formations of the same rock among the other isles. A juster idea of the amount of these projected rocks may however be had from a survey of the great mountain masses which they form in Scotland. The following consist of trap overlying red sandstone:

Dunsinnan hill 1012 feet; Kinnoul hill 632. The chain of the Ochill hills is similar. Here are Bencleugh 2200 feet high, Dalmyatt 2500, Alva hill 1600.

Among the Pentlands; Logan Househill 1700; Caernethy 1500; Kirklow hill 1120; Castlelaw 1390; Kirkyetten 1500; Dalmahoy 680.

Trap elevations near Edinburgh: Braid 480; Salisbury craig 550; Arthur seat 800; Corstorphine hill 470; Castle rock of Edinburgh 335; North Berwick law 940; Taprain law 700.

In the Clyde district: Tinto 2306; Eildon 1360; Campsie 1500; Neilston hill 820; Mistylaw 1240; Castle rock Dumbarton 500; Garroch head island of Bute 700. The highest points of the southern trap hills of Arran and Rasay, are 1000 feet; Cuchullin chain (hyperstene rocks) Sky, 2000; Benmore and Ben y Chat in Mull; the first 3097, the second 2294; Basaltic cone of Lamlash isle 1009; Duncan hill in Rasay about 1500; Storr in Trotternish in the isle of Sky about 2000; Oreval and Benmore, isle of Rum, near 2000; Isle of Canna 800; Scuir of Egg, a beautiful pitchstone porphyry, 1339; northern district of Mull about 1400; district of Gribon in that island 2000; the steep escarpments of the same 1000; Isle of Gometra 800; Isle of Ulva 1300; Isle of Kerrera, in centre, 1200. Combining the extent with the altitude of these mountain masses, we may conceive what enormous convulsions must have accompanied their eruption under the surface of the primeval ocean.

What a vast body of lava must have been ejected to cover an island like Sky, 50 miles long and 20 broad, with coulées in many places 1000 feet deep! The basaltic mantle of the island of Mull is in like manner enormous; and from these two central foci the antique lava seems to have spread over the whole district of the trap isles; of which only small fragments are exposed to view, the great portion being now engulphed in the deep; forming dark basaltic caves, and submarine causeways.

The basaltic mountains differ much in shape. They sometimes appear in massive, irregular groups of angular rocks; as in the

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vicinity of Achnacraig in Mull. But most commonly they present a series of terraces placed one above another, capped by slightly rounded surfaces, or in small peaks or detached cones, having a pointed or rounded aspect. Occasionally they compose small mounds flattened on the top, like Macleod's tables in Sky. The ordinary altitude of the terraces is from 20 to 80 feet, and their slope depends partly on the greater or less degradation which the rocks have undergone, and on the nature of the constituent beds. Now and then pretty gentle declivities covered with turf are observed between the precipices. In this way, a trap mountain may consist of two or three vertical sections, joined by inclined planes; a character which serves to distinguish these masses from the primitive and secondary mountains, and to mark their mode of deposition. At other times, the cliffs run nearly straight down from top to bottom. Thus the summits of Storr in Trotternish are cut almost vertically on the eastern side into a façade of nearly 500 feet, presenting detached rocks which by their square or pointed forms, resemble at a distance, castles, towers and spires.

The inclination of the basaltic beds naturally depends on the surface on which they repose; thus in Trotternish, the position of the subjacent gryphite limestone, gives them an inclination to the west, in the isle of Egg they incline to the south, in Staffa they slope down to the east at an angle of 9°, while they lie almost horizontally over the upright edges of the primitive rocks, or over one another, in Mull and its dependencies.

The Campsie hills form a long ridge of overlying trap in the centre of the Scottish Lowlands. They proceed from west-southwest to east-north-east, beginning in the valley of red sandstone near Dumbarton, and ending in the strath of Stirling. On the north from Drymen to Stirling they are bordered by a nearly flat formation of sandy diluvium; and on the south, by coal-measures which connect the grand coal basins of Forth and Clyde. The greatest length of the ridge is about 22 miles, and its average breadth 10, having a surface finely waved with undulations running from south to north. The eminences are in general rounded and have their convex parts turned to the west. On the east, the ridge slopes down in a plane inclined at an angle of 20° or 30°. The summits are in general 1200 or 1500 feet high; but the MeikleBen rises to 1800 feet, and gives origin to the rivers Carron and

Endrick; which run to opposite seas. The tops exhibit small escarpments, or steep cliffs on the western side, as do the trap hills in general of the great valley of Scotland, with an occasional display of irregular prismatic colonnades. On the sides of the transverse valleys of Campsie and Fintry, pretty extensive sections of the superposition of several basaltic beds on one another appear, similar to what forms so picturesque an object above Kilpatrick on the north banks of the Clyde, near Dumbarton.

We are indebted to Dr. Berger and the Rev. W. Conybeare for a very interesting account of the basaltic area of the north of Ireland, presenting facts demonstrative of its igneous origin, and the extensive convulsions which must have attended its eruption.

The opposite points of Scotland and Ireland on the Antrim coast, correspond exactly in geological structure. The Mull of Cantyre which faces Tor Point resumes the chain of mica slate which was there broken off. The Cantyre hills are connected with the Grampians, a chain strikingly similar to the mica-slate mountain range of the county Londonderry, which is succeeded on the south by a conglomerate, perfectly resembling that, which is so well known as skirting the Grampians on their southern border.

Of the basaltic group of hills in Ireland, Knocklead in the northern extremity of the eastern chain offers the highest summit, rising 1820 feet above the level of the sea; but its basis is occupied to the height of 500 feet by primitive rocks, leaving 1320 feet for the thickness of the overlying strata. Divis hill, near the southern extremity of the chain, is wholly composed of these strata, and attains an elevation of 1475 feet above the level of the sea. Cragnashoack at the southern extremity of the western chain has a height of 1864 feet. The basaltic covering seems to acquire its greatest thickness on the north, where the basaltic cap of Benyavenagh,

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the most northern summit of the western chain, measures more than 900 feet. The average depth of the basaltic superstratum may be safely estimated at 545 feet, and its superficial extent at the prodigious area of 800 square miles, a solid mass of magnificent dimensions.

From the absence in Scotland of the chalk formation the trap lies there in immediate contact with secondary sandstone for the most part, but in Ireland it has substrata of chalk, and other members of the English secondary formations. The chalk in Ireland does not average in thickness above 200 feet, while in England its mean value is 800. It agrees exactly with the lower beds of the English, and consequently of the French chalk, which are distinguished from the higher beds of the same formation, by their greater consolidation. It is impossible for any two portions of the same formation to be more entirely identified, by every external character, and by the fossils and organic remains contained in them, than are these Irish beds, with the English beds in the isles of Wight and Purbeck. In both localities the lower beds are destitute of flints, which the upper contain in abundance; and beneath both of them, the green sand is found, called Mulatto in Ireland. But the numerous beds of coarse calcareous oolites, which in England succeed this green sandstone, are entirely wanting in Ireland, and the mulatto reposes immediately on the lias limestone. This again rests, as in England, on beds of red and variegated marl, containing gypsum, and is further distinguished by numerous salt springs. Under the marl, lies a thick deposit of red and variegated sandstone containing clay galls. These four formations, which

together with the basalt, constitute the whole mass of the mountains associated with the eruptive class, cannot be estimated as possessing a less average thickness than from 800 to 1000 feet. This whole system of beds, at its north-eastern and south-western extremities, appears to repose on the coal formation, and its associated limestone; which in their turn repose on primitive rocks. The proofs which the deep valleys, separating the detached eminences characteristic of the basaltic system, afford of their formation by diluvial action, excavating portions of the solid strata, where no rivers run, are complete, and have been ably illustrated by Dr. Richardson in the Appendix to the Statistical Survey of Antrim. Rolled fragments of primitive rocks are often found on the summits of high basaltic ridges which are at present cut off from all communication with the primitive districts, by numerous intervening valleys; indicating at once the state which preceded or constituted the deluge, and the ravages of its retiring torrents.

The chalk stratum is frequently traversed by basaltic dykes, and often exhibits a remarkable alteration near the point of contact. In such cases, the change sometimes extends 8 or 10 feet from the wall of the dyke, being greatest where in contact with it, and thence gradually passing into the ordinary appearance. The extreme alteration presents crystalline flakes; then a saccharine grain, lastly a porcellanous density; and the altered chalk emits a vivid phosphorescence when moderately heated. In the insulated mass of secondary strata which cap the primitive mass of Slieve Gallion