myself of examining the vein, states that it is nearly thirty feet in width; that the middle of it is filled with pyrites, which in some places is nearly eight feet in thickness; that the sides are composed of a soft ochry clay, the ore being surrounded on each side by matter seemingly arising from its own decomposition *.

MASSES INTERSECTING THE STRATA, OR CROSSING THEM IN THEIR COURSE.About a mile to the west of the head of Quendal Bay, and about the same distance from the extremity of Garthsness, is a thin mass of limestone, apparently stratified, which in its course intersects the strata of mica-slate among which it is found. It appears to be scarcely more than eight or ten yards in width, running from east to

west.

In the clay-slate of Fitfiel Head, at the Girths of Quendal, is found a vein of iron-mica. It is about twelve feet thick. Dr FLEMING examined this vein with much attention, and justly considered it as entitled to consideration, since ores of this kind contain from 70 to 80 per cent. He supposes it to traverse the hill in an easterly direction.

T 2

• See Dr FLEMING's Mineralogy of Shetland, in SHERRIFF'S Agricultural Survey, p, 125.

FORM OF THE HILLS.-The form of the hill of clay-slate, known by the name of Fitfiel Head, is represented in the Geological Plate of the Appendix, Fig. 4. It shews in its outline the long section of an ellipse, one extremity of which is broken off. This hill of clay-slate, when traced from its contact with the epidotic sienite, rises by a gradual ascent towards the south, when it is abruptly terminated by the invasion of the sea.

The elevation of its ridge is perhaps from 700 to 800 feet *.

The easterly face of Fitfiel Head has a gradual inclination; it is separated from the contiguous strata of schistose gneiss and mica-slate which form Garthsness, by the intervention of a sulcated hollow, parallel to the direction of the strata. The westerly and southerly faces of the hill are, by the inroads of the sea, formed into stupendous cliffs, almost vertical.

The small ridge of Garthsness appears to be elevated in its greatest summit about 150 or 200 feet above the level of the sea.

1

CHANGES INDUCED BY EXTERNAL AGENTS.-All the rocks of Garthsness and Fitfiel Head are very prone to decomposition. The cliffs of Fitfiel have yielded more to the ocean than the other rocks, from the facility with which the strata are disintegrated under the form of polyedrous fragments.

STRATA OF CLAY-SLATE, HAVING THEIR LINE OF DIRECTION OPPOSED TO THE NORTHERLY BOUNDING LINE OF THE EPIDOTIC SIENITE OF DUNROSSNESS.

Consist chiefly of Clay-Slate, (the Phyllade of the French,) and form the Cliff Hills,

GEOGRAPHICAL LIMITS.-These rocks consist of clay-slate, being diversified by interstrata of quartz and hornblende. Their outgoings are to be traced near Spiggie and the Loch of Lunabister, from the northerly bounding line of the epidotic sienite. It is probable, also, that the strata are opposed to that part of the northerly line of epidotic sienite which appears at the head of the Bay of Quendal; the junction being

• From a misfortune which befel my Barometer, very early on landing in Shetland, where it could not be repaired, most of my calculations on the heights of the Shetland hills must be considered as conjec

tural.

partly concealed by deep coverings of blowing sand, and partly by the superposition of rocks of a conglomerate structure. From Dunrossness, we are enabled to pursue the strata of clay-slate in their lines of bearing, through a long tract of country to Quarf, where there is an interruption to the ridge which they form, by the occurrence of a narrow deep valley that intersects the hill in a direction nearly east and west, crossing at right angles the lines of bearing common to the strata.

Near the head of Dale's Voe, an easterly ramification of the ridge is given off, being continued to Kibister's Ness. The clay-slate, lastly, appears at the small island of Greenholm, where it is associated with limestone.

The geographical limits of the clay-slate, that forms the Cliff Hills, may be again stated, by supposing a straight line drawn from a point a little to the north of Quendal Bay, in a direction of N. by E. to the head of the Bay of Channerwick, thence to Quarf, and thence to the head of Dale's Voe. We must here extend the EASTerLY BOUNDARY LINE of the clay-slate, by supposing two small lines to be given off to the north-east, within the included space of which is the subordinate ridge or crest of the Cliff Hills that is continued to Kibister's Ness; the strata re-appearing from beneath the bed of the ocean at the small island of Greenholm. After this ramification has taken place, the line is continued in its wonted direction of N. by E. to Hawksness.

THE WESTERLY BOUNDING LINE of the clay-slate originates from a point a little to the east of the small inlet of Spiggie, where the junction takes place of the clayslate and the epidotic sienite. From this point the line is continued in a direction of N. by E. to the east of the isthmus that divides the Peninsula of St Ronans from the Mainland; forming the easterly limits of Cliff Sound, it may be traced from thence to Scalloway, when it is prolonged in a more easterly direction to Hawksness.

EXTENT OF THE STRATIFIED MASS.-The Cliff Hills have been described as forming a long ridge which extends from Dunrossness to Hawksness, interrupted only by a transverse valley which occurs at Quarf. This ridge is about twenty-four miles in length. The breadth of the base of the hills is at Dunrossness about two miles; but the width gradually decreases from south to north, until at Hawksness it does not exceed half a mile. Kibister's Ness, which has been described as a small ramification of the Cliff range, is, when reckoned from a point east of Dale's Voe, about two miles in length, and about half a mile in breadth.

CHEMICAL INGREDIENTS.-D'AUBUISSON has very properly considered, that clay-slate has for its base the same principles as mica, and the graduation of one substance into the other, is well inferred from their respective analyses. The same quantity of silex, which is nearly one-half of the whole result, is contained both in clay-slate and mica. Clay-slate yields one-twentieth part more of the oxide of iron

than mica, but it contains one-tenth part less of aluminous matter, and one-twentieth part less of potash. The important fact, that clay-slate generally contains less aluminous matter than mica, but never in any respect exceeds it, considerably affects the correctness of common geological nomenclature. Certain French naturalists have lately complained, with much justice, of the inconvenience of attaching the name of Clay-slate to a rock which is not distinguished from several other mineral substances, by any notable quantity of argillaceous matter which it contains. D'AUBUISSON justly observes, "Le mot argileux éveille nécessairement l'idée d'argile, et semble dire que notre roche est composée de cette terre; elle ne l'est pas plus que le schiste-micacé: elle a seulement un aspect moins cristallin, et par suite plus terreux *."

There is also another urgent reason for abandoning such a term as clay-slate. A more than common degree of subtlety is required for pointing out the verbal difference between the name of clay-slate (schiste-argilleux,) as applied to a certain rock of the primitive class, and slate-clay, (argile-schisteuse,) as applied to a certain rock of a newer formation. Terms of intended distinction, such as slate-clay or clayslate are unworthy the nomenclature of any department of Natural Science; they only find parallels in the equivocal terms of common law, that rise into importance among none but the lowest order of legal quibblers. Reasons not far remote from these induced BROCHANT and D'AUBUISSON to change the name of Clay-slate into that of PHYLLADE, (from the Greek word quaads,) expressive of the foliated structure of the rock. The same term will be as much as possible employed in these researches; but, although the continued use of the term Clay-slate is more honoured in the breach than the observance, any change for a better expression must be unavoidably gradual. The rejection, therefore, of a word leading in geological description to infinite confusion, and the substitution of one that is less ambiguous, will be best promoted by encreasing our familiarity with both appellations, as they are associated together under the character of Synonyms, the one being English and the other French.

The mineralogical character of the phyllade or clay-slate of the Cliff Hills is as follows: Its colour is of a light blue or greenish blue; its fracture is completely earthy, and it often affords a dark streak. Scales of mica are diffused in minute particles throughout most parts of the rock. Between the lamina of the strata, white quartz is very frequently interposed in thin layers, generally about an inch in breadth. It may be lastly observed, that there is scarcely any specimen to be detached from the rock in which traces of iron-pyrites in very small portions are not more or less discernible.

The strata of the Cliff Hills differ from those of Fitfiel Head, in evincing more decidedly the qualities of clay-slate. But since mica-slate may be occasionally found both on the east and west of the Cliff ridge, the transition of one rock into the other

• Geognosie, par J. B. D'AUBUISSON, tome ii. § 182.

is so imperceptible, that the materials of certain parts of the hill may, in a geological sense, be regarded as constituting neutral ground.

The particular variety of mica-slate into which the clay-slate graduates, contains a great excess of mica, either under the form of minute scales or of fine pellicles.

When mica occurs under the form of minute scales, mineralogists have detected in them figures that are imperfectly crystalline. Along with these semi-crystalline scales, particles of quartz are blended, but in so minute a state of division, that their form and magnitude are almost imperceptible. The transition, therefore, of the mica-slate into phyllade or clay-slate, consists in the particles of the mica and quartz becoming gradually blended together in one apparently homogeneous and compact mass. In the first stage of the transition, it is often impossible to distinguish the high state of glistening lustre that might be ascribable to some varieties of clay-slate, from the pseudo-metallic lustre peculiar to mica. In the last stage, where the scales of mica and particles of quartz become apparently homogeneous and compact, the lustre acquires a pearly or glimmering character, and scales of mica are sparingly disseminated in the earthy base of undisputed clay-slate.

In another variety of mica-slate graduating into phyllade or clay-slate, the small scaly particles of which it is partly composed cohere with various degrees of firmness, and in this state of mutual connection, manifest a foliated arrangement, and separate under the form of micaceous pellicles. The ultimate layer of a very common variety of mica-slate, may, according to this view, be regarded as consisting of a micaceous pellicle. This notion is in conformity with the microscopic observations of D'AUBUISSON, who detected upon the surface of a schist an incalculable number of distinct scales of mica of an imperfectly crystalline form, and of extreme tenuity. He therefore conceived that such scales, when united by different degrees of cohesion, formed a foliated tissue or pellicle. "Presque toujours" remarks this excellent geologist, "les écailles, ou cristaux imparfaits, sont tellement tissues et fondues les unes dans les autres, que leur ensemble forme des feuillets continus semblables à des pellicules."

We have thus shewn, that when mica occurs in the form of fine pellicles, we are entitled to consider that a number of these ultimate layers make up a schist of the dimensions usually found in mica-slate. The quartz is then interposed in the form of parallel plates, often an inch in thickness. We shall now inquire into the circumstances attending the transition of this variety of mica-slate into clay-slate.

In the first stage of the transition, many of the scales of mica are so intermixed with each other as to lose their semi-crystalline form, and to resolve themselves into

* See D'AUBUISSON, for instance, Traité de Geognosie, Tome ii, § 173. "Presque toujours les ecailles, ou cristaux imparfaits," &c.