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Whin-sill* of Yorkshire. The latter is an enormous basaltic dyke, which traverses the island from the Tees to Robin Hood's Bay, and intersects all the strata from the lowermost beds of the coal-measures to the oolite inclusive.

27. TRAP-DYKES IN THE ISLE OF SKYE.-In the Isle of Skye the intrusions of basalt are on a large scale, and present many important and instructive examples of the disturbance and altered character of the sedimentary rocks, that have been exposed to their influence. From the numerous sketches that illustrate Dr. Macculloch's work on the Western Isles,† I have selected the one before us (Lign. 208), as exhibiting vertical, oblique, and horizontal

d

LIGN. 208.-TRAP-DYKE ON THE COAST OF TROTTERNISH. IN THE ISLE OF SKYE.

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veins or dykes; a large mass of trap is seen abutting, at a, against the sandstone-strata e; and giving off a thick horizontal stream, which sends off branches both upwards (6) and downwards (d), and finally divides into three small veins (c, c, c).

In the cliffs at Straithaird, in the Isle of Skye, the sandstone-strata are traversed by numerous vertical dykes and veins of trap; and the latter in many places have decom

See Prof. Sedgwick's valuable Memoirs, on the Trap-rocks of Yorkshire and Durham, in the Transact. Cambridge Phil. Soc. vol. ii. p. 21, and p. 139.

†This work should be referred to, in order to obtain an adequate idea of the extent and complexity of the trap-dykes and veins in the Isle of Skye and others of the Hebrides. See also Mr. Geikie's Memoir on the Geology of Strath in Skye, Quart. Journ. Geol. Soc. vol. xiv. p. 15

posed, and left perpendicular fissures, as is shown in the annexed sketch (Lign. 209); the reverse of the phenomena observable in the Val del Bove (p. 865).

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LIGN. 209.-VERTICAL CHANNELS IN SANDSTONE-STRATA, LEFT BY LECOMPOSED TRAPDYKES; AT STRAITHAIRD, ISLE OF SKYE.

(Dr. Macculloch's Western Isles.)

Porphyritic dykes and veins also occur abundantly in the same island, in some instances protruding through, and in others spreading over clay-slate, red sandstone, and shelly limestone.

In some of the slate-districts, where the trap has burst through and overflowed the strata, fragments of slate are found imbedded in the basalt, appearing to have been detached from the rock at the intrusion of the lava, and enveloped while the latter was in a state of fusion.

Sometimes the fractures and displacements of the strata are on so small a scale as to exhibit the relative connexion of the separated portions, as shown in this sketch of trap intruded between sandstone, in the Isle of Arran (Lign. 210, fig. 4). This island, which is the largest in the Firth of Clyde, presents, like the Isle of Wight in the south-east of England, an epitome of the geology of the neighbouring mainland. There" the four great classes of rocks-the fossiliferous, volcanic, plutonic, and metamorphic-are all conspicuously displayed within a very small area, and with their peculiar characters strongly contrasted." *

* Lyell's Manual, 5th edit. p. 589. I much regret that my limits will not admit of a detailed notice of this most interesting island; and I

28. GRANITE.-Various modifications of the compound mineral termed granite (see above, p. 882) constitute a great proportion of the hypogene rocks, and are found almost everywhere beneath the gneiss and mica-schist, and often in contact with strata of the secondary and even tertiary formations. In the British Isles granite appears in Galway, Donegal, Armagh, and Down, Wicklow and Carlow; * in Cornwall and Devon; Pembrokeshire; Anglesea; Kirkcudbrightshire; Aberdeenshire, &c.; and forms the nucleus of Skiddaw, Shapfell, Ben Nevis, and other mountain-peaks.

Granite-veins.t-Granite often occurs in dykes and veins which traverse not only other rocks, but also the preëxisting masses of granite; proving that the formation of this mineral has taken place at various and distant periods. Veins are fissures or chasms produced in rocks either by mechanical disturbance, or by contraction of the mass during its consolidation or refrigeration, and which have been filled by subsequent infiltration or sublimation, or by injections of mineral matter in a state of fusion from a subterranean source. Although many metallic veins are synchronous with the rocks they traverse, having been formed by segregation during the consolidation of the mass, yet the veins and dykes of volcanic matter are obviously of later

must refer the reader to the work cited, and to the excellent guide to the Geology of the Isle of Arran, by Prof. Ramsay. See also Prof. Phillips's "Manual of Geology," 1855, p. 503, &c., for some interesting details and sketches of the general geology and plutonic rocks of Arran.

* Some instructive observations, by the Rev. Prof. Haughton, on the composition of some of the Granites of Ireland, illustrated by numerous analyses, and accompanied by remarks on the relative distribution of the several varieties of potash-granites and soda-granites, will be found in the Quarterly Journal of the Geological Society, vol. xii. p. 171, and also in a second Memoir read before the Society in January, 1858.

† See De la Beche's "Geol. Observer," 2nd edit. p. 575, for descriptions and illustrations of these phenomena. Also Phillips's " Manual," p. 508.

origin than the beds in which they are intruded. Thus the granite-veins represented in this diagram (Lign. 210, fig. 1) are newer than the slate-rocks which they penetrate.

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LIGN. 210.-INTRUSIVE VEINS OF TRAP, GRANITE, PORPHYRY, &c.

Fig. 1. Granite-veins (a, a) traversing schist, Isle of Arran.*

2. Veins of granite (a, a) traversing schist, themselves crossed by veins of two different kinds of porphyry (b, b, b)..

- 3. Gneiss (b, b, b, b) shifted by a granite-vein (a, a, a).

4. Intrusion of trap between layers of sandstone (a, a), presenting an example of fracture and displacement so small, as to admit of the reädaption of the separated portions.t

Granite-veins traversing other rocks are themselves sometimes intersected by intrusions of other melted materials. This is frequently the case in Cornwall and Devon, where numerous granitic and porphyritic dykes (called elvans ‡) traverse both the granites and the schists (killas) of that

* Phillips, Encycl. Metrop., and "Manual," p. 508.

+ Dr. Macculloch, Geolog. Trans. and "Western Isles."

See De la Beche's "Report on Cornwall," &c., and that excellent work on Physical Geology, the "Geol. Observer," by the same talented author.

district. This sketch (Lign. 210, fig. 2) represents a mass of schistose rock crossed by granite-veins (a, a) in one direction, and again by veins of porphyry (b, b, b), which cut through both the schist and the granite. When gneiss is intersected by granite, it becomes shifted, as in this example, in which the granite-veins (Lign. 210, fig. 3, a, a, a) have displaced the lamina of gneiss (b, b, b). Thus, by numerous observations of phenomena of a like nature, it is now clearly established that granite has been ejected during the Silurian, Carboniferous, Jurassic, Cretaceous, and even Tertiary epochs.

Where granite has been erupted in a fluid or softened state among secondary strata, the latter are invariably altered near the line of junction; but when consolidated masses of granite have been protruded, no such change is observable. Into the slate-rocks of the Cumbrian chain syenite, porphyry, and greenstone have been injected in a melted state, and now fill up fissures produced during the general movements of those strata; but the central nucleus of crystalline rock exhibits no such appearance.

29. GRANITIC ERUPTIONS.-In the Isle of Arran, the granitic rocks were evidently erupted in a state of fusion, for the slates are penetrated by veins of granite (Lign. 210, fig. 1); and in some instances are changed into fine-grained mica, or hornblende-slate.

M. Dufrenoy describes granite-veins traversing chalk, in the Pyrenees, which have converted the cretaceous rock into crystalline limestone, and generated in it veins of iron-ore. The following instructive fact is noticed by M. Elie de Beaumont: in the environs of Champoleon, where granite comes in contact with Jurassic limestone, whatever may be the position of the surfaces in contact, the limestone and the granite both become metalliferous near the line of junction, and contain small veins of galena, blende, iron- and copperpyrites, &c.; and at the same time the secondary rocks are

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