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the superincumbent rocks are frequently penetrated by granite veins, and it is obvious that every vein must be of a date posterior to that of the body which contains it; and further, as the veins are often observ. ed to proceed from the main body of the granite, into the superincumbent strata, it may be argued, that the mass of granite, and the veins proceeding from it, are coeval, and both of later formation than the immediately superincumbent strata.

Veins of granite, however, are frequently discovered, which cannot be traced to any original mass, or mountain; they seem to be insulated, as it were, among other strata. This is the case at Portsoy, and in Glentilt; and in some of the Western Isles of Scotland, especially Tiree and Coll; and is also observed in many parts of Cornwall. Dr. Hutton, from collateral evidence, conceives that these are always united to some granitic mass, though too deep, or at too great distance, to be traced and discovered

It may now be asked, how the pupil of Werner accounts for phænomena of this kind? I have already said that he regards granite as having been deposited before all other rocks, though its irregularity and its general want of stratification are decided objections to such an idea, and that the other substances were precipitated upon it in the order we find them. In these strata, cracks and fissures occurred, and a new deposition of granite took place from the chaotic fluid, confined to the said cracks and fissures, and producing the appearance of granitic veins; and the hardening of the neighbouring rocks, referred by the Huttonians to the heat of the injected granite, is accounted for by the infiltration of the aqueous solution, which has, as it were, lapidified the softer materials. Now, though we may imagine granite to have been in igneous fusion, we cannot easily conceive it susceptible of aqueous solution; and if so dissolved, why should its second deposition have been confined to the cracks and fissures? Why should it not have formed a new stratum ? With these facts before us, it is useless to enter into further comments, and we can only embrace that hypothesis, (for, after all, it is but hypothesis,) which appears best supported by evidence derived from actual observation*.

2071. The aspect of a granite district in nature is subject to variation; it, however, exhibits traits sufficiently peculiar, which are readily recognised by the traveller in his approach to it.

In Cornwall, and in some parts of Ireland, especially in the county of Donegal, the granitic rocks are marked by the bold and abrupt precipi ces which they present to the attacks of the ocean; and by the barren and dreary aspect of the inland plains that seem like fields, in which blocks of the stone have been torn from their beds, and indiscriminately scattered over the moss-grown surface. The elevation of these districts is not considerable, the granite is coarse grained, and splits into immense blocks, separated from each other by natural seams, and appearing like the ruins of edifices constructed by a giant race. In

* Some have regarded granite as a congeries of crystals of mica, feldspar, and quarta, accidentally blended and united; the inspection, however, of the rock, clearly proves that all its materials have been together in fusion; for we find in some granites the quartz impressed by the crystals of feldspar, and in others the feldspar receives impression from the quarts Dr. Hutton has looked upon this as demonstrating the igneous fusion of granite, for, (says Mr. Playfair,) "had the materials been dissolved in water, one kind of crystal ought not to me press another, but each enjoy its own peculiar shape." This, however, I do not hold to we sound argument.

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other cases, granite forms irregular and broken peaks, of prodigious elevation, and does not split into the blocks and masses just alluded to. This is the case in the Alps and Pyrenees, in the highest Scotch mountains, in the Hartz, and in the Tyrol.

In Asia and Africa granite constitutes the Uralian, Altaian, and Himáláyán chains, and the Atlas mountains; and in South America, the lofty ranges of Cordilleras are chiefly of a similar description. The wood-cut at the head of this section shows the appearance of the alpine and of the massive granite. The sketch is taken from that of Mont Blanc, and of the Land's-End, in Cornwall.

2077. Some kinds of granite are prone to decomposition, crumbling down into a fine clay containing siliceous particles: this probably arises from a peculiarity of the feldspar, afterwards to be noticed. In general, granite is the most durable of nature's productions, and long re sists the destroying hand of time; as a building material, therefore, granite is almost unrivalled: and, though in common cases its extreme hardness is against its employment, its use should be enjoined for public edifices. Dublin furnishes some noble examples of buildings constructed of granite, which is there procured in the immediate vicinity of the city, and of a very beautiful kind.

In Wales there is very little granite; in the north of Scotland it is abundant; and in England it occurs in Cornwall, Devon, Westmoreland, and Cumberland. It is also met with in smaller quantities in Worcestershire, at the Malvern Hills; and in Leicestershire, in Charnwood Forest.

2078. Although granite probably exists in great abundance below the earth's surface, the quantity visible above ground is comparatively small, perhaps not amounting to a hundredth part of the other primi. tive and transition rocks. In some parts of Scotland the granite superficies, however, is very considerable, and much exceeds the limits assigned to it by Dr. Hutton. Upon this subject a very acrimonious controversy arose between Dr. Hutton and Mr. Kirwan; the general statements, however, of the former, in this and other cases, commonly make much nearer approach to truth than those of the latter; but as human reason is not infallible, he who always contradicts must sometimes be right, and thus the mere cavilling disputant may occasionally discover the errors of the slow and cautious observer of nature.

2079. To the class of massive unstratified rocks belongs PORPHYRY, a substance which is ranked by Werner among the primitive formations. Its essential constituent is feldspar; and genuine porphyry may be defined as massive feldspar, containing embedded crystals of the same substance. Any rock including distinct crystals of feldspar, is called porphyritic, as porphyritic granite, &c. The colour of porphy

ry,

which is usually reddish, brown, and green, is principally derived from the base, or paste including the crystals. The common aspect of porphyry is that of blocks and masses, not very unlike some of the varieties of granite, but its fragments are generally smaller, and are in a more decaying condition. Porphyry is an extremely durable material for architectural purposes, and as such was highly esteemed among the nations of antiquity. It is met with in many parts of Britain: and in the north, the porphyry districts are of singular grandeur, as at the base of Ben-Cruachan, on the banks of the Awe; and amidst the precipices of Ben-Nevis, the highest of the British mountains,

The British porphyries are many of them of great beauty, and might well be substituted for all ornamental purposes, for the more rare and expensive foreign varieties.

2080. Granitic rocks frequently contain a large proportion of hornblende, a mineral of a greenish black-colour, which sometimes forms prismatic crystals; it consists of silica and alumina, with magnesia, and appears to derive its colour from oxide of iron, of which it contains from 20 to 30 per cent. Hornblende sometimes passes into mica; and if the component parts of the two bodies be compared by analysis, the principal difference will often be found to consist in the excess of iron in the former.

These aggregates are termed SYENITES, or syenitic rocks, and are of various hues, according as one or other of the constituents predominates. Sometimes the place of the quartz is wholly occupied by hornblende, and the rock is principally an aggregate of feldspar and hornblende. The term syenite is derived from Syene, in Upper Egypt, where this rock is plentiful and was used for architectural purposes by the Egyptian and Roman sculptors. The aspect of syenitic rocks is allied to that of granite and porphyry. They may be observed rising from the slaty district of St. David's in Pembrokeshire; and in Cumberland, near Wastdale and Buttermere. A beautiful syenite is noticed by Mr. Bakewell, as occurring in Leicestershire, at MarkfieldKnowle, a hill on Charnwood Forest. Syenite very often contains magnetic oxide of iron.

2081. Another substance belonging to the class of rocks we are now describing, is SERPENTINE; its appearance is singularly picturesque and beautiful; and it forms a delightful contrast to the sublimity of granitic districts. Serpentine has its name from the variety of tints which it exhibits, such as bright red, green, brown, yellow, and their various shades, and it often is prettily traversed by veins of a soft substance, to which the term steatite or soapstone has been given (698)*.

Some of the varieties of serpentine admit of a tolerable polish, and such are very desirable for many ornamental purposes.

Serpentine is seen in Cornwall in characteristic beauty, forming part of the Lizard promontory on the southern coast of the county, where its general aspect is shown in the following sketch. It appears in variously shaped and coloured blocks and masses; it forms natural arches, columns, and caves; and the district is of very singular interest from many concomitant circumstances, especially from the blocks of porphyry upon which the serpentine is incumbent, and the veins of granite associating with those of steatite, which pervade it.

* Serpentine has been repeatedly analyzed; but the results are very discordant; no doub owing to the indeterminate nature of the rock. See JAMESON'S Mineralogy, 2d. edit., Vol. i., p. 509. Its principal constituents appear to be silica, magnesia, oxide of iron, and a line carbonate of lime. See the analyses of Serpentine, given above (1410).

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Serpentine is met with also in the Isle of Anglesea, upon the northern coast near the celebrated Parys Mine. Some of the serpentine of this district is of more brilliant colours, more hard and translucent than the ordinary serpentine; it belongs to the species called by mineralogists noble serpentine; the same rock occurs at Portsoy, on the Murray Frith in Banffshire, where it is associated with granite.

The composition of serpentine, as relates to its proximate components, has been variously described. It is generally so fine grained as to appear of an uniform texture; but in Cornwall a coarsely aggregated rock, consisting of feldspar, talc and schiller spar, may be traced passing into the fine-grained serpentine. I have already alluded to the nature of feldspar. Talc is a body somewhat resembling mica in appearance, but the plates into which it is divisible are not elastic. Its usual colours are various shades of green. It consists of nearly equal parts of silica and magnesia, with a little lime; not more than six per cent. It is met with in small tabular crystals.

Schiller stone, or schiller spar, is a term from the Germans, implying glistening or changeable spar: it is one of the varieties of diallage of the French authors; it is a silico-ferruginous fossil, containing

44 silex

24 iron

18 alumina

12 magnesia.

Its colour is dark green: its usual lustre is semi-metallic, varying according to its position in regard to incident light.

Steatite is a substance of different tints of grey and green, and from its very singular unctuous feel has been called soap-stone. It is somewhat abundant in the serpentine of Cornwall, one of the masses of which is called the soapy rock; it is here carefully collected for the porcelain works of Worcester and Swansea, in which it forms a very important ingredient. It also occurs in the serpentine of Banff. According to Klaproth, Cornish steatite consists of

[blocks in formation]

22. MASK 543 is the last of the rocks belonging to the class I am now descrizing. It is also very abundant in the secondary rocks, Jus as characters are there different. Among primary rocks, marble is asercuted with mica slate, gneiss, serpentine, and quartz rock, and st fufers from marole belonging to other rocks, in its granularly foJuted sexcare and in the absence of organic remains. The most esDeemed varieties are perfectly white and free from veins; somewhat transincent, and susceptible of a good polish. These marbles are imported for ornamental purposes, especially for those of the sculptor. Nearly all the sublime works of the Grecian artists were sculptured in the marble from the isle of Paros in the Archipellago, and from the Pentelic mount near Athens; but the marble of Carrara is now in highest estimation, and is almost exclusively used by the European sculptors of the present day of the coloured varieties, that of the sue at Tiree is extremely beautiful; it is of a pale red, spotted with green horabiende. Marble is found in several parts of Scotland, and in some places of characteristic beauty, and alternating within small hots, with other rocks. Dr. Mac Culloch, in his Sketch of the Minerawingy of Skyt, has described several beautiful varieties found in that island, and has adverted to the economical uses to which they are appicable. In Inverary park primary marble may be seen in contact with mica slate and porphyry. Serpentine and marble are sometimes blended together, and they then form a valuable compound for ornamental purposes, which has been called Ferd Antique. In the serpentine of Anglesea, patches of marble are found which much enhance its beauty.

A very remarkable marble quarry is that of Icolmkil, or Iona. Gneiss rocks constitute the leading feature of this island, but at the south-west point is a bed of marble, about 40 feet wide bounded by vertical walls of hornblende rockst. Near it is a mass of hornstone, and above the whole protrudes an immense vein of granite, surrounded

• Vide KLAPROTH'S Beitrage, V. Band, S. 24.

+ Geological Transactions, iii. p. 1.

↑ The marble is of the species called dolomite (698), distinguished from the true primary marble or granular lime-stone, by the tardy effervescence excited by pouring muriatic acid upon it, and by its containing magnesia; it is also finer grained, and its fracture more splia tery, than that of common marble. The dolomite of Iona yielded to Mr. Tennant,

Carbonic acid
Lime.

[blocks in formation]

48.82

[blocks in formation]

The dolomite of the Apennines yielded to Klaproth,

Carbonate of lime

Carbonate of magnesia

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