view only that it appears detached, as if occupying an accidental, and not its natural and original place. Its general figure is irregularly prismatic and foursided, having at its lower part that protuberance on which it is poised So inclined is the plane on which it rests, that it appears at first sight as if a slight alteration of its position, would cause it to slide along the plane into the sea, standing as it does within two or three feet of the edge of the precipice. The breadth of the apparent contact between the plane and the centre of motion of the stone is about a foot and a half. As this support is curved only in one direction, being of a cylindrical, and not of a spheroidal figure, the motion of the stone is consequently limited to a vibration in one direction, which is nearly at right angles to its longest dimensions. It is said that the motion is now much more limited than it has been within the memory of those who live near it; a circumstance rendered very probable by the progress of disintegration at those points of contact where water can be detained... In the trials which I have at different times made on it, the greatest force that three persons could apply to it, was sufficient to make its outward edge describe an arc whose chord was of an inch at 6 feet distance from the centre of motion.... The weight appears to be 65.8 tons, a deduction if not precise, sufficiently accurate at least to satisfy general curiosity.'

The Cheese Wring is another object of attraction to travellers, presenting the appearance of five thick blocks of stone, balanced upon each other in such a manner, as considerably to overhang their base; and fancied, by Dr. Borlase, to have been formed into a Druidical statue of Saturn. Dr. Mac Culloch, however, satisfactorily shews, as indeed De Lucand others have before explained, that these appearances, with all the fancied stratification of granite, are owing to various stages of disintegration, to which that rock is liable when exposed to the air, exhibiting another trace of similarity between the granite and the basaltic rocks.

IV. Notes on the Mineralogy of the Neighbourhood of St. David's, Pembrokeshire. By J. Kidd, M. D.

The more interesting of these rocks belong to that well known but imperfectly described series of compounds of hornblende and felspar, commencing with syenite and greenstone, and passing over into serpentine and steatite. They are ably described by the Author, but prove the want of a nomenclature, to whose terms ideas within the reach of definition are attached.

V. An Account of the Brine Springs at Droitwich.
By Leonard Horner.

These springs are in the same red stratum which elsewhere produces salt and gypsum, and is probably identical with Werner's old red sandstone. The sandiness of this stratum is, how

ever, a quality so inconstant, while its colour, arising from oxide of iron, is so permanent, that we would rather adopt one of the appellations given by our English geologists, than that of German naturalists.

The brine springs at about fifty yards below the surface, and the four pits produce about sixteen thousand tons annually, at thirty one pounds per ton, including thirty pounds per ton duty. The greatest produce of a pint of the water is 2289. grs. so that it is somewhat weaker than a saturated solution of salt. Mr. Horner's analysis gives-muriate of soda or common salt 96.48, sulphate of lime, or gypsum 1. 63, sulphate of soda 1.82, and muriate of magnesia 0.07 per cent.

VI. On the Veins of Cornwall. By William Phillips.

This paper contains a considerable number of facts, collected partly from the verbal accounts of captains of mines, and personal observation, and partly from the descriptions of former authors; and is elucidated by plans of some of the more remarkable veins but the materials are of such a nature, as not to admit of a compressed abstract; and the results do not throw any light on the impenetrable obscurity which envelops the theory of veins; nor furnish any just grounds for establishing general rules. No spot more completely convinces the geologist of his ignorance, than the mound of rubbish round a Cornish mine.

VII. On the Freshwater Formations in the Isle of Wight ; with some Observations on the Strata over the Chalk in the South East Part of England.

The researches of M. M. Cuvier, and Brongniart, into the geology of the neighbourhood of Paris, have excited considerable attention. The strata are superior to the chalk; they are of limited extent, appearing to have been formed in a hollow excavated in the upper part of the chalk stratum. They are well marked by distinguishing mineralogical characters; and yet more by a multitude of reliquiæ of shells, and even of quadrupeds, in such a state of preservation, as to be referrible to their places in the system of the present creation, though almost universally differing in species.

These shells and other animal remains, appear to have been deposited, partly by the ocean, or, at least, salt water, and partly by fresh, and these depositions alternate several times. The naturalists who investigated them, were not only men of distinguished abilities, but were assisted in their comparisons, by the immense collection of the French National Mu

seum. All these circumstances have contributed to stamp the investigation with a high degree of interest; and the French geologists are, perhaps, as far before those of this country, in a precise knowledge of these strata, as we exceed them, in accurate acquaintance with those beneath the chalk.

The organic remains of Hordwell, Harwich, and Sheppey Island, which are well known to collectors, indicated that there are two depressions in the chalk strata of this country, containing formations in some degree similar to those of the Paris basin. Mr. Webster, in this paper, traces the boundaries of the British depressions, which are indicated on three maps; and compares the stratification more particularly of the Isle of Wight, with that of the vicinity of Paris. The similarity of the fossils affords a means of identifying several of the fresh and of the salt water formations, but there is a very considerable difference in their character, more particularly from the want of the beds of gypsum, none having hitherto been found in this island, above the chalk. The sandstone of Fontainbleau seems to belong to the same stratum which produced the partial concretions termed Grey Weathers, or Wethers, scattered over the surface of the chalk in some of the midland counties, and admirably displayed in the paradoxical erections of Stonehenge.

No withstanding Mr. Webster's diligence and ability, much still remains to be done, in exploring the geology of these portions of our island, the surface of which exhibits the traces of the last great revolution which this earth has undergone, while the strata themselves teem with the remains of plants, quadrupeds, reptiles, fishes, shells, corals, &c., to the amount of some thousand species, the greater part of which are distinct from any that now exist, and the rest connected with the present creation only by a doubtful and imperfect resemblance.

VIII. Remarks on the Vitrified Forts of Scotland.
By J. Mac Culloch.

The attention of antiquaries has been long attracted by the remains of edifices, evidently intended for the purpose of martial defence, which bear unequivocal indications of having been subjected after their erection to the influence of intense heat. The opinions of different observers and narrators, have differed respecting the date of their formation, and the manner in which the fire was applied; some attributing it to the accidental conflagration of a superstructure of wood; others, to the intentional combustion of fuel heaped around the edifice, either with the intention of consolidating, or of consuming it; while a third party sought to account for the heat by which the materials were affected, by the hypothesis of volcanic agency. The dis

putants having exhausted their arguments, the contest appears to have been for some time relinquished. Dr Mac Culloch, in this paper, kindly brings them a fresh supply.

His remarks are chiefly drawn from personal and attentive consideration of two of these works; one, on the hill of Dun Mac Sniochain; the other, on Craig Phadric. The first consists of a series of parallelogramic works, capable of containing about six hundred men, occupying the summit of a hill precipitous along three quarters of its circumference, and carefully guarded at the other end, where it descends gradually to the plain; displaying in the whole arrangement of its parts, marks of military design and experience, which assign their erection to an age of some talent and improvement.

The ignorance and rudeness attributed to nations of mere hunters and warriors, is falsely assigned. The history of infant society shews, on the contrary, instances of acute reasoning, of ready invention, of perseverance and prowess, which would be in vain sought among the enlightened populace of modern times, nay even among those who are far removed above that rank. But this ability and vigour of mind, have been necessarily directed to those objects only, which were useful or honourable, or were then in fashion. The abilities of infant nations require to be compared with their necessities, and to be measured by their best works, not by their worst.' p. 259.

The walls are about twelve feet in thickness, bearing marks of vitrification throughout their whole extent, but not more than a foot or two from the foundation, the effects of fire diminishing as we proceed upwards. The heaps of loose stones, accumulated on both sides of the parts still standing, do not appear to indicate a greater elevation than five feet, or just sufficient for a man to overlook; and this Dr. Mac Culloch supposes to have been the height of the ancient British field works, from the perfect remains of Castle An-Dinas, in the parish of Ludgvan, in Cornwall.

The hill on which this fort stands, is composed of limestone alternating with schistus; the surrounding plain is alluvial; the mountains of Benediraloch to the west, consist of primitive rocks, but are skirted for a considerable space, by a mountain of trap, and trap breccia or pudding stone, consisting of rounded nodules of trap cemented together by calcareous spar Of all the different species of stone in the neighbourhood, this is the only one which vitrifies by the action of fire; but it no where occurs within half a mile of Dun Mac Sniochain; yet in the construction of the walls, fragments of it are every where interspersed among the gneiss, granite, and other primitive rocks, of which they are composed, while the lime-stone of which the hill itself consists, has been used in very small quantities.

Hence it appears at least a probable conclusion, that the builders were acquainted with the effect of fire in destroying limestone, and that intending to erect a vitrified wall, they rejected that which was unfit for their purposes, however conveniently placed. Had the object been to erect a dry wall of stone and wood, the limestone This notion of a dewould have equally answered their intentions.

sign to vitrify, seems to receive additional strength from the apparent solicitude and labour employed in introducing so much pudding-stone into the work.' pp. 265, 266.

From the change produced on the pudding-stone in the walls of the fort, compared with the results of experiments on the same kind of rock in the laboratory, Dr. Mac Culloch found that the heat requisite to vitrify and consolidate the materials, must have been at least 60 of Wedgewood's scale; 30, or the heat of melting brass, was insufficient to produce any degree of vitrification. The combustion of a wooden superstructure, or of wood introduced into the walls, could not have produced so intense a degree of heat: it is, therefore, probable, that a sort of furnace was constructed for the purpose, by means of a double earthen wall;-a supposition the less improbable, as the Africans effect the reduction of iron from its ores, by a similar contrivance.

In the walls of Craig Phadric, the cementing vitrifiable substance, is a pudding-stone perfectly distinct from that of Dun Mac Sniochain, being destitute of calcareous matter. The walls consequently contain none of the porous scoriæ of the latter place, which have been circulated as pumice; but the heat requisite for their agglutination, cannot have been less. A third fort, in the parish of Amworth, built of yet more refractory materials, afforded traces of vitrification only in a few detached spots where calcareous particles abounded in the grauwacke employed in its construction, though it appears that the same attempt was made, but failed for want of a sufficiently yielding ingredient.

The idea of intentional vitrification, receives additional support, from the Hindoo practice of building with clay, and subsequently burning the walls to brick; and from a solitary instance of a dwelling-house, the walls of which appear to have been intentionally vitrified. This curious relic of antiquity, Gatacre-house, in Shropshire, is now unfortunately destroyed.

IX. On the Sublimation of Silica. By J. Mac Culloch. This phenomenon occurred in an experiment on the oxides of tin and lead. It will be highly acceptable to the Huttonians.