BOOK XI. of pure quartz; strong veins of it are seen traversing even the granite mountains, and the vast wall of quartz upon the mountain of Felsberg, near Manheim, excites the wonder of the naturalist.* But to the admiration of these phenomena succeeds the spirit of disputes, classifications, and nomenclatures. It is often impossible to discover the facts in the midst of the geological discussion to which they have given rise. The rock which most nearly resembles the granite, unites to the granulated structure of the latter an arrangement of parts, which gives it a foliated appearance; the quartz and feldspar are found there in grains, but the mica forms bands, or very thin layers, in which the two other substances are contained. This is the micaceous rock of Haüy, the gneiss of Werner, and the fo- Micaceous liated rock of Saussure; but frequently talc, or even ar- Gneiss. gil, supplies the place of mica in these foliated rocks, a circumstance occasioning continual mistakes. There are some mineralogists who give to the name of gneiss, an extension which renders all definition impossible. rock. schists. The micaceous schists are rocks which occur in great Micaceous Jaminæ, composed of quartz and mica. It appears that they recede by imperceptible gradations from the granulated texture of granites, and that they become more and more mixed with alumina or argillaceous earth, an earth, the presence of which seems to prevent crystallization from taking place. ceous The argillaceous schists, in which alumina or argilla- Argillaceous earth predominates over the silex, occurs in the form schists. of large unbroken lamine; a circumstance which constitutes what we call the schisteous structure. These imperceptible transitions from one kind of aggregation to another, render all classifications uncertain. No one has yet been able precisely to determine the four or five kinds of rocks, to which the names of horny schists and of wacke‡ schists. Faujas, 1, c. p. 94. + Saussure, Voyages, ◊ 1359, 1679, &c. Hauy Mineralogie, iv. 432. Comp. Delamétherie, Théorie de la Terre, ii. p. 337-380. Werner, cité par Saussure, Voyages, 1304. Horny BOOK are given; they are merely combinations of silex and alumina, with a little lime and iron. The horny texture is found in part in certain rocks, composed as it were of a paste, in which masses of a more perfect crystallization are Porphyry. imbedded. These are the porphyries, rocks which baffle more than any other kind the zeal of classifiers.* Sometimes it is the paste which is not homogenous, and sometimes the imbedded crystals which constitute the species or the variety. The porphyries, properly so called, are masses of imbedded felspar, coloured by a metallic oxide, and containing crystals of the same kind. Such is the superb red antique porphyry, originally from Egypt, and of which only imperfect varieties are to be found in Europe. Such is still the ophites, or serpentine antique porphyry, although chequered with plates of amphibole; a substance which abounds more and more in black porphyries. All the real porphyries commonly found in the vicinity of granite, are distinguished from it only by that pasty substance which serves as their base; but they differ also from pudding stones, or conglobated rocks, by the perfect crystallization of this paste or cement. One may easily conceive how difficult it is to fix the innumerable shades which occur in the crystallization of these rocks; it is this which Porphyroi- has led geologists to create a class of porphyroidal rocks. dal rocks. The disorder, or rather the capricious order, according to which the particles of these rocks have crystallized, sometimes occasions the most brilliant and fanciful appearances. Who would not admire the orbicular porphyry of Corsica,‡ which, on being polished, displays circles composed of little yellowish red leaves, arranged in rays around a reddish brown kernel, and which presents to the eye a transverse section of some unknown and delicious fruit? Trapp. Trapp is composed nearly of the same substances as * Comp. Haüy, iv. 435. Faujas St.-Fond, Essai, ii. 405, &c. + Saussure, Voyages, 149, sqq. Faujas St.-Fond, Essais de Géologie, ii. 245, and planche xx. bis. ♦ A name derived from the Swedish trapp, a stair or ladder. Saxum trapezium, Waller, sp. 220. XI. porphyry. They are united in the same proportions, only BOOK the iron abounds more in trapp; but this arrangement in small grains, united sometimes by a cement, does not present the shining crystallization of granite and porphyry.* Trapp rocks have a tendency to separate and subdivide into rhomboidal fragments, resembling the steps of a staircase, which often gives them the appearance of rocks called basalt, by the moderns. The celebrated Werner, indeed, considers these two kinds of rocks as the same, and comprehends them under the name of primitive trapp; an opinion which, at present, seems to be received only in the school of that mineralogist. jade, &c. The rocks of petrosilex and of jade, as well as that com- Petrosilex, posed of amphibole and mica, which Werner names grunstein, that is, greenstone, forms small varieties, resembling trapps and porphyries in their relative positions, although the amphibole rock, amongst others, often has the crystallized texture of granite. Of all the elementary earths, the calcareous, and, next to it the magnesian, have the least contributed to form crystallized rocks. limestone. The primitive limestone, or that in which there is not a Primitive vestige of animal remains, is not very extensively distributed; it is rarely found amongst the foliated granites, and the micaceous schists; the calcareous banks, in the argillaceous schists, have a texture less perfectly granulated, and present a compact or lamellated fracture. It is amongst the stratified rocks that chalk shews itself in abundance; it is at the head of these aggregations, as granite is at the head of rocks purely crystallized. Magnesian earth appears to predominate in the talc rocks, Magnewhether compact or schistous, in the serpentine, the ollair sian rocks, or potstone, steatites, or soap rocks, and some other aggregations, which, in truth, often occur in the primordial mountains; but which form in general only small subordinate masses. There are rocks of serpentine, however, * Saussure, Voyages, 145. Werner, cité par Steffens, 1, c. 21. XI. BOOK which rise to a very great height. May we not imagine that the alcaline particles, existing in feldspar, one of the elements of granite, may be separated from it to form a series of insulated rocks? It is at least very remarkable, that magnesian earth seems in a manner to arrest the progress of pure crystallization, and to disappear with the primordial rocks, not to shew itself again either in stratified mountains, or in accumulations, but only in the waters of the ocean. We ought to pause a little to consider these crystallized aggregations, which seem to have preceded the birth of our mountains, of which they form the most solid part. We leave, however, to the naturalists by profession, the task of classifying the innumerable modifications of agConglobat- gregate substances, which we have named conglobated ed rocks. rocks. We may conceive, that the circuit of the globe, scarcely consolidated in the bosom of primitive chaos, must soon after its formation have begun to waste away. The action of different elements, probably the rapid alternations from excessive heat to intense cold, and even the very weight of the masses themselves, but recently crystallized, could not fail to produce explosions and downfalls, and consequently fragments, which, in reuniting by means of the crystallization of the fluid substances which surrounded them, have given birth to some of the rocks termed variegated marbles and pudding stones. These rocks differ from porphyries, in this, that they are composed of fragments of other masses already crystallized, while porphyry consists of crystals which have been formed in the midst of the cement which unites them; hence the cement of porphyry should be designated only under the name of basis. The conglobated rocks differ from the flint, (grès) in as much as they contain larger grains, or do not present regular homogeneous beds, or a foliated structure. Brèche, or Custom appears to have appropriated the word Brèche, variegated borrowed from the Italian, to aggregations of fragments of marbles. Saussure, Voyages, 19. Faujas, Essais, ii. Saussure, Voyages, 149. XI. stone. calcareous rock, while the term pudding stone, for which BOOK the more classical appellation allatoides, may be substituted, appears to be reserved for rocks formed by the reunion of a great number of small flints. It would be bet- Pudding ter to call "Brèches" the aggregations of angular fragments, and "pudding stone," those of rounded fragments, which probably have been rolled along by the waters. The etymology even of the term in Italian, Breccia, borrowed from the Goths or Lombards, indicates a violent bursting. Occurrence. We may easily imagine, that these assemblages would vary in a thousand ways, either with respect to the nature of the reunited fragments, or of the cement which unites them. Of all the substances, however, which serve to conglutinate the fragments of primitive rocks, it is quartz, or siliceous earth, which is most abundant, and of most frequent This element, which, appears coexistent with our globe, may be considered as the source of all crystallized formation. Sometimes the brèches, in their turn, have been reduced to fragments, which, agglutinated by a new cement, have produced brèches composed anew, which have been called double brèches. We place next to these brèches, and among the conglobated rocks, those which the naturalists have called Amygdaloïdes, from their similarity Amygdato almonds imbedded in paste. The amygdaloïdes are composed of any paste whatever, in which are found knots or glands of the same substance, or of another. The whole is united by a confused crystallization; sometimes the spaces occupied by the knots, are found empty, the substance which once filled them having been destroyed by an unknown cause; this makes the amygdaloïde resemble porous lava. It is extremely difficult to distinguish the amygdaloides from certain varieties of porphyry. Without involving ourselves in discussions to which the formation of * Du mot Grec αλλας, genitif αλλάζος. (The genitive of αλλας is αλλαντος, Τ.) poudingue. + Saussure, 197. Romé de l'Isle, Christallographie, ii. 573. Delamétherie, Théorie, 531. Comp. Haüy, iv. 463. Du verbe Brechen, Allem. Brekke, Dan. briser, rompre, French. loïdes. |