II. VEINS WHICH CONNECT MINERAL BEDS TOGETHER, NOT

CONFINED TO TRAP-ROCKS.

In countries where trap-rocks abound, we sometimes observe in two beds of greenstone which are separated from each other by strata of sandstone, slate-clay, limestone, or other rocks, that they are connected by means of veins that shoot from the floor of one bed into the roof of the other. The Fig. 4. Plate III., represents such an appearance; a a, the beds of greenstone, cc, strata of limestone, and a the connecting vein of greenstone. This arrangement is said to be peculiar to rocks which have been projected from below in a melted form, and hence Volcanists, I presume, would confine it to trap-rocks, as all of these are with them lavas. But this neighbourhood, so rich in illustrations of many contested points in geology, affords us examples of rocks, not of the trap series, with communicating veins. Salisbury Craigs, one of our most interesting hills, is a mass of sandstone with subordinate beds of greenstone, and occasional layers of limestone, slate-clay, and clay-ironstone. On the south-east angle of the hill, in one of the quarries at present working, the following arrangement is to be observed. Greenstone, the uppermost stratum; immediately below it a bed of coarse siliceous limestone; under the limestone a bed of greenishgrey slate-clay; and below the clay, strata of sandstone. Several veins of siliceous limestone shoot from the floor or under side of the bed across the slate-clay, and form a continuous mass with the floor or upper side of the sandstone; and veins shoot from the roof of the bed of sandstone across the slateclay into the floor of the siliceous limestone, and form with it a continuous mass. In other parts of the same quarry, branches or veins are observed shooting from the roof of the sandstone, and gradually terminating in the bed of slate-clay. These appearances are represented by the plan, Fig. 5., a, greenstone; b, siliceous limestone; c, slate-clay; d, sandstone; e, veins shooting from limestone across the slate-clay into the sandstone; f, veins shooting from sandstone across the slateclay into the limestone; and g, veins shooting from the sandstone, and terminating in the slate-clay.

I have observed connecting veins of the same description in layers of fibrous gypsum contained in the compact kinds. This appearance is represented in Fig. 6. Plate III. where a is a mass of compact gypsum; b b layers of fibrous gypsum included in it, and c c communicating veins. In an extensive limestone quarry about a mile to the eastward of Burntisland, there is a fine example of a communicating vein between two beds of a curious kind of amygdaloid. The beds are separated from each other by numerous strata and beds of bituminous shale, slate-clay, clay-ironstone, sandstone, and limestone, but in one place a vein of amygdaloid shoots across all these rocks, from the lower to the upper bed of amygdaloid.

How are we to explain the formation of such veins? Volcanists say, that communicating veins are proofs of the igneous origin of the rocks in which they occur, and that they are formed by the spouting of the fluid lava through cracks in the rock, at the time when the lower bed was forming, and before the upper one was formed. But this hypothesis will not explain the communicating veins of limestone, sandstone and gypsum, because these, on the volcanic system, are not lavas. We are inclined to view them as illustrations of the simultaneous crystallization of rocks of the same formation.

III. TRAP-VEINS (WHIN-DIKES) PROBABLY OF COTEMPORANEOUS FORMATION WITH THE TRAP-ROCKS WHICH THEY

TRAVERSE.

Veins are tabular masses that in general traverse mineral strata and beds of different kinds. According to the Volcanic or Plutonic hypothesis, they were originally open rents or fissures, wide below, but terminating above in the form of a wedge, which were afterwards filled from below with melted mineral matter, projected from the interior of the earth. The Neptunists again maintain, that these fissures and rents were wide above, but terminated below, and were filled from above with their mineral contents. The Plutonists adduce as proofs of their opinion, veins shut above, and widening below, as sometimes happen with metalliferous veins, and also with those formed of mountain rocks, such as granite or trap; while the Neptunists offer as illustrations of the truth of their hypothesis, VOL. I. NO. I. JUNE 1819.

K

numerous instances of veins, both metalliferous and saxigenous, in which the crop or outgoing is wide, but the lower part narrow, and terminating in a wedge, or in numerous small branches.

Both statements are correct, and, therefore, the two opinions appear to be plausible. But there is a fact which cannot be reconciled with either of the hypotheses, and which forces us to have recourse to some other mode of explaining the formation of these veins. The fact we allude to is the occurrence of veins that terminate both above and below in the rock which they traverse, in short, are completely inclosed in it. Fig. 7. Plate III., represents the three kinds of veins; a, vein open at top; b, vein shut above, and c, vein terminations in the rock. Such a vein, it is evident, could not have been filled from above, nor is it possible to conceive that it could have been filled from below. We are therefore led to the conjecture, "that such veins must have been formed at the same time with the rock in which they are contained." This view receives considerable support by a careful attention to the appearances presented by the distinct concretions into which many trap-rocks are naturally divided. These concretions, at their line of junction with the bounding trap-rock, exhibit the same phenomena as occur at the meeting of the sides and walls of trap-veins in trap: and they differ from veins only in shape; the concretions being short and massive, whereas the veins are long and tabular. A vein of trap (or a dike of trap) we consider as merely a series of concretions, arranged in a tabular form. But, it will be inquired how, on this principle, we can explain the crossing of trap-veins? This arrangement we consider as an effect of crystallization, and of precisely the same nature as the crossings observed in crystals, or the intersections in tabular concretions. The crossings in crystals require no illustration, but those of concretions may shortly described. In sections of trap-rocks, as of greenstone, sometimes the cliff is naturally divided into tabular concretions, that extend from the top to the bottom of it. These concretions occasionally vary in their direction, some being horizontal, and others vertical; but the remarkable fact, and that which is highly illustrative of our present view, is this,-that the same table or tabular concretion will continue for some feet or yards parallel with the bounding concretions, and then suddenly turn and intersect these for several yards or fathoms, and then terminate, as aa, Fig. 8. Plate III. Now, this concretion in one part of

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its course is parallel with the others, and terminates amongst them; in another part of its course it intersects these same concretions, and then it ends. It is evident, that if this concretion had continued parallel with the others throughout its whole extent, we would not hesitate to consider it as of cotemporaneous formation with them. If this inference be admitted, and we do not see on what ground it can be refused, it is evident, that, if the intersecting portion of the concretion is a continuation of that parallel with the other, both must be considered as having crystallized at the same time, and as a simultaneous formation with the whole rock in which it is contained. But the tabular concretions intersected by the one part of the concretion, are equivalent to veins or dikes, because they are tabular masses intersected by another tabular mass; and as all these concretions are of simultaneous formation, it follows, that the crossing concretion and that which is crossed, which are equivalent to two veins, of which the one crosses the other, have been formed at the same time. EDINBURGH, April 1819.

ART. XXVIII.-Notice respecting a Singular Optical Property of Tabasheer. By DAVID Brewster, LL.D. F.R.S. Lond. and Edin. Communicated by the Author.

THE substance called Tabasheer, has been long known in eastern countries, and formed an important article in the Materia Medica of the Arabian Physicians. In the Gentoo language it is called Vedro-Paloo, or Bamboo milk; in the Malabar, Mungel Upoo, or Salt of Bamboo; and in the Warriar, Vedroo Carpooram, or Bamboo Camphor. It is found in the joints of the female bamboo, sometimes in a fluid state like milk, sometimes with the consistency of honey, but generally in the form of a hard concretion. Some specimens of it are transparent, and resemble very much small fragments of the artificial pastes made in imitation of opal; others are exactly like chalk; while a third kind is of an intermediate character, and has a slight degree of translucency.

Pliny clearly describes Tabasheer under the name of Sugar. The word is derived from the Persian Scher, or the Sanscrit Kschiram, signifying milk. See Humboldt on the Natural Family of the Grasses.

The first person that examined the properties of this substance was Mr Macie (now Mr Smithson), who analysed a portion of the Tabasheer from Hyderabad, which Dr Russell + had the preceding year presented to the Royal Society. "From its indestructibility by fire;-its total resistance to acids ;—its 'uniting by fusion with alkalis in certain proportions into a white opaque mass, in others into a transparent permanent glass, and its being again separable from these compounds entirely unchanged by acids," he considers it "as perfectly identical with common siliceous earth.”

In the year 1804, Messrs Humboldt and Bonpland brought with them from America some specimens of Tabasheer, called "Guaduas butter by the Creoles, taken from the bamboos which grow to the west of Pinchincha in the Cordilleras of the Andes. These specimens were analysed in 1805, by Messrs Fourcroy and Vauquelin §, who found them to be different from the Tabasheers of Asia. Instead of being wholly composed of silex, they contained only 70 per cent. of this earth, and 30 per cent. of potash, lime and water.

The Tabasheer which I employed in my experiments, was sent from Nagpore by Dr Moore to Dr Alexander Kennedy, who was so kind as to favour me with a considerable portion of it. It had the same general chemical characters as the Tabasheer from Hyderabad, which was used by Mr Smithson, the same specific gravity nearly, and the same external appearance; so that I have no hesitation in considering it as also composed principally of silex.

When the semi-transparent specimens of this substance are immersed in water, they imbibe it with great rapidity, emitting numerous bubbles of air. The transparency increases whenever the air has been discharged, and after a few minutes the water pervades, and renders transparent the whole mass.

If a small portion of water, on the contrary, is laid upon the Tabasheer when dry, instead of adding to its transparency as might have been expected, it actually renders it as opaque and white as chalk; and, from the same cause, the Tabasheer which has been saturated with water becomes opaque, as the water eva

See Philosophical Transactions, 1791, p. 368. +See Philosophical Transactions, 1790, P1 273.

Humboldt's Personal Narrative, vol. i. Introd. p. xiii. Note.

& Memoires de l'Institué, tom. vi. p. 382.