the Fins, forms no contiguous mountains nor carth; it is de- BOOK composed very slowly.* The formation of porphyries is continued through the period of stratification; it has principally produced jaspers and quartz-agates. XI. rated rocks. The decomposition of stratified rocks, and the reunion of their fragments, or of those even of the most ancient rocks, by a stratified gluten or cement, forms what we call the conglomerated rocks. This formation appears to extend Conglomethrough a variety of periods, and to present a long series of shades which it is almost impossible to define. The brèches, or variegated marbles, and pudding-stone, composed of flint, jasper, shell marble, and others which belong to this class, are often confounded with the variegated marbles and pudding-stones composed of parts of crystallized rocks. The countries which have been most carefully examined with this view, namely, Thuringia and Silesia, present no other constant rule than the identity of the conglomerated fragments with rocks, either crystallized or stratified, which border upon them. The pale-red of Thuringia is a varie- Different gated marble of sandy quartz, in which are found granite, these rocks. porphyry and schists, according as the neighbouring mountains contain them.t In Silesia conglomerate-rocks accompanying the primordial mountains, vanish when they disappear, and contain only fragments analogous to the stones which compose them. At the foot of Mount Altai, in Siberia, we find whole mountains of quartz and jasper in small fragments conglomerated by a cement sometimes quartzeous, and sometimes argillaceous. In Chili, enor. mous masses of pebbles reunited by a black clay, rest upon the most elevated of the Cordilleras. How are we to trace Bergmann, Géograph. Physique, i. 214. Comp. Patrin, Dictionnaire d'Histoire Naturelle, de Déterville, x. 81, au mot Granit d'Ingrie. + Heim, Correspondance de Zach, vi. 535. Léopold Buch, Description Géognostique de Landeck, p. 19. Id. Observations Géognostiques, vol. i. passim. Schangin, dans le Journal des Mineurs. (Bergmannisches Journal,) 1791, vol. i. p. 83, 89. Molina, Histoire Naturelle du Chili, p. 83. (trad. All.) situation of BOOK ΧΙ. Coagulated rocks, basalt. nature of basalt. the innumerable formations which have successively produced these beds of broken remains spread over the whole surface of the globe? We have not been able to comprehend in any of the preceding classes, a species of rock which is evidently formed by an operation different at the same time both from crystallization and stratification. We allude to the celebrated basalt, which has been the subject of so many discussions among geologists, and which has made them split, as it were, into two churches, each attached to its particular opinions with all the keenness of theological zeal. These rocks, compact or porous, assuming prismatic or globular forms, or without any definite form, black, brown, greyish, and bluish, resemble in their texture corneous or horny rocks, and those which Werner designates primitive Chemical trapps. Silex and iron appear to be the principal chemical elements of basalt. So are they equally of trapp, but exposed to strong heat, the trapp affords a glass of a greenish transparency, whilst that of basalt is black and opaque. Besides, the basalt incloses crystals of "peridote," which are not found in the trapps.* The most striking character Configura of basaltic rocks is their configuration. Nothing of this kind is more celebrated than those prismatic columns, of an immense height and thickness, of which the Cave of Fingal in the island of Staffa is composed. tion. On the coast of Ireland, is another admirable collection of basaltic rocks, placed horizontally, and forming together what is called the Giants' Causeway. Masses of a similar description, though less considerable in size, are to be seen in Iceland, and are called in that country the Walls of the Devil. It has been remarked, particularly in the Cave of Fingal, that though the prisms themselves are unequal, the faces of each are equal to the corresponding faces. of the adjacent prisms, that the inequalities which are in relief upon the top or end of one of the prisms, are adapted to the depressions or little concavities, on the top or end of the next opposite prism, as if the one was moulded Faujas Saint-Fond, Géologie, ii. 269. XI. basalt. into the other; lastly, it is worthy of notice, that in the isl- BOOK and of Staffa, where the prisms are raised one upon another, like a series of columns, the convex base of the one is inserted into the concave summit of the other; so that the columns appear to be articulated. Even when the basaltic rocks present themselves under a less regular structure, their position alone is sufficient to attract the particular at- Position of tention of the naturalist. Those masses which, in a confused manner, lie over granite, gneiss, the primitive and secondary schists; those summits, which, sometimes conical, sometimes pyramidal, shoot up, in an insulated form, above rocks of a totally different nature; those cements which unite the basalts to various crystallized rocks; the successive transition of argillaceous siliceous schist to basalt, and from basalt to a kind of schorly rock, named grunstein; lastly, the numerous instances of the disintegration and decomposition of basalt itself, imparting fertility to the soil of the earth; these are facts, the explanation of which, has for many years exercised and baffled the ingenuity of geologists. canic ori The volcanists, with Desmarets, Faujas Saint-Fond, and of the vol Dolomieu at their head, regard basalt as lava melted by gin of bathe heat of a volcano, and which, while slowly cooling, has salt. acquired by contracting those prismatic forms by which it is distinguished.* But this explanation labours under great difficulties. We ask the volcanists, Why these supposed lavas have an appearance similar to rocks, the aquatic formation of which is generally admitted? Why do they present no trace, either of vitrification, or of tumefaction? Why do they envelop untouched crystals, and other substances which must have originated from a state of fusion? Dolomieu answers, that the heat which melted these basaltic lavas, had very little intensity; that the fusion of these substances was only a simple dilatation, which, in separating the particles, allowed them merely to glide one over another, without, in any degree, changing their * Dolomieu, Mémoires sur les les Ponces, p. 100, sqq. Desmarets, Memoires de l'Acad. des Sciences, 1771, p. 273. Faujas Saint-Fond, Histoire Naturelle des Laves Prismatiques, &c. XI. BOOK nature. He supports this opinion by his own observations; according to which, the ordinary lava, even while it is flowing, is sufficiently firm and cool on its surface to admit of being walked upon without much inconvenience.* Facts opposed to A multitude of other facts, however, seem to leave no the volca- subterfuge to the volcanists. In the first place, the basalnic origin. tic cones, placed above all kinds of aucient and secondary rocks, and often forming the most elevated summits of chains of mountains, present, from their situation, from their entire structure, and from the absence of craters, characters very contrary to those of lavas and volcanic accumulations. Basalt is decomposed daily, which happens to no species of lava. In Ringuerike, in Norway, the soil appears almost throughout entirely to consist of decomposed basalt. Near Christiania, small fragments of this rock are spread over the fields, under the conviction, that it will form an earth proper for cultivation. If we examine the texture of these rocks, we find it very sensibly approach that of the schistous porphyries and grunstein. Upon the Meisner in Hesse, we have observed, with a great degree of attention, the transition of basalt into grunstein. The third fact, which is inconsistent with the volcanic origin of basalt, is its relative position in regard to coals. Not only in Hesse does basalt cover an immense bed of coals, but in Sudoree, one of the Feroe Islands, a mine of coal is seen in the midst of the basalt. It is evident that these masses of coals would have changed their nature considerably, if the lava in melting had flowed around them. Neptunian origin of basalt. The Neptunian origin of basalt appears to be attended with much probability. Bergmann, who was the first to prove by analysis the identity of trapp with basalt, is content with supposing, "that the substance of basalt, penetrated and softened by humid vapours, was converted * Dolomieu, Journal de Physique, Fructidor, an. ii. p. 408. ibid. Pluvoise, même année, p. 118. + Strom. Description du Canton d'Eger, p. 47, sqq. (in Dan.) Daubuisson, Traduction de la Théorie des Filons, p. 94, note 2. Mem. de la société d'Histoire Naturelle de Copenhague. See in the course of this work the description of the Feroer Islands. XI. into a clammy and liquid mass; that this mass having, in BOOK the progress of drying, suffered contraction, but not equally throughout the whole, had formed ruptures, and that it was thus subdivided, with a kind of regularity, into prisms of different kinds."* of Werner. A celebrated mineralogist, Werner, from considering the Hypothesis singular nature of basaltic rocks, and particularly the position of those of Saxony and Bohemia, thought that he had discovered, that the formation of these substances constitutes a distinct epoch, and requires causes quite different from those which have produced the ordinary kinds of earths and rocks. This philosopher imagined that a mechanico-chemical solution of a particular nature had once covered the globe. This solution produced precipitates analogous to its own nature; at first, gravel, clay, and sediment, purely mechanical; very soon after these, came the argilo-siliceous schists, named wackes, which already indicate the commencement of confused crystallization; then appeared the basalts. The mass of these being deposited upon almost the whole of the surface of the globe, the solution arrived at a state purely chemical, and afforded nothing but crystallized precipitates, such as the schistous porphyries, and the rocks called greenstein.† According to this hypothesis of Werner, there is a possibility of explaining why isolated basaltic columns are found projecting from the midst of a soil of a different nature, for we may conceive that the parts the least crystallized of the basaltic solution, that is to say, the parts mixed with clay and gravel, after having been deposited and dried, were again decomposed and carried along, with other earths, by currents of water, far from the sides of the mountains which once were covered with them. In some places, this mass of decomposed basalt is extended more equally over considerable tracts of ground. The masses *Bergmann, De Productis Vulcaniis. Werner, Classification des Roches, &c. &c. Daubuisson, Journal des |