Magnesian limestone.-This characteristic rock is well developed in a continuous band, extending from the Tyne southwards through Yorkshire, and descending into the counties of Derby and Nottingham. Towards the northern and southern extremities the strata rise into a series of round-topped hills; but the centre sinks to the general level of the country. The appearance of stratification is very distinct, and several varieties of structure occur-some beds being granular, others imperfectly crystalline, and others cellular. In the latter variety, abundant in the county of Durham, where it bears the name of the honeycomb limestone, the cells are lined with crystallised carbonate of lime. The rock, as its name imports, is a compound of the carbonate of lime and the carbonate of magnesia, in various proportions; but calcareous beds are common, in which there is little or no admixture of magnesia with the lime. It forms one of our most durable building stones, and has been largely used in the construction of some of our finest edifices, as York Minster and Westminster Hall, which have admirably withstood the wear and tear of the atmosphere. It is now employed in the erection of the new houses of Parliament, derived from the Bolsover quarries in Derbyshire. An analysis of the stone of York Minster gives→→

The stone of Westminster Hall contains about 2 per cent. less of magnesia, but the proportion is often far greater, amounting to more than 50 per cent. Magnesia in minute quantities is very extensively distributed; it occurs more largely in the mountain limestone, oolites, and chalk; but is only found in such abundance in the limestone of this system as to become a characteristic. The two constituents of the rock, the carbonates of lime and magnesia, some suppose to have been deposited contemporaneously; but Von Bûch considers the magnesian limestone to be a metamorphic change of common limestone, effected by the disengagement of vapours containing magnesia from plutonic masses. But it has been justly remarked, that, according to this theory, we ought to expect all limestones to become magnesian in the neighbourhood of igneous rocks, which is far from being the case.

Gypsum. This sulphate of lime, one species of which is commonly known under the name of alabaster, frequently appears in association with the red marls and sandstones of the upper part of the series. It occurs in detached nodules of fibrous structure, in horizontal seams equally fibrous, varying from one to three inches in thickness, and in amorphous granular masses capable of being worked into columns. The red marly banks of the Trent exhibit beautifully white and translucent seams of gypsum; and at Chellaston, not far from the borders of the river, it appears in a granular mass, from which the beautiful pillars at Kedleston Hall, near Derby, were wrought. Its origin has been referred by some to segregation from the surrounding sedimentary matter; but its common connection with rock-salt, wherever the latter mineral is found, is considered by others as indicating its formation from the same cause.

Rock-salt.—The mineral, chloride of sodium, is not peculiar to the series of deposits under notice, for salt springs occur in the coal-measures, and in strata belonging to the modern volcanic period; and salt is procured from the chalk and oolite systems. Former writers, therefore, were too hasty in referring the production of all the saliferous deposits upon the face of the globe to the geological era of the new red sandstone. Still, saline strata-clear, white cubically crystallised masses of salt, and brine springs issuing from such rocks are so remarkable and frequent in it, as to be justly regarded as one of its distinguishing features, and hence the common application of the term saliferous to the entire series. By far the finest example of rock-salt in Europe, on account of its position at the surface, is at Cardona, a small town in the interior of Catalonia, sixteen leagues

from Barcelona, and seven from the central ridge of the Pyrenean chain. The formation in this locality includes the hill on which the town is situated, and the environs to a considerable extent, which are diversified by various eminences composed of the mineral, but one in particular, called the mountain of red salt, from that colour predominating, is most prominent, from its insular character, its great mass, its sharp forms, and the red and white hues, the vivacity of which contrasts with the grey and sallow tints of some surrounding rocks. It has been well described by M. Cordier, in the Annales des Mines for 1817, and by Count A. Laborde, in his Travels in Spain. "The colours," says the latter, " vary with the altitude of the sun, and the greater or less quantity of rain. At the foot of the mountain a spring of water issues, which comes through a fissure we perceive on the summit. The rivulet runs all along the valley from the east, but passes under ground in part of its course, particularly under the hill where the rock-salt is mined; it rises again to the surface at a little distance, and, after running along the plains, discharges itself into the river Cardona. This brook in rainy seasons swells the waters of the river, which then become salt, and destroy the fish ; but at three leagues lower, the water has no perceptible taste of salt. All the salt mountains are intersected by crevices and chasms, and have also spacious grottoes, where are found stalactites of salt, shaped like bunches of grapes, and of various colours;-nothing can compare with the magnificence of the spectacle which the mountain of Cardona exhibits at sunrise. Besides the beautiful forms which it presents, it appears to rise above the river like a mountain of precious gems, displaying the various colours produced by the refraction of the solar rays through a prism."

Saliferous strata occur upon an immense scale in Hungary and Poland, and in Western Asia, referable to various geological eras; but the extensive beds of rock-salt and gypsum near Bex in Switzerland, so long considered as a decided example of such rocks occurring in the more ancient deposits, have been shown by Dr. Buckland to belong to the new red sandstone formations. The chief site of the mineral in England is in the marl of Cheshire, where, with a few exceptions, it is found in the valleys of the Weaver and its tributary streams, in some places manifesting its presence by springs impregnated with salt; in others, being known by mines carried down through the substance of the strata. At Northwich the brine springs are very abundant, formed by the penetration of spring or rain waters to the upper surface of the rock-salt, in passing over which they acquire a degree of strength, modified by various circumstances, in some instances approaching to the point of perfect saturation of the brine. Here, also, many mines have been sunk for the purpose of working out the fossil salt, which was first discovered in the year 1670 in searching for coal. It forms two great strata or beds, lying nearly horizontal, but on different levels, the superincumbent being separated from the subjacent stratum by several layers of indurated clay or argillaceous stone, irregularly penetrated by veins of salt. The upper stratum is found from 28 to 48 yards below the surface of the earth, the superior soil consisting of white clay and gypsum, the latter appearing in connection with the salt of Hungary, Poland, and Transylvania. This bed, varying in thickness from twenty to thirty yards, has a reddish-brown colour, not much unlike sugar-candy, and consists of muriate of soda, mixed with a small portion of oxide of iron, which gives it its tint. The lower stratum, which has never been entirely perforated, is remarkably different from the upper in appearance, consisting of almost pure muriate of soda, generally perfectly white and clear as crystal. The quantity carried down the river Weaver from April 5, 1855, to April 5, 1856, amounted to 53,256 tons of rock-salt, and 708,358 tons of white salt, manufactured from brine.

The formation of rock-salt and gypsum is in general referred to deposition from the waters of the sea. Dr Holland, from the appearance of the plain constituting the salt

district of Cheshire, supposes that the sea once flowed up the valley of the Weaver, and was cut off from it by the growth of a bar, again making its appearance by bursting through the barrier, and again the communication ceasing from the same cause. He conceives, therefore, the two beds of rock-salt to have arisen from the two salt lakes thus successively formed, whose waters were dissipated by the natural process of evaporation, and their salt deposited, the intervening and superior strata of indurated clay proceeding from earthy sediments in the lakes, deposited after the precipitation of the salt. This hypothesis is supported by the phenomena of many salt lakes in the present day, whose waters are lowering through the supply from springs not keeping up with the expenditure through evaporation, and whose beds consist of layers of salt, deposited by the overcharged water. There can be little doubt that this is the principle upon which the saliferous deposits have been formed, accelerated in its action by a higher atmospheric temperature, and the frequent play of igneous forces, in earlier ages. But the detail of the theory in the case of the Cheshire salt deposits is open to the objection, that it "employs data drawn from the present relations of land and sea to elucidate the phenomena of a period long gone by, and when from unquestionable evidence it is certain that their relations were generally very different." At the same time, it is quite possible, that in the district in question these relations may have been much the same then as at the present.

It has been observed, that the new red sandstone series, taken as a whole, is remarkably deficient in the traces of organic life, though, locally, some of its members, as the muschelkalk of Germany, are rich in fossils. The vegetable luxuriance of the carboniferous epoch appears to have died away, though it is true, the paucity of life may be more apparent than real, for it may have arisen from the unfitness of the new red sediments to preserve organic remains in the fossil state. The most numerous relics are those of marine life, which present a new phase in the instance of fishes. Agassiz established the curious generalisation, that the heterocercal tail is universal in the magnesian limestone, and all the older formations; while in all strata above the magnesian limestone, the

homocercal tail predominates, as at present. Reptiles seem to have been on the increase. In the year 1834, two species were discovered on Durdham-down, near Bristol, in strata belonging to the magnesian limestone-the Palæosaurus and The

state.

Labyrinthodon pachygnatus.

codontosaurus. Relics of a very singular reptile of the lizard tribe were found by Dr. Ward in quarries of the new red sandstone at Grinsill, near Shrewsbury - the Rhynchosaurus — with foot-prints upon the layers of stone in the quarries, supposed to have

In

been impressed by the animal while walking over the surface of the strata, when in a soft Parts of the skeletons of Batrachian reptiles,-the Greek name for the frog,but of a gigantic size, have been taken from the sandstones of Guy's Cliff, near Warwick and Leamington, of which the cut exhibits a restoration of one species by Professor Owen. The generic name, Labyrinthodon, refers to the labyrinthine inflections of the teeth. the upper part of the series on the continent, the remains of reptiles multiply; and here, in the muschelkalk, occur the bones of some large animals of that class—the Protosaurus and Phytosaurus. But the most striking peculiarity yet observed is the repeated occurrence of fossil footsteps, or tracks on the sandstone, affording evidence of the existence, at the era of its deposition, of birds belonging to the tribe of Waders, the first indications we have of that highly organised class of vertebrated animals, as tenants of the globe. Ichnites, traces or foot-prints, are characteristic of the new red sandstones,

and appear to have been impressed generally by reptiles and birds. It was long sus pected that such impressions were of organic origin; but geologists hesitated to admit this opinion till the evidence was complete, preferring to consider them as the effects of disintegration or aqueous action, by which the softer parts of a rock are worn away before the harder yield. We may notice such impressions under two general divisions.

Ichnolites, foot-prints on stone.-In the year 1828, Dr. Duncan gave an account, with drawings, to the Royal Society of Edinburgh, of the tracks of an animal on new red sandstone, in the quarry of Corn Cockle Muir, in Dumfriesshire. The tracks were found there in great abundance, on many successive layers of stone, to the depth of forty-five feet, or as low as the quarry had been opened. After removing a large slab which

presented foot-prints, perhaps the very next stratum, at the distance of a few feet or inches, exhibited the same phenomenon. Hence the process by which the impressions were made on the sand, and subsequently buried, must have been repeated at successive intervals. In another quarry in similar strata, near the town of Dumfries, the same marks were discovered, and in one instance a track extended from twenty to thirty feet in length. Dr. Buckland refers these impressions to land tortoises. In 1834, an account was published of some remarkable fossil footsteps in the new red sandstone at Hesseburg, near Hildberghausen in Saxony. The largest track appears to have been made by an animal whose hind foot was eight inches long. It has received, from Professor Kaup, the name of Chirotherium, founded on the resemblance of its impressions to the shape of the human hand; but some of the tracks appear to have been made by tortoises, and M. Link suggests, that others are to be referred to gigantic batrachians, or frogs and salamanders. The annexed cut shows a few tracks of the Chirotherium on a sandstone slab from Hesseburg.

Tracks of the Chirotherium.

In the summer of 1838, a variety of tracks, referred to the Chirotherium, tortoises, and saurian reptiles, were discovered in the new red sandstone at the quarries of Storeton Hill, in the neighbourhood of Liverpool. The largest foot-print was nine inches long, and six inches broad,

the length of the step approaching to two feet. Professor Hitchcock notices twentyseven species of tracks, occurring in fifteen quarries, along the banks of the Connecticut river, some of which he called Sauroidichnites, from their resemblance to the tracks of saurians; and Mr. Scrope found abundant foot-prints, along with ripple-marks, on layers of the forest marble, to the north of Bath. These are conjectured to have been made by crustacea, crawling along the bottom of an estuary, for between the rows of the foot-marks the impression of the stomach, or the trail of the tail, is sometimes visible.

Ornithichnites, stony bird tracks.-A communication made to the American Journal

of Science, in January, 1836, by Professor Hitchcock, brought before the attention of European savans some very distinct tracks in the red sandstone of the Connecticut valley, first observed by Dr. Deane of Greenfield, who immediately noticed their similarity to the impressions left on the muddy banks of the river by the living aquatic birds common to the locality. Subsequent examinations discovered similar foot-marks in several quarries in the same valley, and other parts of Massachusetts, and several specimens are now in the British Museum, the most remarkable of which is a slab, eight feet by six, which exhibits traces of various sizes, belonging to different individuals. Subjoined is a representation of this slab-that which arrested the attention of Dr. Deane, at Turner's Falls, Massachusetts.

All these footmarks are referred to that class of birds called Waders; and while some are very small, others are of enormous size, indicating proportions equal nearly to twice the magnitude of the ostrich. In one species, Ornithichnites giganteus, the imprint of the foot measures fifteen inches in length, and ten inches in width, excluding the hind claw, which is two inches long. The distance of the impressions from each other vary from four to six feet. The former may be taken as the length of the stride of the bird when walking at an ordinary pace, and the latter when proceeding more swiftly. These dimensions, so far surpassing those belonging to any known birds, led many geologists to conceive that some mistake had been made respecting the nature of the impressions, till convinced by an actual inspection of the specimens transmitted to Europe, and by the discovery of the bones of the Dinornis, fearfully great bird, in New Zealand, an individual much larger than the existing ostrich.

But a still more remarkable feature of the new red sandstone formations is the preservation, in connection with foot-prints, of very distinct impressions of rain drops upon the strata, as represented upon the preceding page. Dr. Deane discovered a stratum containing, in all, more than one hundred marks of the feet of four or five species of birds, the whole surface having also been pitted with the marks of a shower of rain. The same obserPortion of a slab of new red sandstone from Turner's Falls, vation has been made in the Storeton quarry, Massachusetts, with imprints of the footsteps of a large near Liverpool, where tracks of the Chirotherium are found. "The under surface of two strata, at the depth of thirty-two or thirty-five feet from the top of the quarry, presents a remarkably blistered or watery appearance, being densely covered by minute hemispheres of the same substance as the sandstone. These projections are casts in relief of indentations in the upper surface of a thin subjacent bed of clay, and owing, in Mr. Cunningham's opinion, to drops of rain." The impressions are sometimes perfect hemispheres, indicating a vertical fall of rain; but in

bird.