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Tweed. The length of the outcrop is about 8o m., but in places it is covered with superficial deposits or may be actually discontinuous. Near Haltwhistle, however, it is visible for about 20 m., and as it lies among softer rocks (limestones and shales), it weathers out on a bold craggy See also:

ridge or escarpment. When it crosses the streams the resistant character of the igneous rock is indicated by waterfalls or " forces," e.g. High Force in Teesdale. The thickness varies from 20 to 150 ft., but averages 90 ft. In some places the Whin Sill splits up into two or more smaller sills which may unite, or one of them may See also:

die out and disappear, and often small attendant sills, resembling the See also:

main mass in petrographical character, appear in association with it.

It is difficult to estimate the See also:

AREA

area over which it extends, as it dips downwards from its outcrop and is no longer visible, but we may conjecture that it spreads over .no less than 4Q00 sq. m. underground. The rocks in which it lies belong to the Carboniferous See also:

Lime-See also:

stone See also:

SERIES (a Latin word from serere, to join)

series, and the Sill is probably one of the manifestations of the volcanic activity which occurred during the later See also:

PART

part of the carboniferous See also:

period. Many similar sills, often of large See also:

SIZE

size, though none so great as the Whin Sill, are found in the Scottish coalfields. There are few lavas or ash beds at or above the horizons on which these intrusive rocks See also:

lie, and hence it has been concluded that towards the See also:

close of that volcanic See also:

EPISODE

episode in See also:

BRITISH

British See also:

GEOLOGICAL

geological See also:

HISTORY

history the molten magmas which were impelled upwards towards the surface found a See also:

PLACE (through Fr. from Lat. platea, street; Gr. IrAar6s, wide)

place of See also:

REST (O. Eng. rest, reste, bed, cognate with other Teutonic forms, e.g. Ger. Rast, Riiste, rest, and probably Gothic Rasta, league, i.e. resting or stopping place)

rest usually within the sedimentary rocks, and rarely flowed out as lavas on the sea-bottom (the intrusive succeeding the effusive phase of volcanic See also:

ACTION

action). In the Carboniferous rocks the Whin Sill lies almost like an interstratified See also:

bed, following the same See also:

HORIZON (Gr. 6pfTwv, dividing)

horizon for many See also:

MILES, NELSON APPLETON (1839— )

miles and hardly varying more in thickness than the sedimentary bands which accompany it. This, however, is true only on a large See also:

scale, for where the junctions are well exposed the igneous rock frequently breaks across the layers of stratification, and sometimes it departs quite suddenly from one horizon and passes to another, where again for a See also:

time it continues its apparently See also:

REGULAR

regular course. Its intrusive character is also shown by the emission of small See also:

VEINS

veins, never very persistent, cutting the sediments above or below it. In addition, it bakes and hardens the adjacent rocks, both below and above, and this proves that the superjacent beds had already been deposited and the molten diaoase forced its way along the bedding planes, as natural lines of weakness. The amount of contact alteration is not usually great, but the sandstones are hardened to quartzites, the shales become brittle and splintery, and in the impure limestones many new calc-silicates are produced. The Whin Sill consists of a dark-See also:

green granular diabase, in which quartz or micropegmatite appears as the last product of See also:

CRYSTALLIZATION

crystallization. It is not usually vesicular and is not porphyritic, though exceptions may occasionally be noted. At both the upper and the under surface the diabase becomes much finer grained, and the finest intrusive veinlets which enter the surrounding rocks may even show remains of a glassy See also:

BASE

base.

These phenomena are due to the rapid cooling where the magma was in contact withthe sediments. No ash beds accompany the Whin Sill, but there are certain dikes which occur near it and probably belong to the same set of injections. In many places the diabase is quarried as a road-mending stone. The great Palisade See also:

TRAP (0. Eng. treppe or traepee, properly a step, as that or which an animal places its foot and is caught, cf. Ger. Treppe flight of stairs)

trap of the See also:

Hudson See also:

RIVER

river, which is an almost exact parallel to the Whin Sill, is an enormous See also:

SHEET

sheet of igneous rock exposed among the Triassic beds of New See also:

Jersey and New See also:

York. It has an outcrop which is about too m. See also:

long; its thickness is said to be in places 800 ft., though usually not above 200 to 300 ft. Like the Whin Sill the rock is a quartz-diabase occasionally passing into See also:

OLIVINE

olivine-diabase, especially near its edges. The Palisade diabase is compact, non-vesicular and non-porphyritic as a See also:

RULE

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 See also:

COLD (in O. Eng. cald and ceald, a word coming ultimately from a root cognate with the Lat. gelu, gelidus, and common in the Teutonic languages, which usually have two distinct forms for the substantive and the adjective, cf. Ger. Kolte, kalt, Dutch koude

cold and nearly horizontal strata must have solidified very gradually. Their edges are See also:

FINE

fine grained owing to their having been rapidly chilled, and the whole mass is usually divided by See also:

JOINTS

joints into See also:

VERTICAL (from Lat. vertex, highest point)

vertical columns, which are narrower and more numerous at See also:

TOP (cf. Dan. top, Ger. Topf, also meaning pot)

top and base and broader in the centre.

Where exposed by denudation the rocks, owing to this See also:

SYSTEM

system of jointing, tend to See also:

PRESENT

present a nearly vertical, mural escarpment which seems to consist of polygonal pillars. The name " Palisade trap " expresses this type of scenery, so characteristic of intrusive sills, and very fine examples of it may be seen on the See also:

banks of the Hudson river. In See also:

BRITAIN (Gr. Hperavuml vi7vot, Bperravia; Lat. Britannia, rarely Britannia)

Britain it is no less clearly shown, as by the Sill at See also:

Stirling on which See also:

Wallace's See also:

MONUMENT (Lat. monumentum or monumentum; from monere, to advise, bring to mind, remind; the German equivalent is Denkmal)

Monument is placed; and by the well-known escarpment of See also:

Salisbury Crags which fronts the See also:

TOWN

town of See also:

EDINBURGH

Edinburgh. In the See also:

TERTIARY

Tertiary volcanic See also:

DISTRICT

district of the West of Scotland and North See also:

Ireland, including See also:

SKYE

Skye, See also:

MULL

Mull and See also:

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 See also:

ISLAND (O.E. ieg =isle, +land')

island of Eigg, one of the inner See also:

HEBRIDES, THE, or WESTERN ISLES

Hebrides. At one time it was supposed to be a See also:

LAVA

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 described. Its length is about two and a See also:

HALF

half miles and its breadth about a See also:

QUARTER (through Fr. from Lat. quartarius, fourth part)

quarter of a mile. On the See also:

east See also:

side it terminates in a great cliff from 300 to 400 ft. high, rising from a steep slope below. This cliff is beautifully columnar, and shows also a horizontal 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 See also:

black See also:

PITCHSTONE (German Pechstein, from its resemblance to pitch)

pitchstone, containing large shining crystals of See also:

FELSPAR, or FELDSPAR

felspar; it is dull or cryptocrystalline in places, but its glassy character is one of its most remarkable peculiarities. In the Tertiary volcanic series of Scotland and Ireland intrusive sheets build up a great part of the geological See also:

SUCCESSION (Lat. successio, from succedere, to follow after)

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 perfectly 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 See also:

hill slopes their presence is often indicated by small vertical steps, while on the cliff faces their columnar jointing is often very conspicuous. On See also:

MODERN

modern volcanoes intrusive sheets are seldom visible except where erosion has cut deep valleys into the mountains and exposed their interior structure. This is the See also:

case, for example, in Ireland, See also:

TENERIFFE [Tenerife]

Teneriffe, Somma and See also:

Etna and in the volcanic islands of the West Indies. In their origin the deep-seated injections See also:

ESCAPE (in mid. Eng. eschape or escape, from the O. Fr. eschapper, modern echapper, and escaper, low Lat. escapium, from ex, out of, and cappa, cape, cloak; cf. for the sense development the Gr. iichueoOat, literally to put off one's clothes, hence to sli

escape See also:

NOTICE

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 See also:

Sandwich Islands the craters are filled at times with liquid See also:

BASALT

basalt which suddenly escapes, without the See also:

APPEARANCE (from Lat. apparere, to appear)

appearance of any lava at the surface. The molten rock, in such a case, must have found a passage underground, following some bedding See also:

PLANE

plane or fissure, and giving rise to a See also:

DIKE, or DYKE (Old Eng. dic, a word which appears in various forms in many Teutonic languages, cf. Dutch dijk, German Teich, Danish dige, and in French, derived from Teutonic, digue; it is the same word as " ditch " and is ultimately connected with the ro

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 See also:

rose from beneath but never actually reached the See also:

earth's surface.

The connexion 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 See also:

COMMON

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 See also:

GOOD, JOHN MASON (1764-1827)

good example of the difficulty in identifying intrusive masses. Lavas indicate that volcanic action was going on contemporaneously with the See also:

DEPOSIT (Lat. depositurn, from deponere, to lay down, to put in the care of)

deposit of the beds among which they occur. Sills, on the other See also:

hand, show only that at some subsequent period there was liquid magma working its way to the surface. (J. S.

End of Article: SILL

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SILL (O.Eng. syl, Mid. E. sylle, selle; the word ap...