çades to the ocean. The Giant's Causeway, a small part of which, with the neighbouring coast, is shown in the above wood-cut, consists of three piers of columns, which extend some hundred feet into the

sea.

It is surrounded by precipitous rocks, from two hundred to four hundred feet high, in which there are several striking assemblages of columns, some vertical, some bent or inclined, and some horizontal, and, as it were, driven into the rock. Bengore, which bounds the Causeway on the east, consists of alternate ranges of tabular and massive, with columnar basalt. But amongst the various and grand objects on this coast, Pleskin is perhaps the most striking; it presents several colonnades of great height and regularity, separated from each other by tabular basalt; and at Fairhead there is a range of columns of from 10 to 20 feet in diameter, and between 200 and 300 feet high, supported upon a steep declivity, and forming a terrace which towers nearly 600 feet above the waves beneath. He who would really see the sublime should visit this stupendous promontory.

Another basaltic district, which I am inclined to regard as exceeding the former in magnificent peculiarities, is that which presents itself in sailing down Loch-Nagaul, in Mull, towards the Isle of Tiree. The coast of Mull, upon the right and left, exhibits the step-like appearance of basaltic rocks in great perfection, and has fine caverns and columns; the islands of Ulva and Gometra rise with the abrupt and irregular precipices common to this formation. The Treshamish Isles exhibit columpar and massive basalt, and in the midst of this curious panorama, Staffa presents itself. The columns, which are from 30 to 50 feet high*, are approached by a fine causeway, rising gradually from the deep, and they appear to support an immense weight of tabular basalt. The pillars are perpendicular, inclined, and in places extremely curved; and in the Cave of Fingal the ranges of columns extend in long perspective into the interior of the rock, presenting a scene of such unrivalled grandeur, as hitherto to have foiled all attempts of the poet to describe, or of the painter to represent. The wood-cut at the head of the next Section, copied from Dr. Mac Culloch's sketch, represents the Causeway and entrance of the Cave.

• Se Dr. MAC CULLOCH's Description of the Western Islands of Scotland. Vol. ii. p. 1.

SECTION IV. Of Metallic Veins. Of the General Causes of the Decomposition of Rocks. - Of Volcanoes: and of the Analysis of Soils.

3019. BESIDES the veins of lapideous substances, the fissures filled with debris and rubbish, the dykes, the beds of salt, and the fields of coal, there are diffused through the strata a variety of other treasures, among which the metals are of the utmost interest and importance. By the term Mineral Vein, we mean a separation in the continuity of a rock of determinate width, but extending indefinitely in length and depth, filled with metallic ores, and crystalline substances, differing from the rock itself.

Nearly all rocks are occasionally thus traversed, but the middle series are those in which metals are most abundant. In Cornwall, for instance, tin occurs both in the granite and slate; but it is most abundant in the latter, and the vein occasionally runs between the two rocks, so that one wall consists of granite, and the other of slate. The metal is often separated from the rock by thin layers of clay, or of stony materials, called Deads, which also intermix with the ore, and form its gangue or matrix.

3020. The richest metallic veins run, without exception, east and west. Those which run north and south being usually filled with stony materials. The latter veins appear of posterior date to the former, for they often intersect them throwing them out of their regular course; generally a few inches only east and west, but many fathoms north and south. These cross courses often interfere with the treasures of the metallic vein, though, when solid, they are sometimes of great service in keeping out water.

$021. The extent to which veins may be pursued, is extremely various, and depends much upon accidental circumstances. Sometimesa cross course cuts the vein, and puts an end to the miner's hopes, he being unable to discover its continuation after such interruption; sometimes the depth of the vein becomes so great, that it cannot be prudentJy pursued; sometimes a rich lode of metal suddenly disappears, or

:

vanishes into thin strings, which, though often quite lost, occasionally reunite into a good vein, or bunch of metal. So that, taking all these circumstances into account, between two or three miles is usually the utmost extent to which a vein has been pursued*.

3022. Veins vary in width, from an inch or two, to 30 or 40 feet, but the middle-sized veins are usually most prolific, the larger becoming relatively poor. The influx of water was formerly an insuperable impediment to the pursuit of a vein, and remains now a serious and expensive obstacle to mining. Formerly many veins in Cornwall were only worked for tin, which, at greater depths, have lately yielded abundance of copper; but in Cornwall copper is never found without water, and all the mines of that metal require drainage by engines, or other

means.

3023. Concerning the original formation of metallic veins, there has been considerable collision of sentiment among geologists; but two circumstances seem sufficiently obvious; one, that they are of later date than the containing strata, that they are not contemporaneous; and the other, that their contents have been in a fluid state. The former position is indicated by their intersecting different strata; the latter, by the crystalline forms of the substances they contain. The Neptunians tell us that veins have been filled by metallic and lapideous solutions flowing in from above, but they do not inform us of the nature of the solvent which held the different bodies they present; nor can we guess why its contents are deposited exclusively in the vein, and not found upon the adjacent surface.

3024. The Plutonists consider veins as filled from below, by the injection of matters in igneous fusion; and in the shifting, breaking, and dislodgement of the strata, they read the force with which these operations have been performed. The validity of hypotheses is only to be estimated by their accordance with facts; and although there be many inexplicable phænomena attending metallic veins, yet the nature of their contents is such as to favour the igneous hypothesis, and to lead to the belief that fire, not water, has been the grand solvent of which nature has here availed herself. That the metals have passed from the fluid to the solid state, seems sufficiently obvious, from their crystalline form; and it is much more probable that they should have been liquefied by heat than by any other solvent.

Sulphur is very commonly found united to metallic bodies, and the greater number of metallic ores contain that element. Such compounds are easily produced by the artificial agency of fire, but with great difficulty by any other process.

3025. A very curious fact in the history of veins is, that they are of different dates, for one vein often intersects another, and we are thus enabled to judge of their relative ages. In the county of Cornwall, one of the richest mining districts of the world, we observe some remarkable circumstances of this kind. Where a copper and a tin vein, for instance meet, the former always cuts through the latter, and generally throws it out of its old course, greatly to the distress of the miner, who sometimes cannot find its continuation, or at least is put to

much difficulty and expense to do so. It appears, therefore, that tin veins are invariably older than those of copper. Sometimes, as in Derbyshire, the metallic ores lie in large longitudinal cavities, called pipe veins.

3026. In searching for veins of the useful metals, there are certain indications of which the experienced miner sometimes profitably avails himself. Thus, a green earthy matter is a good symptom in a tin mine; a brown ochrey earth, and compact iron pyrites, are regarded as favourable omens in a copper mine.

Detached pebbles of ore, or fragments of vein-stones, have sometimes led to the riches of the vein, and tin has especially been thus discovered in Cornwall.

In older mineralogical works we read much upon these and other subjects. Flames of light have been described as playing over a district which afterwards has been found to contain subterranean riches, and this may have arisen from the good electrical conducting powers of the vein. The waters issuing from the soil sometimes hold metallic salts in solution, and repositories of the metals have been discovered by circumstances of this kind. Copper veins tinge waters blue, and a piece of grease put into them becomes rapidly stained of that colour.

There is no popular notion more common than that metals grow in the veins: an idea which may very probably have originated from observing the depositions of one metal by the introduction of another into its solution, as when silver is precipitated by the introduction of a plate of copper into its solution, or copper by iron.

3027. Districts rich in the metals are generally barren, and seem peculiarly dreary and desolate to the traveller. This partly arises from the nature of the strata; partly from the heaps of rubbish and hills of stone thrown upon the surface: and partly from the operations carrying on in the vicinity, being inimical to vegetation. The high road through Cornwall, especially near Redruth, is an excellent specimen of this kind of country; while, at the same time, the romantic beauty and luxuriant vegetation of many parts of that county, and of Devonshire, prove that exterior cultivation is not always incompatible with internal riches. The neighbourhood of the Parys Mountain, in Anglesea, is singularly marked by sterility and gloominess. The soil, naturally unproductive, is rendered more so by the poisonous waters that traverse it, and the sulphurous vapours that float around. There are not only no shrubs and trees, but the barrenness is unrelieved even by a single blade of grass, or the rusty green of a hardy lichen.

3028. I have hinted above at the relative permanence and durability of the different kinds of rocks, and it has been found that the unstratified, or primary, substances, are least acted upon by the elements; that these have retained their great and pristine elevation, while the secondary strata have been washed from their sides and summits, whose rugged and abrupt outline records this devastation. Every one who views the mountain side strewed with immense blocks of materiala transported from distant summits, and discovers the dells and valleys filled with fragments and pebbles of the neighbouring rocks, will allow that a constant system of disintegration and decay is here carrying on; but the geologist, not content with the mere observance of the fact, will endeavour to trace it to its source, and follow it up to its ultimate effect.

3029. The change of temperature to which the earth's surface is constantly submitted, is one great cause of the slow destruction of its most solid and durable constituents; and when to this is added the gigantic power with which water, in becoming ice, opposes the obstacles to its expansion, we have an agent nearly resistless. The fissures that occur between the blocks and masses of the granites, porphyries, and similar rocks, become filled with water, which, in the act of freezing, expands so as slowly to remove them from each other; their edges and angles become thus open to the attacks of the weather, and by a slow dislodgement they fall into the valleys or rivers, or are at once cast into the ocean. Where the materials are of a more yielding and frangible texture, this destruction is proportionally rapid, and the influence of the weather upon slate mountains, is often such as to produce hills of fragments at their feet: the softer substance of the secondary and horizontal strata is, of course, yet more easily and quickly degraded.

3030. Masses of rock, thus loosened from their original beds, become new and powerful instruments of destruction; they roll down the precipices, wearing themselves and the surface that bears them, and, if near the sea, or carried thither by rivers, they become " a part of the mighty artillery with which the ocean assails the bulwarks of the land;" they are impelled against the coasts, from which they break off other fragments: and the whole thus ground against each other, whatever be their hardness, are reduced to gravel; the smooth surface and rounded masses of which are convincing proofs of the manner in which it was formed.

3031. It is by operations of this kind, not performed in a day, but in ages, that nature has indented and carved out the earth's surface; that the rivers seem to have cut their own beds; that the land is undergoing gradual demolition; and that the materials which we have elsewhere considered as consolidated at, and elevated from, the bottom of the ocean, are gradually restoring to the parent deep. These are mechanical agents, but they are not unassisted by the chemical energies of matter; and, in this respect, the solvent powers of water may be contemplated as effecting most important changes. -KIDD's Essay, p. 181.

3032. By impregnation with carbonic acid, water acquires a great solvent power over carbonate of lime (642,) and in trickling through, such strata becomes saturated with it, and, on exposure, again deposits it, in consequence of the escape of the gaseous solvent; it is thus that the stalactitical concretions of limestone caverns are produced, as in the Fluor Mine, and Peak Cavern of Derbyshire; and, in many cases, the once empty chasms are entirely choaked up by this sparry deposit. The power of incrustation, thus possessed by some waters, is such as rapidly to cover extraneous bodies thrown into them with a calcareous coating, of which the petrifying spring of Matlock furnishes a good example.

3033. The sands upon flat coasts are sometimes agglutinated by this action of water, so as to produce a new rock; or, as the Wernerians would call it, a new formation. This has probably been the case with the stone in which the galibi, or human skeletons of Guadaloupe, are found (Phil. Trans. 1816,) and the process is constantly going on upon the coast of Cornwall, in the parish of St. Columb, where the water, having percolated the neighbouring rocks, becomes slightly carbonated