PSEUDO-STRATIFICATION OF TRAP. 405 veins; the former of which run sometimes parallel to the strata for a certain way, then break through into a superjacent plane in which they proceed as before. Irregular masses of trap are seen lying beneath the stratified rocks in some places, from which veins shoot up to the surface, or run in other directions. Pseudo-stratification of a basaltic vein, injected between beds of sandstone on the east coast of Trotternish. The apparent stratum of trap changes its line of direction, passing up through the planes between which it had formerly spread, and then holding a similar course between others.-Macculloch's Western Isles. Large beds of trap are often subdivided into prismatic or cubical blocks, which get rounded by the weather into forms hardly distinguishable by the eye from granite. But the traps more usually affect the prismatic figure, a tendency first displayed by vertical lines, which graduate into distinct columnar fissures. The effect of architectural regularity is produced when prisms are crossed by horizontal bands rectangularly, an arrangement conspicuous in Staffa, (see figure at the commencement of this section). The prisms differ in form, and vary in the number of sides from three to twelve, their most usual number being five or six. Their diameters are from an inch tonine feet; and their lengths from a foot to upwards of three hundred; in which space, there are often transverse fissures constituting natural joints, either oblique or at right angles, to the axis. This columnar structure is not restricted to basalt; it appears in syenite and claystone in Ailsa, Rum, and Arran; in augite rock in Sky; in porphyry in Arran and Egg, and in greenstone in several localities. The predominant substance in the members of the trap family, is a simple rock, of which indurated clay or wacke may stand at one end of the range, and compact felspar at the other; the intermediate body being claystone and clinkstone. In some cases, the simple rock composes the whole mass; in others, it is mixed with other minerals in various modes and proportions; producing great diversities in the external aspect, without any essential difference in the fundamental nature of the compound. Hornblende is the most common crystalline ingredient of these overlying rocks; and it gives character to three of the leading members of the family, basalt, greenstone, and syenite; in each of which the basis of claystone, clinkstone, or common felspar is united to it. Those rocks in which any of these three bases has a palewhite, yellowish, or reddish hue, are ranked as syenites; and those where it is gray, greenish, or dark-coloured are greenstones. The latter has, moreover, a larger proportion of hornblende than the former. Augite and hyperstene are also two minerals which enter largely into these unstratified rocks, and communicate in each case very peculiar characters. The most interesting feature of the trap forina VERTICAL VEINS IN STRATHAIRD. 407 tion is to be found in the veins of it, which pass through the subjacent or surrounding strata. Perhaps the most remarkable example of these veins is that described by Dr. Macculloch as occurring in Strathaird on the coast of Sky. They traverse in Basaltic veins descending through Sandstone from a superjacent nearly a vertical direction, and parallel to each other, the sandstone strata of the cliffs; being so numerous in many places, as nearly to equal, when collectively taken, the mass of the stratified rock itself. Thus 6 or 8 are sometimes observed within the space of 150 feet; and their aggregate magnitude is apparently 60 or 70 feet. The veins do not vary in size through the visible part of their course, nor do they throw out ramifications; their average breadth being 10 feet, though they vary from 5 to 20. The basalts of an earthy fracture are liable to decomposition by the weather, while the vitreous traps are very durable. In the instance of Strathaird, they seem to decay very fast, and occasion caves and fissures in the sandstone walls. At the spar cave, this wasting effect of the sea and air has corroded the basalt to a depth of 250 feet from the external cliff. The sandstone partitions that remain, look like terminal abutments, and when cut off behind by similar corrosion, they seem to stand forth in square pillars of masonry. In one of these great veins, another smaller one is seen passing through it. They all seem to disappear in the superincumbent mass of trap; suggest ing the idea, that they penetrated downwards through the unindurated sand beds, from a stream of lava effused above. What gives probability to this opinion, is that the strata remain here quite undisturbed, though they are very liable to disorder from venous intrusions. The sandstone is much harder on the façade crossed by the basalt than elsewhere. The above figure represents a trap vein, passing perpendicularly through the calcareous sandstone of Strathaird, and again pierced nearly in the line of its axis, by a second smaller vein of basalt, singularly undulated; somewhat like the zigzag line of an electric spark in the atmosphere. Among the limestone beds of Borrereg an instructive phenomenon occurs. A bed of basalt after flowing extensively in a parallel course among the strata of limestone, makes a sudden flexure into the RECURRENCE OF TRAP ERUPTIONS. 409 oblique position, which shortly becoming vertical, runs away beyond reach of examination in the capricious manner of a trap vein, having in its progress intersected at right angles in one place, those strata to which it lay parallel in another. The last trap vein deserving specific notice in the Strathaird district, is one found near Loch Oransa, remarkable for the mixture which it presents within itself of all the ordinary varieties of trap; being a fine grained basalt, at the edge, and passing by degrees into greenstone, porphyry, and amygdaloid. Trap formations like the common volcanic eruptions, have a tendency to recur in the same spot. The alternations found in Canna prove that the several formations visible, there at least, are as distinct in time, as they are in place; since lapse of time is evidently implied in the formation of conglomerate rocks. Here we see rolled masses of basalt aggregated into a breccia, which is again covered by a fresh bed of basalt. In the same island, cavities have been observed in the trap rocks exactly similar to those included in the scoriæ of volcanoes, or in cellular lavas. Cells of the same kind occur abundantly in Mull, in the trap near Oban, and they are particularly conspicuous in some parts of the little Cumbray. Such analogies complete the chain, which links the trap formation to visible volcanic products. The overlying rocks occupy a large portion of the Hebrides. They form the mountain masses of Cuchullin, and the vast cliffs of nearly the whole coast of the large island of Sky. Rum, Egg, Canna, and Mull are nearly in the same predicament. The south ern half of Arran is also basaltic; not to mention many partial |