Part 60 (1/2)
[Ill.u.s.tration: Fig. 445. Greenstone dike, with fragments of gneiss.
Sorgenfri, Christiania.]
The fact above alluded to, of a foreign fragment, such as _b_, fig.
444., included in the midst of the trap, as if torn off from some subjacent rock or the walls of a fissure, is by no means uncommon. A fine example is seen in another dike of greenstone, 10 feet wide, in the northern suburbs of Christiania, in Norway, of which the annexed figure is a ground plan. The dike pa.s.ses through shale, known by its fossils to belong to the Silurian series. In the black base of greenstone are angular and roundish pieces of gneiss, some white, others of a light flesh-colour, some without lamination, like granite, others with laminae, which, by their various and often opposite directions, show that they have been scattered at random through the matrix. These imbedded pieces of gneiss measure from 1 to about 8 inches in diameter.
_Rocks altered by volcanic dikes._--After these remarks on the form and composition of dikes themselves, I shall describe the alterations which they sometimes produce in the rocks in contact with them. The changes are usually such as the intense heat of melted matter and the entangled gases might be expected to cause.
_Plas-Newydd._--A striking example, near Plas-Newydd, in Anglesea, has been described by Professor Henslow.[381-A] The dike is 134 feet wide, and consists of a rock which is a compound of felspar and augite (dolerite of some authors). Strata of shale and argillaceous limestone, through which it cuts perpendicularly, are altered to a distance of 30, or even, in some places, to 35 feet from the edge of the dike. The shale, as it approaches the trap, becomes gradually more compact, and is most indurated where nearest the junction. Here it loses part of its schistose structure, but the separation into parallel layers is still discernible. In several places the shale is converted into hard porcellanous jasper. In the most hardened part of the ma.s.s the fossil sh.e.l.ls, princ.i.p.ally _Producti_, are nearly obliterated; yet even here their impressions may frequently be traced. The argillaceous limestone undergoes a.n.a.logous mutations, losing its earthy texture as it approaches the dike, and becoming granular and crystalline. But the most extraordinary phenomenon is the appearance in the shale of numerous crystals of a.n.a.lcime and garnet, which are distinctly confined to those portions of the rock affected by the dike.[382-A] Some garnets contain as much as 20 per cent. of lime, which they may have derived from the decomposition of the fossil sh.e.l.ls or Producti. The same mineral has been observed, under very a.n.a.logous circ.u.mstances, in High Teesdale, by Professor Sedgwick, where it also occurs in shale and limestone, altered by basalt.[382-B]
_Antrim._--In several parts of the county of Antrim, in the north of Ireland, chalk with flints is traversed by basaltic dikes. The chalk is there converted into granular marble near the basalt, the change sometimes extending 8 or 10 feet from the wall of the dike, being greatest near the point of contact, and thence gradually decreasing till it becomes evanescent. ”The extreme effect,” says Dr. Berger, ”presents a dark brown crystalline limestone, the crystals running in flakes as large as those of coa.r.s.e primitive (_metamorphic_) limestone; the next state is saccharine, then fine grained and arenaceous; a compact variety, having a porcellanous aspect and a bluish-grey colour, succeeds: this, towards the outer edge, becomes yellowish-white, and insensibly graduates into the unaltered chalk.
The flints in the altered chalk usually a.s.sume a grey yellowish colour.”[382-C] All traces of organic remains are effaced in that part of the limestone which is most crystalline.
[Ill.u.s.tration: Fig. 446. Basaltic dikes in chalk in island of Rathlin, Antrim. Ground plan, as seen on the beach. (Conybeare and Buckland.[382-D])]
The annexed drawing (fig. 446.) represents three basaltic dikes traversing the chalk, all within the distance of 90 feet. The chalk contiguous to the two outer dikes is converted into a finely granular marble, _m m_, as are the whole of the ma.s.ses between the outer dikes and the central one. The entire contrast in the composition and colour of the intrusive and invaded rocks, in these cases, renders the phenomena peculiarly clear and interesting.
Another of the dikes of the north-east of Ireland has converted a ma.s.s of red sandstone into hornstone.[382-E] By another, the slate clay of the coal measures has been indurated, and has a.s.sumed the character of flinty slate[383-A]; and in another place the slate clay of the lias has been changed into flinty slate, which still retains numerous impressions of ammonites.[383-B]
It might have been antic.i.p.ated that beds of coal would, from their combustible nature, be effected in an extraordinary degree by the contact of melted rock. Accordingly, one of the greenstone dikes of Antrim, on pa.s.sing through a bed of coal, reduces it to a cinder for the s.p.a.ce of 9 feet on each side.[383-C]
At c.o.c.kfield Fell, in the north of England, a similar change is observed.
Specimens taken at the distance of about 30 yards from the trap are not distinguishable from ordinary pit coal; those nearer the dike are like cinders, and have all the character of c.o.ke; while those close to it are converted into a substance resembling soot.[383-D]
As examples might be multiplied without end, I shall merely select one or two others, and then conclude. The rock of Stirling Castle is a calcareous sandstone, fractured and forcibly displaced by a ma.s.s of greenstone which has evidently invaded the strata in a melted state. The sandstone has been indurated, and has a.s.sumed a texture approaching to hornstone near the junction. In Arthur's Seat and Salisbury Craig, near Edinburgh, a sandstone which comes in contact with greenstone is converted into a jaspideous rock.[383-E]
The secondary sandstones in Skye are converted into solid quartz in several places, where they come in contact with veins or ma.s.ses of trap; and a bed of quartz, says Dr. MacCulloch, found near a ma.s.s of trap, among the coal strata of Fife, was in all probability a stratum of ordinary sandstone, having been subsequently indurated and turned into quartzite by the action of heat.[383-F]
But although strata in the neighbourhood of dikes are thus altered in a variety of cases, shale being turned into flinty slate or jasper, limestone into crystalline marble, sandstone into quartz, coal into c.o.ke, and the fossil remains of all such strata wholly and in part obliterated, it is by no means uncommon to meet with the same rocks, even in the same districts, absolutely unchanged in the proximity of volcanic dikes.
This great inequality in the effects of the igneous rocks may often arise from an original difference in their temperature, and in that of the entangled gases, such as is ascertained to prevail in different lavas, or in the same lava near its source and at a distance from it. The power also of the invaded rocks to conduct heat may vary, according to their composition, structure, and the fractures which they may have experienced, and perhaps, also, according to the quant.i.ty of water (so capable of being heated) which they contain. It must happen in some cases that the component materials are mixed in such proportions as prepare them readily to enter into chemical union, and form new minerals; while in other cases the ma.s.s may be more h.o.m.ogeneous, or the proportions less adapted for such union.
We must also take into consideration, that one fissure may be simply filled with lava, which may begin to cool from the first; whereas in other cases the fissure may give pa.s.sage to a current of melted matter, which may ascend for days or months, feeding streams which are overflowing the country above, or are ejected in the shape of scoriae from some crater. If the walls of a rent, moreover, are heated by hot vapour before the lava rises, as we know may happen on the flanks of a volcano, the additional caloric supplied by the dike and its gases will act more powerfully.
[Ill.u.s.tration: Fig. 447. Trap interposed between displaced beds of limestone and shale, at White Force, High Teesdale, Durham.
(Sedgwick.[384-A])]
_Intrusion of trap between strata._--In proof of the mechanical force which the fluid trap has sometimes exerted on the rocks into which it has intruded itself, I may refer to the Whin-Sill, where a ma.s.s of basalt, from 60 to 80 feet in height, represented by _a_, fig. 447., is in part wedged in between the rocks of limestone, _b_, and shale, _c_, which have been separated from the great ma.s.s of limestone and shale, _d_, with which they were united.
The shale in this place is indurated; and the limestone, which at a distance from the trap is blue, and contains fossil corals, is here converted into granular marble without fossils.
Ma.s.ses of trap are not unfrequently met with intercalated between strata, and maintaining their parallelism to the planes of stratification throughout large areas. They must in some places have forced their way laterally between the divisions of the strata, a direction in which there would be the least resistance to an advancing fluid, if no vertical rents communicated with the surface, and a powerful hydrostatic pressure was caused by gases propelling the lava upwards.
_Columnar and globular structure._--One of the characteristic forms of volcanic rocks, especially of basalt, is the columnar, where large ma.s.ses are divided into regular prisms, sometimes easily separable, but in other cases adhering firmly together. The columns vary in the number of angles, from three to twelve; but they have most commonly from five to seven sides.
They are often divided transversely, at nearly equal distances, like the joints in a vertebral column, as in the Giant's Causeway, in Ireland. They vary exceedingly in respect to length and diameter. Dr. MacCulloch mentions some in Skye which are about 400 feet long; others, in Morven, not exceeding an inch. In regard to diameter, those of Ailsa measure 9 feet, and those of Morven an inch or less.[385-A] They are usually straight, but sometimes curved; and examples of both these occur in the island of Staffa.
In a horizontal bed or sheet of trap the columns are vertical; in a vertical dike they are horizontal. Among other examples of the last-mentioned phenomenon is the ma.s.s of basalt, called the Chimney, in St.
Helena (see fig. 448.), a pile of hexagonal prisms, 64 feet high, evidently the remainder of a narrow dike, the walls of rock which the dike originally traversed having been removed down to the level of the sea. In fig. 449. a small portion of this dike is represented on a less reduced scale.[385-B]