A Manual of Elementary Geology Part 59

Diallage 60 -- 275 -- -- -- 105 -- -- (Klaproth) --(mean of 3 4333 22 2641 558 -- -- 1153 -- 854 W.

a.n.a.lyses)

Epidote 37 21 -- 15 -- -- 24 15 -- (Vauquelin)

Felspar, 6283 1702 -- 3 - 13 -- 1 -- -- common (Vauq.) --(Rose) 6675 175 -- 125 12 -- 075 -- -- --(mean of 7 6404 1894 -- 076 1366 -- 074 -- -- a.n.a.lyses)

Garnet 3575 2725 -- -- -- -- 36 025 -- (Klaproth) --(Phillips) 43 16 -- 20 -- -- 16 -- --

Hornblende 42 12 225 11 a -- 30 025 -- (Klap.) trace --(Bonsdorff.) 4569 1218 1879 1385 -- -- 732 022 15 F.

Hypersthene 5425 225 14 15 -- -- 245 a 1 W.

(Klaproth) trace

Labrador- 5575 265 -- 11 -- 4 125 -- 05 W.

felspar (Klap.)

Leucite 5375 2462 -- -- 2135 -- -- -- -- (Klap.)

Mesotype 5464 1970 -- 161 -- 1509 -- -- 983 W.

(Gehlen)

Mica 425 115 9 -- 10 -- 22 2 -- (Klaproth) --(Vauquelin) 50 35 -- 133 -- -- 7 -- -- --(mean of 3 4583 2258 -- -- 1108 -- 14 145 -- a.n.a.lyses)

Olivine 50 -- 385 -- -- -- 12 -- -- (Klaproth)

Schorl or 3548 3475 468 -- 048 175 1744 189 402 B.

Tourmaline (Gmelin) --(mean of 6 3603 3582 444 028 071 196 1371 162 -- a.n.a.lyses)

Serpentine 4307 025 4037 05 -- -- 117 -- 1245 W.

(Hisinger) --(mean of 5 3729 497 368 289 -- -- 314 -- 1277 W.

a.n.a.lyses)

Steat.i.te 64 -- 22 -- -- -- 3 -- 5 W.

(Vauquelin) --(mean of 3 483 618 2665 -- -- -- 2 -- 95 W.

a.n.a.l. by Klap.)

Talc. 6175 -- 305 -- 275 -- 25 -- -- (Klaproth)

In the last column of the above Table, the letters B. C. F. W. represent Boracic acid, Carbonic acid, Fluoric acid, and Water.

FOOTNOTES:

[368-A] For a description and theory of active volcanos, see Principles of Geology, chaps. xxiv. to xxvii.

[374-A] G. Rose, Ann. des Mines, tom. viii. p. 32.

[374-B] Geol. Trans. vol. ii. p. 211. 2d series.

CHAPTER XXIX.

VOLCANIC ROCKS--_continued_.

Trap dikes--sometimes project--sometimes leave fissures vacant by decomposition--Branches and veins of trap--Dikes more crystalline in the centre--Foreign fragments of rock imbedded--Strata altered at or near the contact--Obliteration of organic remains--Conversion of chalk into marble--and of coal into c.o.ke--Inequality in the modifying influence of dikes--Trap interposed between strata--Columnar and globular structure--Relation of trappean rocks to the products of active volcanos--Submarine lava and ejected matter corresponds generally to ancient trap--Structure and physical features of Palma and some other extinct volcanos.

Having in the last chapter spoken of the composition and mineral characters of volcanic rocks, I shall next describe the manner and position in which they occur in the earth's crust, and their external forms. Now the leading varieties, such as basalt, greenstone, trachyte, porphyry, and the rest, are found sometimes in dikes penetrating stratified and unstratified formations, sometimes in shapeless ma.s.ses protruding through or overlying them, or in horizontal sheets intercalated between strata.

_Volcanic dikes._--Fissures have already been spoken of as occurring in all kinds of rocks, some a few feet, others many yards in width, and often filled up with earth or angular pieces of stone, or with sand and pebbles. Instead of such materials, suppose a quant.i.ty of melted stone to be driven or injected into an open rent, and there consolidated, we have then a tabular ma.s.s resembling a wall, and called a trap dike. It is not uncommon to find such dikes pa.s.sing through strata of soft materials, such as tuff or shale, which, being more perishable than the trap, are often washed away by the sea, rivers, or rain, in which case the dike stands prominently out in the face of precipices, or on the level surface of a country. (See the annexed figure.[378-A])

[Ill.u.s.tration: Fig. 439. Dike in inland valley, near the Brazen Head, Madeira.]

In the islands of Arran, Skye, and other parts of Scotland, where sandstone, conglomerate, and other hard rocks are traversed by dikes of trap, the converse of the above phenomenon is seen. The dike having decomposed more rapidly than the containing rock, has once more left open the original fissure, often for a distance of many yards inland from the sea-coast, as represented in the annexed view (fig. 440.). In these instances, the greenstone of the dike is usually more tough and hard than the sandstone; but chemical action, and chiefly the oxidation of the iron, has given rise to the more rapid decay.

[Ill.u.s.tration: Fig. 440. Fissures left vacant by decomposed trap.

Strathaird, Skye. (MacCulloch.)]

There is yet another case, by no means uncommon in Arran and other parts of Scotland, where the strata in contact with the dike, and for a certain distance from it, have been hardened, so as to resist the action of the weather more than the dike itself, or the surrounding rocks. When this happens, two parallel walls of indurated strata are seen protruding above the general level of the country, and following the course of the dike.

[Ill.u.s.tration: Fig. 441. Trap veins in Airdnamurchan.]

As fissures sometimes send off branches, or divide into two or more fissures of equal size, so also we find trap dikes bifurcating and ramifying, and sometimes they are so tortuous as to be called veins, though this is more common in granite than in trap. The accompanying sketch (fig.

441.) by Dr. MacCulloch represents part of a sea-cliff in Argyles.h.i.+re, where an overlying ma.s.s of trap, _b_, sends out some veins which terminate downwards. Another trap vein, _a a_, cuts through both the limestone, _c_, and the trap, _b_.

In fig. 442., a ground plan is given of a ramifying dike of greenstone, which I observed cutting through sandstone on the beach near Kildonan Castle, in Arran. The larger branch varies from 5 to 7 feet in width, which will afford a scale of measurement for the whole.

[Ill.u.s.tration: Fig. 442. Ground plan of greenstone dike traversing sandstone. Arran.]

In the Hebrides and other countries, the same ma.s.ses of trap which occupy the surface of the country far and wide, concealing the subjacent stratified rocks, are seen also in the sea cliffs, prolonged downwards in veins or dikes, which probably unite with other ma.s.ses of igneous rock at a greater depth. The largest of the dikes represented in the annexed diagram, and which are seen in part of the coast of Skye, is no less than 100 feet in width.

[Ill.u.s.tration: Fig. 443. Trap dividing and covering sandstone near Suishnish in Skye. (MacCulloch.)]

Every variety of trap-rock is sometimes found in these dikes, as basalt, greenstone, felspar-porphyry, and more rarely trachyte. The amygdaloidal traps also occur, and even tuff and breccia, for the materials of these last may be washed down into open fissures at the bottom of the sea, or during eruptions on the land may be showered into them from the air.

Some dikes of trap may be followed for leagues uninterruptedly in nearly a straight direction, as in the north of England, showing that the fissures which they fill must have been of extraordinary length.

_Dikes more crystalline in the centre._--In many cases trap at the edges or sides of a dike is less crystalline or more earthy than in the centre, in consequence of the melted matter having cooled more rapidly by coming in contact with the cold sides of the fissure; whereas, in the centre, the matter of the dike being kept long in a fluid or soft state, the crystals are slowly formed. In the ancient part of Vesuvius, called Somma, a thin band of half-vitreous lava is found at the edge of some dikes. At the junction of greenstone dikes with limestone, a _sahlband_, or selvage, of serpentine is occasionally observed.

[Ill.u.s.tration: Fig. 444. Syenitic greenstone dike of Naesodden, Christiania.

_b._ imbedded fragment of crystalline schist surrounded by a band of greenstone.]

On the left sh.o.r.e of the fiord of Christiania, in Norway, I examined, in company with Professor Keilhau, a remarkable dike of syenitic greenstone, which is traced through Silurian strata, until at length, in the promontory of Naesodden, it enters mica-schist. Fig. 444. represents a ground plan, where the dike appears 8 paces in width. In the middle it is highly crystalline and granitiform, of a purplish colour, and containing a few crystals of mica, and strongly contrasted with the whitish mica-schist, between which and the syenitic rock there is usually on each side a distinct black band, 18 inches wide, of dark greenstone. When first seen, these bands have the appearance of two accompanying dikes; yet they are, in fact, only the different form which the syenitic materials have a.s.sumed where near to or in contact with the mica-schist. At one point, _a_, one of the sahlbands terminates for a s.p.a.ce; but near this there is a large detached block, _b_, having a gneiss-like structure, consisting of hornblende and felspar, which is included in the midst of the dike. Round this a smaller encircling zone is seen, of dark basalt, or fine-grained greenstone, nearly corresponding to the larger ones which border the dike, but only 1 inch wide.

It seems, therefore, evident that the fragment, _b_, has acted on the matter of the dike, probably by causing it to cool more rapidly, in the same manner as the walls of the fissure have acted on a larger scale. The facts, also, ill.u.s.trate the facility with which a granitiform syenite may pa.s.s into ordinary rocks of the volcanic family.