164. Serpentine, a greenish rock containing a large proportion of magnesia, sometimes traverses the strata in dikes and veins, but occurs more frequently as a metamorphic rock.

165. Trap rocks very frequently present themselves in dikes, injected into fissures in the stratified and unstratified rocks. Fig. 64, § 106, presents a powerful trap dike, penetrating the strata, in New South Wales. When such dikes pass through soft strata, as shale, they are sometimes left prominent, by the more rapid wearing away of the soft rocks. In some instances, however, they are decomposed more rapidly than the containing rock, in consequence of the oxidation of the iron in their composition; in such cases they leave open fissures.

166. The most conspicuous effects produced by trap rocks are the changes which their intense heat has effected on adjacent rocks. A trap-dike in Anglesea, passing vertically through shale, has altered the structure of the strata to the distance of thirty-five feet from itself, converting the shale into a porcellaneous jasper, and obliterating the fossil shells of the strata. In the north of Ireland basal

tie dikes traversing the chalk have converted it into granular marble to the distance of ten

feet. No traces

of the organic re

Trap-dike.

thus crystallized. Fig. 80 represents a trap dike displacing and altering the structure of strata. Secondary sandstones have been converted thus into quartz rock. Beds of coal, from their combustibility, exhibit the agency of these melted rocks in a striking manner. A greenstone dike passing through a coal-bed in Ireland has reduced it to cinders, through a space of nine feet on each side. In the north of England a similar effect has been produced by a trap-dike passing through the coal series. At a distance of one hundred and fifty feet, the structure of the coal is changed. At a certain distance it is coked, while in the immediate vicinity of the dike it is burned to soot. This dike extends seventy miles and is fifty feet wide.

167. Masses of trap rock are frequently met with, intruded between strata, covering extensive areas, closely resembling beds. They take this position when the resistance offered to their progress laterally between the strata, is less than that which they encounter from above. In some instances, having spread out in horizontal sheets upon the

bottom of the ocean, they have been subsequently covered by sedimentary deposits.

168. The structure of the trap rocks, especially of basalt, is very frequently columnar, dividing immense masses into regular prisms, and constituting some of the most remarkable features of the earth's scenery. The "Giant's Causeway" is the legendary name applied to the most regular series of the magnificent basaltic columns which abound in the north of Ireland. The columns vary in the number of angles from three to twelve, but have most commonly from five to seven sides. They are divided transversely into joints, whose upper and lower surfaces are alternately concave and convex, as is shown in figure 65, § 107. These joints vary in length and diameter, but from six to eighteen inches are the prevalent dimensions. The surfaces of these Fig. 82.

segments are so nicely adapted to each other, that neither between joints nor adjacent columns can a knife-blade be introduced; and yet each segment is completely dissevered

from the others, so that it may be lifted out of its place. The Causeway extends under the sea. The neighboring headland seen at the left in Fig. 81, consists of alternations of basaltic columns, with beds of ochre interposed. As the ochre is worn away by atmospheric agencies, the columns are left unsupported, and many of them have fallen to the base of the precipice.

169. The Western Islands of Scotland consist almost exclusively of basaltic or trap rocks, which rise to the height of one hundred and fifty feet above the level of the ocean. The isle of Staffa is distinguished for its basaltic pillars and caves. The Scallop Cave, so called because the sides are formed of curved columns, extends one hundred and thirty feet into the rock, is thirty feet in height and twenty in breadth, at the entrance. But the principal Fig. 83.

object of attraction in this island is the Musical Cave, or as it is more commonly known by its legendary name, Fingal's Cave. This cavern is two hundred and twentyfive feet in length, with an average height of sixty-five feet; the breadth at the entrance is a little more than forty feet, and at its farthest recess is about twenty. The sides are columnar and the roof is partially so. The columns have been broken away in many places by the waves, presenting lateral recesses and jutting colonnades.