almost all the mountain chains of the south of this country The district of Leadhills is nearly in the central portion, where Lauder rises to 3130 feet. This chain of greywacke crosses the island, extending from sea to sea. It abounds in metallic veins grouped together, especially in its western part. Various ores of lead, zinc, iron, nickel, and copper occur, of which the first alone is an object of commercial importance. It is superfluous to attempt an enumeration of the places where this transition schist is found; as it is an essential constituent of almost every country on the surface of the globe, demonstrating the universality of those interior convulsions which were accessory to its production.

LIMESTONE.-The limestone of the submedial class is to be distinguished from the other limestones, chiefly by its position. Its grain is scarcely so highly crystalline as that of the limestone found among the primitive rocks. More frequently its fracture is scaly, approaching to compact, and even completely so. It is uniformly translucent on the edges, unless when occasionally mixed with too large a quantity of foreign matter. Its colour is very various. Most part of the marbles employed in architecture, are from the intermediate rocks; the primitive being alone adapted for statuary, by the fineness of its grain, and the purity of its white. In the mixed beds of slate and limestone, each of the two substances is formed by itself; the limestone occurring in flattened, ovoid, or lenticular masses, disposed in nearly parallel planes, separated and enveloped by the schistose mass. Among the

ORGANIC REMAINS OF TRANSITION LIME. 149

minerals contained in the intermediate limestone, we may distinguish hyaline quartz, lydian stone, (flinty slate,) and mica, or talc, passing into steatite. In some of the black coloured beds, of the North of France, the carbonaceous matter is in such quantities, as to form masses of stone coal (anthracite), approaching occasionally in its nature to common coal.

In the older limestones of Flanders, a great quantity of zoophytes have been found. These with madrepores, and millepores, are sometimes so abundant in certain intermediate strata, that M. Schlottheim was led to believe them the work of these animals, like the limestone reefs of the South Sea. The orthoceratites, which approach to that order of animals, also occur in considerable abundance, along with fragments of entrochi and encrinites. The shells most frequently found there, are terebratulites, turbinites, certain ammonites, and belemnites. But the only fossil which appears to be characteristic of the transition limestone, is the trilobite, a singular extinct animal of either the crustaceous or insect tribe.

In a great many points of the Hartz mountains, particularly towards Blankenberg, it furnishes a fine marble like that called by the Italians, rosso corallino. In Saxony, it occurs in the vicinity of Kalkgrün and Wildenfels, from which it extends even into the territory of Bayreuth. There it forms different marbles, one of which is black, and approaches

4

closely to that known in Italy, under the name of Nero d'Egitto. It is full of fragments of entrochi. In the South of France, among the Pyrenees, the transition limestone is very abundant; constituting the principal mass of the intermediate formations of these mountains. In the north, it forms a portion of the great transition zone, which stretches from Flanders into the Hartz. Here it alternates repeatedly with roofing slate. It furnishes Paris with black marbles (of Namur and Dinant). The granite marble, of the Ecaussines, 4 leagues north of Mons, has white spots, which are fragments of shells, particularly encrini, converted into lime-spar. These transition rocks, serve as a basis to the coal district of Flanders. The Alps are flanked by a prodigious calcareous belt, which extends from France into Hungary, from 8 to 15 leagues in breadth, rising into mountains upwards of 12,000 feet high. A portion in contact with the primitive formations, belongs to the intermediate class, and alternates with clay slate. A few trochites and encrinites occur in it.

In England, one narrow chain of submedial limestone, runs through South Wales, in a south-west direction from near Wolverhampton, south of St. David's; another in North Wales passes from near Ormeshead, south through Llanrwst to Bala; a third goes from Shap towards Ulverstone, through Westmoreland; fourthly, there are several ridges of it near Exeter and Plymouth; besides a few smaller separate patches. It occurs in Scotland, near the Crook on the road to Moffat, and in many other situations.

GYPSUM is very common in the intermediate

strata.

TRANSITION GYPSUM FORMATION.

151

That of the Alps is usually of a very white colour, a fine grain, and sometimes compact, containing also particles of limestone, mica, or talc, sal gem, and sulphur. The salt mines and springs of Bex, near Geneva, are supposed to belong to a saliferous gypsum lying in beds, among a submedial limestone impregnated with argillaceous and coaly matter. Among these are strata of clay-slate passing into greywacke, containing a pretty large quantity of salt in grains, nodules, and veins, 7 or 8 inches thick. The gypsums of Tyrol, Salzbourg, Wieliczka, &c. lie in a mass of clay; they are anhydrous in the interior of the mines, but at about 60 feet from the surface of the ground, water has been introduced, converting them into ordinary Paris plaster. The gypsum of the environs of Tarascon, in the Pyrenees, rests on the primitive rocks, and is covered by a limestone containing ammonites.

CHAP. III.-MEDIAL OR CARBONIFEROUS STRATA.

We now approach a subject of great geological interest, the coal-measures; the main spring of the manufacturing prosperity of Britain. On entering on these formations, a few remarks on their general features may be useful. We no longer find ourselves among rocks, consisting of such a variety of minerals, and mineral elements, which being differently combined, according to complex laws of affinity, and peculiar local circumstances, produced a great diversity of compounds. Here we shall have more uniformity; we shall observe masses relatively simple, deposited one over another in

the form of layers of considerable extent. The superpositions will be very manifest, the relative ages incontestible, and the systems of beds constituting a whole, or the formation-suites, will be much easier to recognise and determine.

Strata of limestone, alternating with strata resulting from the debris of primitive rocks, make up the entire body of secondary formations.

To the limestone properly so called, gypsum is sometimes joined. The debris of ancient rocks unite here to form breccias, puddingstones, sandstones, sands, clays and marls. These several matters, by the differences which they present in their associations, and in the substances which accompany them, or which they include, enable us to determine their diversities of epoch or deposition.

The numerous vestiges of animals and vegetables, which occur in secondary districts, will afford us a great facility for effecting these determinations; the different formations possessing always some fossil peculiar to them, which enables them to be distinguished, even when they occur in insulated spots, so that their true geological locality could not otherwise be known. The secondary formations which have been most studied, are those of England, the centre of Germany, and the North of France.

In the central parts of Germany, in Thuringia, Mansfeld, &c. a great many mines of coal, copper, and salt, have made the strata be well understood. They present four great formation-suites. The first which reposes immediately on the primitive or transition beds, is chiefly composed of sandstone, and bears the name of red sandstone, or coal sandstone,