The great Palisade trap of the Hud son river, which is an almost exact parallel to the Whin Sill, is an enormous sheet of igneous rock exposed among the Triassic beds of New Jersey and New York. It has an outcrop which is about Ioomi. long; its thickness is said to be in places 800ft., though usually not above 200 to 30o feet. Like the Whin Sill the rock is a quartz-dolerite occasionally passing into olivine-dolerite, especially near its edges. The Palisade dolerite is compact, non vesicular and non-porphyritic as a rule. It follows the bedding planes of the sedimentary rocks into which it was injected, but breaks across them locally and produces a considerable amount of contact alteration. In New Jersey, however, there is also an extensive development of effusive rocks which are olivine-basalts, and by their slaggy surfaces, the attendant ash-beds and their strictly conformable mode of occurrence, show that they were true lavas poured out at the surface. There can be little doubt that they belong to the same period as the Palisade trap, and they are consequently later than the Whin Sill.
These great sheets of igneous rock intruded into cold and nearly horizontal strata must have solidified very gradually. Their edges are fine grained owing to their having been rapidly chilled, and the whole mass is usually divided by joints into vertical col umns, which are narrower and more numerous at top and base and broader in the centre. Where exposed by denudation the rocks, owing to this system of jointing, tend to present a nearly verti cal, mural escarpment which seems to consist of polygonal pillars. The name "Palisade trap" expresses this type of scenery, so char acteristic of intrusive sills, and very fine examples of it may be seen on the banks of the Hudson river. In Britain it is no less clearly shown, as by the sill at Stirling on which Wallace's Monument is placed ; and by the well-known escarpment of Salis bury Crags which fronts the town of Edinburgh.
In the Tertiary volcanic dis trict of the west of Scotland and north Ireland, including Skye, Mull, and Antrim, innumerable sills occur. Perhaps the best known is the Sciur of Eigg, which forms a high ridge terminating in a vertical cliff or Sciur in the island of Eigg, one of the inner Hebrides. At one time it was supposed to be a lava-flow, but A. Harker has maintained that it is of intrusive origin. This sill occupies only a small area as compared with those above des cribed. Its length is about two and a half miles and its breadth about a quarter of a mile. On the east side it terminates in a great cliff from 30o to 400ft. high, rising from a steep slope be low. This cliff is beautifully columnar, and shows also a hori zontal banding, simulating bedding. The back of the intrusive sheet is a long ridge sloping downwards to the west. The rock of which the Sciur of Eigg consists is a velvety black pitchstone, containing large shining crystals of felspar; it is dull or cryp tocrystalline in places, but its glassy character is one of its most remarkable peculiarities.
In the Tertiary volcanic series of Scotland and Ireland, intru sive sheets build up a great part of the geological succession. They are for the most part olivine-basalts and dolerites, and while some of them are nearly horizontal, others are inclined. Among the lavas of the basaltic plateaus there is great abundance of sills, which are so numerous, so thin and so nearly concordant to the bedding of the effusive rocks that there is great difficulty in distinguishing them. As a rule, however, they are more per fectly columnar, more coarsely crystalline and less vesicular than the igneous rocks which consolidated at the surface. These sills are harder and more resistant than the tuffs and vesicular lavas, and on the hill slopes their presence is often indicated by small vertical steps, while on the cliff faces their columnar jointing is often very conspicuous.
On modem volcanoes intrusive sheets are seldom visible except where erosion has cut deep val leys into the mountains and exposed their interior structure. This is the case, for example, in Ireland, Teneriffe, Somma and Etna and in the volcanic islands of the West Indies. In their origin the deep-seated injections escape notice; many of them in fact belong to a period when superficial forms of volcanic action have ceased and the orifices of the craters have been obstructed by ashes or plugged by hard crystalline rock. But in the volcanoes of the Sandwich islands the craters are filled at times with liquid basalt which suddenly escapes, without the appearance of any lava at the surface. The molten rock, in such a case, must have found a passage underground, following some bedding plane or fissure, and giving rise to a dike or sill among the older lavas or in the sedimentary rocks beneath. Many of the great sills, however, may have been connected with no actual volcanoes, and may represent great supplies of igneous magma which rose from beneath but never actually reached the earth's surface.
The connection between sills and dikes is very close; both of them are of subterranean consolidation, but the dikes occupy vertical or highly inclined fissures, while the sills have a marked tendency to a horizontal position. Accordingly we find that sills are most common in stratified rocks, igneous or sedimentary. Very frequently sills give rise to dikes, and in other cases dikes spread out in a horizontal direction and become sills. It is often of considerable importance to distinguish between sills and lavas, but this may be by no means easy. The Sciur of Eigg is a good example of the difficulty in identifying intrusive masses. Lavas indicate that volcanic action was going on contem poraneously with the deposit of the beds among which they occur. Sills, on the other hand, show only that at some subsequent period there was liquid magma working its way to the surface.