VII. BOOK surements, whether taken trigonometrically, or from the comparative height of the barometer, have been made to no great extent, except in Europe and America. But in such general observations upon the whole of the globe, Europe cannot be regarded as a very important part, or as a standard of comparison much to be depended upon; because the summits of our Alps, such as Mount Blanc, Mount Rósa, the Ortelos, are elevated only about 14 or 15,000 feet, while those of the Cordilleras, Chimborazzo, Antisana, and Pichincha, tower to the height of 19 or 20,000, is that a reason to conclude, that the new world has in general loftier mountains than the old, or that the mountains increase in elevation as they approach the equator? One of these conclusions is, and will continue to be hazarded, until the intrepid Humboldt has measured the Alps of Thibet, more elevated still, perhaps, than Chimborazzo; the other is decidedly false, since the Andes of Chili are reckoned to be as high as those of Peru ;* the volcanos of Mexico are but very little inferior to those of Quito; and the enormous peaks of Spitzbergen and Greenland, appear to equal the Alps, and very much to surpass the mountains of Norway and Russia, to which, according to the hypothesis, they ought to be inferior. It is proper, therefore, to reserve, until we enter upon the description of the several parts of the world, the few general comparative observations suggested by those mountains whose heights have been already determined. *Moline, Hist. Nat. de Chili, BOOK VIII. Continuation of the Theory of Geography. Of the Interior Of Banks or LEAVING the surface of the earth, which itself is so imperfectly known to us, and descending into its interior, our knowledge of which is still more confined, we are now about to consider the solid crust of the globe, in so far as it has been examined in reference to its interior structure, and the substances of which it is composed. beds, &c. To whatever depth excavations have been made in differ- Strata, ent countries, they have uniformly shewn that the greater portion of the earth consists of strata of materials of different natures, irregularly disposed. When these strata are of a similar nature, and of great thickness, they are called beds or banks (bancs.) If they preserve a horizontal position, we call their subdivisions layers (assizes.) But, although we find, even upon the highest mountains, banks, if not layers, the almost vertical position of the principal masses of most of the higher mountains, obliges us to give them the name of blocks (blocs,) although this name by no means points out the nature of these masses, which some regard, with much probability, as enormous crystals, while others affect to consider them only strata changed in their position. Sometimes the blocks, or beds, are divided by Delamétherie, Théorie de la Terre, sec. 1339, el seq. Patrin, Hist. Nat. des Mineraux, i. 103. + Saussure, Voyage dans les Alpes, sec. 604-669. Compare his opinions, sec. 1691 and 2300. Deluc, Lett. Géolog. à Blumenbach, p. 123. (En Fran.) We have adopted the terms of our author, though they by no means coincide with those used by our own writers upon geology and mineralogy; but, in order to prevent mistake, we have in general given the French term also.-Translator, BOOK vertical fissures, and then their portions may be called VIII. leaves, or laminæ, (feuillets ou lames.) We regret that these terms convey no precise and fixed meaning; but we also acknowledge, that, in contemplating the objects of nature, it is impossible to submit them to a more rigorous classification.* The different masses which we are about to describe, are thrown one upon the other in every possible manner, both horizontally and at every angle. Frequently in moderate elevations, and more particularly in low lands, the different strata preserve, for hundreds of leagues, a parallel position; thus the limestone or calcareous strata containing numerous shells, upon which the city of Paris is built, extend across what was formerly called the Isle of France, as far as Belgium. The gypseous strata of Montmartre, and of the heights of Belleville, have the same degree of elevation, though separated by a valley. In Champagne, a large bed of chalk runs nearly upon the same level from Rhetel to Sens. Even the most perfectly crystalized rocks appear sometimes to follow a horizontal direction. A ridge of granite seems to extend straight from the Limosin, by Poitou, to Cherbourg in Normandy. Another girdle or band of granite follows the valley of the Upper Loire from Creuzot and Mount Cenis to Saint Etienne through a space of seventy leagues. Rocks of trapp similar to those in Westrogothia are found again upon the same level in some mountains separated by extensive plains. In the island of Rugen in Pomerania, in the Danish island of Moen, and at Stevens in Zealand, the strata of chalk and flint correspond with each other, though an open sea flows between their bases. What confusion, nevertheless, do we *Bergmann, Géog. Phys. i. 197. † Cuvier et Brongiart, Annales du Muséum, vi. Linck, Voyage en Portugal, i. 48. Dupin, Statist. des Deux-Sèvres, 105. Journal des Mines, No. viii. p. 27-29. Delamétherie, Théorie de la Terre, iv. sec. 954. sec. 1154. Bruslé, Statist. de l'Aube, p. 6. find by the side of this tranquil regularity of formation! BOOK What traces of destruction at once alarm and delight the VIII. observer of nature! verted and shifted. In the plains, and on moderately high mountains, we meet Strata inwith strata that have been entirely inverted, or partly shifted from their first position, bent in every shape, crooked and curved, and returning upon themselves; the mountain of Saint Gilles, near Liege, exhibits all these anomalous appearances.* We find considerable strata in Mount Jura, which, having been overturned or pushed forward upon others, have stopped in a position so precarious, that the application of the least force would put them again in motion. The Alps exhibit a striking spectacle of disorder and confusion. We discover pyramidal mountains, like the needle of the south, the layers of which are ranged round the axis of the pyramid like the leaves of an artichoke, if we may be permitted to Strata concompare those enormous rocks to a small vegetable. Nant d'Arpenaz presents to our view a sort of hemisphere, composed of regularly curved strata. At every step, the rules which appear most generally followed, are broken and set at defiance by the greatest possible diversity. If Mount Blanc be composed of enormous vertical blocks, Mount Rosa, equally gigantic, presents only horizontal strata, a little inclined. centric and of the These strata are almost all intersected by fissures and ca- Cavities vities more or less considerable. Some consist of interstices and fissures left between two ancient rocks at the moment of their crys- globe. tallization; the great majority appear to owe their origin either to the retiring or sinking of the earth. The first of these causes has considerably increased them in the calcareous mountains of secondary formation; they are less frequent in gypsum. Some of these fissures have been filled with metallic substances, some by the filtering of water impregnat * Delamétherie, 1. c. sec. 1383, pl. vi. &c. Bertrand, Nouv. Princ. de Géologie, p. 182 Ibid. sec. 473. BOOK ed with stony matter, others by incrustations, by alluvial VIII. minerals, by vegetable, and animal earths; lastly, some have remained open, and form ravines, precipices, abysses, when they are open to the sky; or caverns and grottos, when they have walls and a natural roof. In another part of this work, we shall point out, and occasionally describe at length the most remarkable caverns and grottos of our globe; but we must here confine ourselves to general views. There are some very considerable caverns; frequently the first excavation is only the vestibule to another much deeper and larger, but the dimensions of caverns have generally been much exaggerated, The depth of that of Eldon Hole, near Castleton in Derbyshire, has not been discovered, though sounded with a line of more than 9600 feet.* Near Frederickshal in Norway, there is a hole, into which, if stones be thrown, two minutes appear to elapse before they reach the bottom, from which it has been concluded that the depth was upwards of 11,000 feet. Among the numerous caverns of Carniola, that of Adelsberg is said to afford a subterranean walk of two leagues; but this computation of rather too enthusiastic a writer requires to be confirmed.‡ Many caverns are remarkable for various natural curiosities. There are some from which, in summer-time, an ice-cold wind issues with astonishing force. Mount Eoto near Turin in Italy is an example of this. There are others, the walls of which are in autumn covered with ice, which melts in December. There are two or three in France that have attracted notice; among others the grotto of Notre Dame de Balme near Grenoble. The little communication which these caverns have with the external air, causes them to change their temperature Lloyd, Phil. Trans. 1771, vol. lxi. part i. No. 31. + Pontoppidan's Natural History of Norway, i. 101. Valvasor, Gloire de la Carniole, 1699. Kircher, Mund. Subterran. lib. iv. 239. (Compare the Grotto of Motiers In Bernoulli's Description of Neuchâtel p. 32.) Mem. de l'Acad. des Sciences, 1755, p. 149, &c. |