DYKES. In volcanic districts, rents frequently occur which are filled with molten materials from below, that subsequently solidify, and form solid walls, filling the fis sures, and separating the edges of the disjointed strata. To these walls, geologists apply the term dyke, a Scottish word for a wall or fence. Similar walls of intruded matter occur in stratified rocks of all ages, and have been connected with volcanic erup tions belonging perhaps to every geological epoch. They consist of similar materials to whatever period they belong—viz., lava, either in a granular, compact, or glassy condi tion. The D. connected with Vesuvius have been minutely described. Those in the great escarpment which Somma presents to the modern crater of Vesuvius permit of a careful examination. They are chiefly vertical, and traverse at right angles the beds of lava, scorim, breccia, and sand which form the ancient cone. They project in relief several inches, or sometimes feet, from the face of the cliff, being extremely compact, and less destructible than the intersected tuffs and porous lavas. In vertical extent they vary from a few yards to 500 ft., and breadth from 1 to 12 feet. Many of them cut all the inclined beds in the escarpment from top to bottom, others stop short before they ascend above haff-way, and a few terminate at both ends, either in a point or abruptly. In mineral composition, they scarcely differ from the lavas of Somma. Their texture is different at the edges and in the middle; towards the center, the rock is larger grained, while at the edge it is always finer grained, sometimes vitreous. This evidently arises from the rate of cooling, it being known that molten trap or lava, when assumes a vitreous structure, while a slow cooling, as it permits the mass to remain in a condition fitted for the operation of the crystalline force, and the segregation of the separate materials, produces a more or le'ss granular structure, in proportion to the time occupied in cooling. The rock forming the D. is far more compact than that of ordinary lava, for the pressure of a column of melted matter in a fissure greatly exceeds that in an ordinary stream of lava; and pressure checks the expansion of those gases which form vesicles in lava. When the fissures have been openings for the egress of molten matter, the surfaces have been worn and smoothed by the current, the intense heat hav ing melted all projections and obstructions to the passage of the incandescent fluid.
The appearances of ancient trap-dykes are very similar to those of recent volcanic dykes. Trap-dykes generally are prominent objects in the landscape, because, while the softer rocks through which they have intruded have been abraded by the sea, rivers, or rain, they, being more compact, stand out prominently in the face of preci pices or on the level surface of a country. Sometimes, however, from chemical action, and chiefly from the oxidation of the iron which all trap-rocks contain to a greater or less extent, the intruded.dyke decomposes more rapidly than the containing rock. It then
for some feet or yards leaves the original fissure again unoccupied. A singular modi fication of this arrangement may sometimes be noticed, when the intrude digneous rock has so indurated the beds through which it passed as to make them less liable to weather than the unaltered portions of the beds, or than even the dyke itself. In such cases, we End two parallel walls of indurated strata rising above the general level of the country, and forming the banks of a ditch produced by the disappearance of so of the dyke. All these appearances may be observed in the island of Arran, a locality unsur passed for observing those remarkable geological phenomena. Some D. have had no apparent influence on the adjoining strata, even when these consist of materials most liable to be affected by heat. Thus, seams of coal sometimes remain unaltered, though in contact with the supposed injected molten matter. Considerable doubt is thus cast upon the generally received opinion, that in all cases D. were intrusions of lava. The effects that have been produced in numerous instances can, however, only be accounted for by supposing that the adjacent rocks have been affected by heat. The writer has observed in Arran, at a place where a dyke cut at right angles an older one, that the edges of the older dyke which had been acted upon by the current of liquid lava, were converted into true obsidian to the depth of nearly half an inch. In Anglesea, shale at the edge of a dyke 134 ft. wide has been converted into hard porcellanous jasper; and argillaceous limestone loses its earthy texture, and becomes granular and crystal line. The chalk, in Antrim, is converted, by basaltic D., into granular marble. -Coal, as might have been expected, is often altered in an extraordinary degree. Witham describes the effects of the Cockfield Fell dyke on a seam of coal through which it passes. It is a nearly vertical wall of trap, 18 or 20 yds. thick, and is traceable to a distance of m., running in a s.e. direction. The coal is about 6 or 8 ft. thick, and is affected about 50 yds. from the dyke. It first loses the calcareous spar, which occurs in the joints and faces, begins to look dull, and loses its quality for burning. As it comes nearer, it assumes the appearance of a half-burnt cinder; and approaching still nearer the dyke, it grows less and less in thickness, becoming a pretty hard cinder only two feet and a half in thickness. Eight yards further, it is converted into real cinder; and more immediately in contact with the dyke, it becomes a black substance resembling soot caked together, the seam being reduced to 9 in. in thickness. The coal thus deteriorated is 25 yds. of bad short coal, half reduced to cinder; 16 yds. of cinder; and 10 of sooty substance.