mately, through the agency of heat evaporating the water, become clay, and finally converted into stone, hence producing this second class of rocks.*

But then these rocks having been formed in horizontal beds, they must have since been acted upon by some powerful volcanic agent from below, by which perhaps they were first softened, and then thrust upwards, to produce the disorder and stratified veins, which give them the various inclinations they now possess. As to the formation of mountains and valleys, when the Deluge took place the earth became immerged in its waters, being "covered with the deep as with a garment,' which on retiring would naturally cause mountains to be formed and valleys to be channelled, in whose sides and cavities evidently various diluvial deposits would take place, and thus other rocks of various kinds might be formed, of different compounds and strata, produced from the convulsion and currents of the floods of waters in their winding passages through the earth, until they finally emerged into the ocean.†

Those stratified diluvian rocks are therefore found to be made up, some of compact clay and pebbles, others composed of clay, pebbles, and small round shells, with various mineral substances; and a third composition of rocks contains a conglomeration of clay, marl, pebbles, marine shells, and bones and teeth of a multitude of animals of various species now entirely extinct.‡ Thus we find there are certain principal groups of petrified substances conglomerated with clay, forming rocks reducible into regular beds, all distinguished by marked peculiar characters, and descending to the primitive rock as a basis. They are as follows:

The Chalk group.

The Oolite (Freestone) group.

The Red Sandstone (red Marl) group.

The Limestone (transition) group.§

The Old Red Sandstone (Conglomerate) group.
The Graywacke Slate group.

The Clay Slate group.

The Granite group.

Now seeing the durability of stones employed in a building must necessarily depend on the chemical compound nature of which those stones are formed, we shall thus describe the leading characters and quality of each group, proceeding in an ascending order from that of the granite or primitive rocks.¶

The present surface of our globe is composed of lapideous materials, the nature and composition of which it is the business of mineralogy and of chemistry to determine; not that the minutiae of either of those studies need of necessity be gone into, either by the practical geologist or by the architect. Though the substances which present themselves are comparatively few in number and

* From the dropping waters of the rock at Knaresborough in Yorkshire, and the petrifying well at Matlock in Derbyshire, various substances have been converted into stone. At Arcueil, about three miles from Paris, there is a spring of water which has the property of converting fruit into stone incrustations.-(R. B.)

+ If this be not the case with the flood, how comes it to pass that marine shells are discovered buried in the earth on the tops of the mountains and at midway, and also various layers of clay deposited there, which had once been in a liquid state, but now become compact stone, and found at various depths in the earth? (B.) The solution of the difficulty to some has been founded upon the supposition that certain powerful agents have elevated our present continents, and at the same time depressed the land of the ocean; what was once therefore the bottom of our antediluvian sea, now appears to be our habitable land, and perhaps the dry land of a very remote period of the world may be the bottom of the present sea.-(Brande.) See Dr. Buckland's Lectures. § The British marbles belong to this class.

| The porphyry verde-antique statuary, and some other foreign marbles, are connected with this primitive class, which we shall describe under the article chimney-pieces.-(R. B.)

¶ Not the least important fact in the history of rocks is their gradual, and often insensible, transition, as it were, into each other downwards; for it furnishes us with a strong argument against many of those speculations into which geologists have entered respecting their original formation. The gradual migration of chalk into clays and sandstone, and of those into oolite deposits, of oolite into lias, and of this again into limestone, is visible in many hard specimens; and there are equally well-marked instances of the transition of clay-slate into red sandstone on the one hand, and on the other into mountain-limestone.-(Brande's Geology, p. 123.)

at the same time simple in their nature, as well as their external character and intimate composition, which are consequently soon known. Thus silicious, calcareous, and argillaceous substances, either pure or nearly so, and in a state of mixture, or loosely and indefinitely blended rather than in strict chemical combination, constitute a very large relative proportion of those rocky masses, scattered or comminuted substances, which form, or have formed, the most extensive constituents of our globe. Now the granite is considered to be the primary rock, although surrounded with others of a primitive nature; that is, such rocks which do not contain diluvian, organic, or volcanic substances, such is that of the clay roofing-slate, the Egyptian red porphyry, statuary, and some other foreign marbles, which are supposed to be a second formation. Granite is also regarded as the primary or original rock, because it is found to be the lowest in the earth, and upon it all other rocks are found to rest, while in some places it is observed to have pierced through the superincumbent substances, forming exposed peaks, and the loftiest summits of the principal mountain-chains in the world.*

We shall first treat of the granite as to its compound nature; secondly, as to its durability; and thirdly, its appropriateness to building. In substance the three component parts of granite are quartz, felspar, and mica,† distinctly or confusedly blended together. The varieties of granite arise solely from the loss of one or other of these ingredients, or from the addition of some other mineral, which is by no means uncommon. The character of granite, therefore, depends much upon the perfection or prevalence of one or other of these three ingredients. Their aggregation in the crystalline form is in some specimens distinct and well defined; in others, imperfect, forming what are usually called fine- and coarse-grained granite; the latter where large crystals of felspar sometimes prevail.‡

As the counties of Devonshire and Cornwall are the most celebrated for this kind of stone, (particularly the former,) we shall confine our examination and remarks on this group chiefly to these two districts. Now, generally speaking, there are two very decided varieties of granite to be met with here. The grey and the red, which last accords with the colour of the felspar which is the predominating constituent.§

Both varieties generally contain black mica, and when the mica is wanting in either, it is commonly replaced by schorl. When worked for the purpose of building, the blocks are readily cleft by a wedge, and the smaller ones thus obtained are shaped to the intended forms by the use of

* The highest mountains in Britain are composed of granite and its associates; but these are mere trifling protuberances upon the earth's face when compared with the exceeding heights of the Alpine chain, or the yet more elevated mountains of South America, and of the Asiatic continent, which consist of the same materials. Ben Nevis, the loftiest of the British mountains, is situated in the south of Inverness-shire, and is four thousand three hundred and seventy feet high. Cairngorum, in the same county, is four thousand and fifty feet high, and Mont Blanc, in Switzerland, has its peak elevated fifteen thousand six hundred feet above the level of the ocean: it is the highest mountain in Europe.- (Brande's Journal.) + Quartz occurs crystallized or massive; felspar crystallized and compact, having a foliated structure and an uneven fracture. Mica foliated and crystallized, soft, and easily scratched with a knife.-(De la Beche's Geology.)

The stones in the carriage-way, and the kerb-stones of the foot-pavements in London, are of granite; the former mostly from Aberdeen and Dundee in Scotland, the latter from Devonshire and Cornwall. In those kerb-stones the crystals of felspar (particularly after rain) may be seen protruding above the surface, and from these the leading lines of the granite are destroyed. Or where mica abounds, the rock acquires a lamellar and slaty fracture, and is then called gneiss: where the felspar is very sparingly disseminated, or otherwise wanting, or where garnets supply its place, granite is said to pass into mica-slate, and this again into quartz-rock, by the partial or entire disappearance of the mica.-(B.)

§ Red granite is so called when the felspar predominates, which is of a red colour. It is common in Scotland, and the cathedral on the island of Icolmkill is built with it. Some have named it porphyritic granite.-(B.) At Troulsworthy, in the parish of Shaugh, in Devonshire, through an extent of about fifty acres, a very fine red variety occurs, much resembling some kinds of Egyptian granite. This admits of an exquisite polish, as may be seen in tables formed of it at Saltramhouse, the seat of the Earl of Morley, Mount Edgecombe, and in the chimneypieces made of it at the Duke of Bedford's cottage at Endsleigh, in Devonshire.—(B.)

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a heavy, pointed hammer or pick. It is worked in this manner when newly detached from the rock with considerable facility, but upon exposure to the atmosphere it becomes extremely hard. The granite on the high hills or Tors on the borders of Dartmoor, frequently seem harder in those places, and also of clearer texture. Hey-Tor, Sheeps-Tor, Collard-Tor, and Pen-Beacon-Hill all verify this remark. The two noted elevations of mountain-granite on Dartmoor are known by the name of HeighTor Rocks, which are peculiarly bold and massive in their character, and rise to nearly one thousand six hundred feet above the level of the sea; and their summits afford one of the finest panoramic views in the county of Devonshire. Those quarries have obtained a high repute amongst architects and engineers for the size, durability, and fineness of the texture of the blocks produced from them. Two or three hundred men are usually employed here, a great portion of the stone being worked to the required sizes, form, and shape on the spot: it is then conveyed to the Storer canal by means of a railway of seven or eight miles in length, formed of the same materials, which winds down between the neighbouring hills and crosses Bovey Heath, joining the canal-head at Ventiford, whence it is conveyed to the new wharf at Teignmouth, and then shipped to the extent of several thousand tons annually, chiefly to the metropolis.

Exclusive of Devonshire and Cornwall, there is little granite in England. The Malvern Hills, Mount Sorrel in Leicestershire, Moss-dale, and a few of the ridges in Cumberland and WestmoreIn the granite district of Cornwall there is nowhere any land afford us specimens of this rock. considerable elevation; its highest part is the hill called Brown Willey, near Bodmin, which is Dartmoor appears to be about one thousand three hundred and sixty feet above the ocean's level.

the principal depôt; a dreary mountainous-tract of land, extending over one hundred and thirty thousand acres, or one hundred and fifty square miles.+. This dreary abode, the subject of a beautiful descriptive poem by the late P. Carrington, of Devonport, was once the great seat of the Druids, but is now become a land of desolation, strewed over with large masses and groups of granite, many of which appear to have been torn from their native rocks by piecemeal, and flung over the heathy hills, setting all cultivation here at defiance..

Some varieties of granite are soft and yielding, while others are extremely hard and permanent in their mountain-masses, and which seem almost imperishable. If we consider them in respect to their uses in the arts, and witness the mighty remains of the Egyptian sculptures, executed in this hard and durable material, or look at the high state of preservation which some ancient temples of a very remote date present, they still stand nearly as perfect as ever, amidst the ruined fragments and dust of contemporary edifices around them, constructed of less enduring kinds of stone. As a material for buildings, granite stands unrivalled, particularly for the basementstory of public edifices, and for which it has lately been used in the King's Library, at the British Museum, and is now using for the subbasement of the new House of Commons. Waterloo Bridge,

* All stones work with greater ease when first taken out of the quarry, and equally become hard when exposed to the sun and the air, probably in consequence of the escape of water previously existing in its pores or interstices.—(B.) + From a survey made some years since by order of the House of Commons.-(B.)

Those persons who have visited any granite country, will not fail to recognize in the scenery of Mucklestone Moor and its vicinity, as described in that exquisite petite romance called "The Black Dwarf," a most accurate representation of the The remote and inaccessible recesses which sheltered the deer appearances and objects which such a country exhibits.

pursued by Hobbie Elliot, the cleugh or wild ravine into which that undaunted borderer followed the game, the extensive waste or moor, interspersed with rarshes and pools of water, over which he returned, the deep purple of the broad outline of heathy mountains which surrounded this desolate spot, are all characters or accompaniments of a district occupied by this rock. The description of scenery in the Black Dwarf must have been drawn, we presume, from the granite moors of Scotland; but we find in their affinity to those in the west of England an evidence of the marked and obvious distinctions of primary districts wherever they occur, and of the influence of the geological structure of a country on the character of its scenery.-(B.)

and the new London Bridge, both furnish splendid examples of the suitableness of granite for that purpose, of which they are both constructed, no less than the grand simplicity of the designs, and the perfection of the execution, which unite to constitute them fit and durable public memorials of the genius and skill of Mr. Rennie, the architect and engineer.

Though granite, as we have observed, is in general a very durable stone, yet let the architect remember we have said there are varieties of granite produced arising from the loss of one or other of the three compounds which we at first described, or from the addition of some other mineral, a circumstance not unfrequent; and that such granite is therefore subject to decomposition and to moulder down, and that with no inconsiderable rapidity.* We have therefore shown how he is to judge of its quality. The decay of granite may further be considered, in relation to its mountain-masses, as effected in various ways, depending upon the structure and composition of the rock. In Cornwall, for example, where the granite is not so excellent as that of Devonshire, there are large groups of granite so soft and yielding that it may be cut asunder with the axe, or rubbed to powder between the fingers, and the grains of quartz and mica may be separated from the pulverulent felspar, which is the ingredient that undergoes decomposition, and forms a fine white powder, which is worked out of the rock by the rains and streams that percolate the district. A very striking assemblage of these decomposing granite hills is presented to our view in an excavation of Carglaire tin-mine. The veins of ore are harder than the rock itself, and easily traced upon its rapidly-decaying surface; and every rill of water that traverses the rock is loaded with that finely divided felspar so abundantly deposited in all the low lands of the neighbourhood, and largely exported under the name of Cornish porcelain-clay.

In an ascending order, the next rock that demands our attention is the great clay-slate formation, which is next in abundance to granite; used for covering the roofs of houses, and which we shall have occasion to mention again more particularly under the article, 'Different Coverings for Roofs.'+ This roof-slate may, without any inconsistency, be referred to the great clay-slate formation, of which in England we have abundant instances, especially in the northern and western parts, and which constitute the greater portion of the mountainous districts of North Wales. In Devonshire and in Cornwall it reaches from Dartmoor to the Land's-end, lying on the back of the granite. All the magnificent scenery of Falmouth, Fowey, Loo, Tintagel, and other places upon the north as well as the south side of Cornwall, derives its grandeur and charms from the various assemblages of slaty headlands and promontories. There are however, in several parts of Cornwall, but more especially about the neck of the Lizard promontory, even rocks which are slaty in their composition and slaty in their exterior, but which, from the fragments and particles distinctly embedded in the main mass, are legitimately allied to the enigmatical family of the graywackes, while the slate at Tintagel is a distinct, unadulterated, and admirably well-defined clay-slate. The goodness of slate is known by

* De Luc informs us of the marble granite of the Hercynian forest; and Saussure describes the mouldering down of the Alps; and we have the like examples on Dartmoor, in Devonshire.-(B.)

Some slates are found to contain abundant minute specimens of minerals and metals, which soon cause them to variegate in colours and tints, after being laid on the roofs; some speckles becoming yellow, some grey, and others drab. But it is rather doubtful whether any ever contained organic remains, though impressions of some bivalve shells are said to have been found in it. I am however rather inclined to believe that all these impressions and mineral substances are either in graywacke-slate, or in that which borders upon and passes into limestone, and that genuine clay-slate is destitute of shells; a fact which appears opposed to its imaginary aqueous origin, and which has given rise to the presumption, that the ocean was inhabited by living beings at the time that this great deposit was produced.-(Brande's Outlines.)

The Dennybole quarry at Tintagel, in Cornwall, produces the finest and best slates in England: surpassed by the Welsh for durability. They divide more equally and uniform in the thicknesses, besides being of a better texture and quite uniform in colour. Mill-hill quarry, near Tavistock, produces excellent slate for roof-covering; and that of Cann quarry, at Borringdon, in Devonshire, is exceedingly good. This quarry is in the midst of an umbrageous wood, and a deep and

its glimmering surface, approaching to that of talc, clearness and uniformity of colour, and freeness from magnesian minerals. Those slates which sound well when struck on a block of wood, that break solid and absorb the least quantity of water when immerged in a vessel, are to be preferred. If water penetrate through, such are decidedly bad. Those on the top of the rock, which are generally of a dusky yellow, are in a state of decomposition, and should therefore be rejected.* Associated with the foregoing rock, and next in order of succession, is the graywacke-slate, a German name for the principal rock among the lowest members of the second series, found lying upon the second primary rock. Gray wacke-stone is considered to be the first mechanical deposit.+ It is an aggregate cemented by ferruginous clay, and is composed from the debris of the primitive rocks. It is sometimes found to form the crust of the roof-slating; it is a schist, and varies in texture: it is also laminated. The colours are various in their different rocks, such as a dun grey, a pale buff, and ferruginous brown. This group occurs extensively in the hilly counties of the south of Scotland, in Cornwall, and Devonshire, where in some parishes, particularly that of Berry-Pomeroy near Totnes, and Tamarton Folliet near Roborough Down, it is the chief building-stone; but it cannot be said to be so durable a material as that of the less stratified and other solid compact stones.§

Old red sandstone. This group is situated upon the lowest secondary rocks, which appear to give it a title to that term. Ranges of this stone are sometimes seen following those of the primitive rocks, where it is evidently composed of their debris. It is characterized by its containing a great number of beds, composed of water-worn fragments, and red sandstone layers of a fine grain, and its being usually of a light red colour, which, like many other rocks, it derives from oxide of iron. It is the principal rock in Herefordshire, but not of very great extent in other English counties, except Somersetshire and Devonshire. It is estimated to be in England about one thousand five hundred feet thick. The rock about Exeter, where there is a quarry at Heavitree, is strictly of the conglomerate formation.T

sombre river passes silently by its side, the whole presenting a very romantic and sylvan scene. The slate-quarry at Larybridge produces large paving-stones for kitchens, larders, dairies, &c.-(Brayley's Geology.)

The simple varieties of roof-slating are formed of a peculiar indurate clay, while the coarser or compound kinds contain in addition quartz and mica; and sometimes, perhaps more frequently than is generally supposed, the mineral called chlorite. Its schistose or slaty texture allows of its division into thin lamina, which in general have a shining surface, and sometimes a more or less silky aspect. The cross-fracture or surface exposed by breaking the rock in the direction across that of the laminæ, is dull and fine-grained, or in some degree earthy: it often possesses the aspect of plates of chlorite, of which the larger planes are disposed with sufficient regularity to impart fissibility. Sometimes this slate has a soapy feel, arising probably from an admixture of talc with the indurated clay forming the basis of the rock. Its colours are bluishgrey, greenish-grey, and reddish-brown.-(Phillips and Conybeare's Outlines of Mineralogy and Geology, edit. 1826, p. 158.)

There are two deposits, the mechanical and the chemical: the first is a substance entirely formed from the debris of other substances. It may be compared to a compact sandbank, thrown up from the depths of the sea and deposited on its shores.(Lyell's Elements of Geology.)

Clay belongs to every formation, being the natural result of the argillaceous substances when decomposed.—(A.) § We must, however, resort to Cumberland for illustrative specimens of this rock, and to the scenery of the Westmoreland lakes for a notion of its mountain-aspects, of which Skiddaw forms the highest elevation. There is something that is exquisitely beautiful in the mountains that environ the southern extremity of Derwentwater: their form, tints, and general association and outline are perfectly peculiar, that belong to a true slaty texture. They show a union of softness and grandeur which marks them as a distinct formation; and if there be a difficulty, which there often is, in deciding respecting hard specimens, graywacke-slate may in general soon be recognized where the form of its hills can be traced.-(Brande.) || The old red sandstone, as it occurs in Devonshire, is best exhibited in the neighbourhood of Torquay and Teignmouth. At Cockington, between Torquay and Paignton, are two quarries of chocolate-coloured micaceous, silicious, and very compact sandstone. In both a slaty variety, splitting easily in the line of the lamina, which are filled with mica, is mixed with compact and micaceous beds. In its great hardness, in its colour, in being micaceous, and in general appearance it differs entirely from the red sandstone associated with the Exeter conglomerate. The old red sandstone passes into graywacke in the high hills north-north-west of Paignton.-(B.)

The quarry at Heavitree is situated about a mile and a half from Exeter, on the road to Honiton. It is worked to