silicious infiltrations in the tissues of sponges, ventriculites, and other zoophytes, and even in the intimate structure of wood, when seen under a highly magnifying power, is that of a viscid fluid, or plastic paste, pressed into the interstices of the tissue, rather than that of the percolation of a mineral solution, or of a metamorphism of the organism, as in other examples of silicified animal and vegetable structures.

Experiments have shown that melted quartz, unlike alumina, retains its viscidity for some time when cooling, and may be drawn out in threads;† and M. Jobert states, that in graphic granite he has found the quartz-crystals in the midst of the felspar flattened and contorted, as if they had been strongly pressed between the felspathic matter.‡

These facts seem to offer an explanation of the flexures and curvatures in quartz-rocks and gneiss: and they are brought forward by Mr. Darwin, with his usual acumen, to illustrate the origin of the remarkable duplications, and abruptly arched positions, of the stratified quartz-rocks in the Falkland Islands. Some of the hills, he observes, are composed of quartz-strata doubled on themselves, with the axisplane thrown quite over,—the quartz must therefore have

* As for example, in some of the fossil wood from Egypt and Australia; see p. 727.

+ M. Gaudin, quoted by Mr. Darwin.

M. Alexarder Brongniart attributes the formation of all agates and chalcedonies to the viscous or gelatinous condition of the mineral matter; and that of hyaline quartz and rock-crystal to the perfect fluidity of the dissolved silica. 66 Lorsque la silice a été complètement dissoute, et par conséquent dans un état de liquidité parfaite, elle a cristallisé et produit le quarz hyalin. Mais lorsqu'elle était en consistance gélatineuse, elle a produit les silex et surtout ceux qu'on désigne par le nom général d'agate et de calcédoine."-Essai sur les Orbicules siliceux. Ann. Sciences Nat. 1831, vol. xxxiii. p. 200.

In the specimen of Trigonia from Tisbury, previously mentioned (p. 527), the branchia are completely silicified; and some of these filaments, when examined under a high power, show the orbicular structure which characterizes chalcedonic silica.

been in a pasty condition when it suffered without fracture such abrupt curvatures. Mr. Darwin states that the detached concentric layers resembled gigantic semi-cylinders of quartz, like draining- or ridge-tiles. One specimen was twenty feet long and twelve in diameter.*

It would appear, therefore, that silex or quartz may be formed in two ways: by the action of thermal waters (p. 93, &c.), and by the influence of intense heat. The remark of Professor Keilhau here applies with equal force as to the experiments which called it forth:-"The greater part of the crystalline rocks have an entirely hidden origin and development. Chemistry alone cannot decide this question, for the same minerals can be composed in nature by different processes. By the side of the celebrated fact which showed the possibility of the formation of felspar by heat, we can now place experiments which prove the possibility of producing felspar in the moist way."

Mica-schists and gneissic rocks are widely expanded over Europe and America; and everywhere abound in metalliferous veins. They are of various ages; and their metamorphic character is proved by the occurrence of gneiss, mica-schist, and talcose-schist in the Alps, Apennines, Andes, and Alleghanies under circumstances showing that their crystalline structure has been acquired since the origin of many of the fossiliferous strata; even in some instances long after the deposition of those which repose directly upon them. On the other hand, the gneiss of Kinnekulle in Sweden, or of the Falls of Montmorenci, and many of the unstratified or plutonic rocks of the Adirondach mountains, west of Lake Champlain, are of older date than any strata in which organic remains have yet been found.†

In the North Highlands of Scotland, as already stated

*Mr. Darwin, On the Geology of the Falkland Islands; Quarterly Geological Journal, vol. ii. p. 267.

+ Sir C. Lyell's Travels in America, vol. ii. p. 129.

(p. 809), we have both an old primordial gneiss and a younger metamorphic gneiss within a section of a few miles in length.*

24. BASALT OR TRAP.-The consideration of the ancient volcanic rock designated by the various names of Whin, Trap, Basalt, and Clinkstone will next engage our attention. Basalt occurs in veins or dykes, which traverse rocks of all ages; it also occurs in layers spread over the surface of the strata, or interposed between them. Many modern lavas differ so little from basalt, that it is unnecessary to adduce proof of the volcanic nature of this rock. Dr. Macculloch observes, that from lava to basalt, and from thence to syenite, porphyry, and granite, there is an uninterrupted succession: as agents in geological changes trap and granite are identical; and that it is a mere dispute about terms to refuse the name of submarine lavas to basaltic dykes. They are as much the product of extinguished volcanos, as the basalts, lavas, and tuffs of Italy are the result, in some cases, of extinct, and, in others, of active volcanos.† Beds of felspathic and basaltic materials, of a friable and coarse texture, are often found in the older rocks; these are volcanic ashes and grits, that have been formed at the bottom of the sea, during the accumulation of the sedimentary matter with which they are associated (p. 818). In some places they appear as currents or sheets of pure volcanic materials; at others they envelope marine remains, pebbles, sand, and fragments of rocks; some layers consist of fine volcanic scoriæ passing into sand; and all these varieties alternate with beds composed exclusively of shelly and marine sediments; so that no doubt can be entertained that the diversified masses, thus arranged in parallel strata, must have been formed during the same period of igneous action. These evidences of ancient volcanic operations are similar to those observable in the modern deposits of Sicily, where banks of existing species * See "Siluria," 2nd edit. p. 199. † System of Geology, vol. ii. p. 114.

of marine shells, now at considerable heights above the sea, are so intercalated with volcanic matter, that no other inference can be drawn than that the whole were of contemporaneous marine formation.*

The most remarkable form assumed by basalt is that of regular pillars, or columns, clustered together; a character also observable in some recent lavas; the columnar basalts of the tertiary epoch have already been noticed (vol.i. p.271). This columnar structure has been proved, by some highly interesting experiments, to have originated in the manner in which refrigeration took place. Mr. Gregory Watt† melted seven hundred weight of basalt from Rowley Regis (p. 813), and kept it in the furnace several days after the fire was

Fig. 1. A block somewhat decomposed, partially exhibiting the primitive spheroidal figure of the prism. 2. Portions of columns, consisting of several joints. 3. The concave surface of a joint.

reduced. It fused into a dark-coloured vitreous mass with less heat than was necessary to melt pig-iron; as the mass cooled, it changed into a stony substance, and globules appeared; these enlarged until they pressed laterally against each other, and became converted into polygonal prisms.‡

Silurian System, p. 75. + Philosophical Transactions for 1804. "Geol. Observer," p. 404.

The articulated structure and regular forms of basaltic columns have, unquestionably, resulted from the crystalline arrangement of the particles in cooling; and the concavities, or sockets, have been formed by one set of prisms pressing upon others, and occasioning the upper spheres to sink into those beneath; thus the different layers of spheres have been articulated together, as in the basaltic columns of the Giants' Causeway (Lign. 204).

Proofs of the correctness of this inference are afforded by the occurrence of a spheroidal nucleus enveloped by a polyhedral block of basalt; and from the fact, that, when this rock is not divided into regular prismatic columns, it often forms laminated spheroids, which, varying in size, constitute by aggregation extensive masses. The position of the columns presents every variety from the perpendicular to

the horizontal; this has arisen from corresponding differences in the direction of the cooling surfaces, for the prisms are found to be always at right angles with the surface of refrigeration; the horizontal, inclined, and curved columns of basalt, which occur in the Isle of Staffa, and elsewhere, have originated from this cause.