duced the various appearances in the basaltic ranges of that island, seems highly probable. In Sicily the connection of basaltic with volcanic rocks has been clearly established by Ferrara, professor of Natural Philosophy at Catania.

In the vicinity of Clermont Ferrand in Auvergne, a thick bed of basalt has once covered an extensive tract of country; it rests upon a bed of volcanic tufa, and the latter frequently covers beds of freshwater limestone. This bed of basalt and the subjacent tufa and limestone, have evidently been furrowed and excavated by the same causes which have excavated valleys in other parts of the world; hence the basalt occurs forming isolated caps on many of the mountains. In some parts a gradation may be traced in the same bed from a compact basalt, similar to that of Arthur's Seat near Edinburgh, to porous basalt approaching more or less to the state of scoriaceous lava. But the basalt of this country belongs evidently to volcanic products, and will be described in the chapter on volcanoes. It may be proper to remark, that as the basalt of Auvergne covers beds of freshwater limestone, which belong to the tertiary strata, its age is evidently posterior to that formation of limestone, which is regarded as the most recent.

Basalt sometimes presents a globular structure, globes of hard basalt being imbedded in a mass of basalt of a softer kind.

Wacke or earthy basalt has frequently a greenish or reddish brown colour; it often contains cavities which are generally filled with nodules of agate, or with zeolite or calcareous spar. The agates are

composed of concentric layers, and have apparently been formed by siliceous infiltration depositing successive coats within each other, until the cavity is filled up. Basaltic rocks of this kind are called amygdaloids. The Hill of Kinoul, in the vicinity of Perth, is formed of basaltic amygdaloid containing agate nodules in great abundance, of various dimensions and beautifully striped. At Woodford Bridge, in Gloucestershire, there is a low rock of amygdaloidal wacke, which is much intermixed with green earth, and has in some parts a saponaceous feel; the agates which it contains are decomposing, and the intense concentric layers are separated from each other, and present the appearance of edges of folded paper, with small interstices between each. examined this singular rock in 1816; it was then quarried for stone to mend the roads. In some parts of the rocks I found masses of corallite of considerable size enveloped in the basaltic amygdaloid. I found also in this rock well defined groups of prehnite, which was not then known to be an English mineral: it has since been discovered in the basalt of Staffordshire.

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The occurrence of organic remains enveloped in basalt, of which there are various instances, may admit of an easy explanation, if we allow that basalt has once flowed like lava at the bottom of the ocean. Modern lavas often envelop bones and other substances that they meet with in their

course.

Imbedded interstratified basalt or trap is some times found alternating with rocks ofundoubted marine origin. In Derbyshire there are three beds of ba

saltic amygdaloid separated by thick beds of transition limestone. Mr. Westgarth Forster has described an enormous bed of basalt in Northumberland and Durham, called the great Whinstone sill, placed between regular strata of limestone and gritstone; this bed varies in thickness from twelve to sixty yards. Other instances might be cited of basaltic beds interposed between regular strata, but frequently the strata are broken and disturbed in the vicinity of the basaltic beds. At Salisbury Craggs near Edinburgh, there is an instance of a bed of sandstone apparently broken by its contiguity to trap; the sandstone near its contact with the trap, is converted into a substance approaching to jasper. A very interesting account is given by Dr. MacCulloch of the partial interposition of beds of trap, in strata of sandstone, on the coast of Scotland, where the trap may sometimes be seen forming apparently regular beds between strata of sandstone, and suddenly rising through part of the upper strata, and forming other beds above. A general idea of this mode of interposition is represented Plate 3. fig. 3. where the strata under a bed of columnar basalt, are intersected vertically by a dyke, and laterally by beds of basalt of limited extent. In such cases we may trace the effect of a disturbing force, which intruded the basalt in a melted or softened state between strata of sandstone.

The beds of Derbyshire toadstone, and the great Whinstone sill in Northumberland and Durham, may have been formed by repeated eruptions of laya over the bed of the ocean; or, what is less probable, they may have been intruded long after the

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formation of the strata, with which they are at present associated.

It has been generally believed, that the veins of lead ore in Derbyshire, which pass through the beds of limestone, are entirely cut off by the toadstone; and if this could be proved, it would favour the opinion, that the beds of toadstone had been intruded between the beds of limestone, after the formation of the metallic veins. In some instances, however, the veins of lead cre do pass into the toadstone, and are rich in ore. It is now even doubted whether all the veins do not pass through the beds of toadstone, though they may become very narrow or yield no ore when in the latter rock. The information which I could collect from the most intelligent miners, when I was last in that county, still leaves the question undecided.

Plate 4. fig. 5. represents the beds of limestone b. b. b. separated by beds of toadstone e. e. In this section the veins are represented as cut through by the toadstone if this be really the fact, it will, as before stated, favour the opinion that they were intruded subsequent to the formation of the veins.

The interesting observations of Dr. Daubeny, in his Sketch of the Geology of Sicily, seem clearly to ascertain, that beds of amygdaloidal trap, alternating with beds of limestone, have, in that island at least, been formed by successive currents of lava flowing over the bed of the sea, at intervals of time so distant, as to allow the deposition or formation of a bed of limestone over each current of lava. A considerable district near Lentini, on the southern side of Mount Etna, and also a part of the island near

Cape Passero, are composed of alternating beds of lava, with tertiary limestone abounding with organic remains of madreporites, nummulites, cerithea, and the remarkable fossil called the Hippurite. Santa Venera, the loftiest mountain in the south of the island, is capped with cellular lava; beneath it is a bed of limestone with minute shells; at a lower level towards Lentini, there is a second bed of volcanic matter similar to the first; and two other similar alternations of beds of limestone and lava, occur still lower down. Dr. Daubeny says that the cellular and semivitreous aspect of many of the volcanic beds associated with the beds of limestone, precludes all doubt respecting the manner of their formation the character of other portions present strong analogies to rocks of the trap family; "they are compact, and have a stony fracture; they contain crystals of olivine, and the cavities are filled with calcareous spar or zeolites, like the amygdaloids of more ancient strata; and in some of the beds, a tendency to a columnar arrangement is discernible."

This account of Dr. Daubeny's affords additional proof of the close connection of ancient volcanic rocks with trap rocks,—may we not add, of their perfect identity: it is beside highly illustrative of the alternation of the beds of basaltic amygdaloid in Derbyshire, with beds of limestone. But in both instances, we must admit that the beds were formed under the ocean, before the present islands and continents had emerged from the watery abyss.

The disturbances and contortions of some of the lower beds of transition limestone, in the vicinity of.