If we direct our attention now to the configuration of mountains, we shall see that this configuration depends essentially on the nature of the rocks which constitute them. Granite, for example, is one of those which offers the most varied outlines, as the reader may see without quitting the United Kingdom, in the rugged, fantastic, broken masses of the Argyllshire Highlands, that hem in the waters of Loch Goil and Loch Long. Granite abounds in the tropical zone, and seems to prefer chains of moderate elevation. Granite heights are generally distinguished by abrupt and polished flanks, pointed or dentelated summits, scarped approaches, deeply fissured slopes, and narrow, wild, and profound valleys.
Gneiss, a felspathic and micaceous rock, of schistous structure, is found in layers sometimes horizontal or gently inclined, sometimes undulating and complicated towards the border. The contours of the gneiss mountains are less cloven than those of mountains of granite; but numerous fissures and indentations are still discoverable.
Porphyry generally occurs in isolated peaks, with almost vertical flanks; seldom in continuous chains. Porphyritic mountains, says M. Maury, imprint on the landscape a peculiarly picturesque character. This rock sometimes appears under the form of tall pillars set in close juxtaposition—it is then known as columnar porphyry; and to groups of these columns have been given in some countries the name of Orgues or Organs, on account of their resemblance to the organ pipes which discourse solemn music in our cathedrals.
Thus: in Mexico two mountains occur distinguished by this appellation, Los Organos; one is that of Mamanchota, situated to the north of the Indian village of Actapan. The portion soaring out of the rock, says Humboldt, is three hundred feet in height; but the absolute elevation of the summit of the mountain, at the point where the Organos begin to shoot aloft, is 1385 toises (about 5310 feet). The other is the Jacal, which is nearly 9600 feet above the sea-level, and crowned with forests of pine and cedar.
But the most celebrated Organ Mountains are those which rear their glittering shafts at the extremity of the bay of Rio Janiero. “It is not only the aspect of these pointed summits,” says Dr. Yvan, “that reminds the spectator of the sublime instrument of our churches; the strange sounds which escape from between these cylinders of rock render the analogy still more striking, and complete the illusion. The voice of the tempest, the lamentations of the forests bowed by the passing winds, the doleful wails of the jaguars, the cries of the howling monkeys passing between these sonorous peaks, produce a harmony before which all human instrumentation loses its grandeur. We feel that it is the universal soul which inspires the chords of the majestic keys. The Serra dos Organos is clothed in virgin forest over three-fourths of its extent; it is only at long intervals, and in obscure valleys, that we encounter any traces of human industry, or that we traverse some circular treeless hollows, in which an abundant herbage flourishes, and feeds the troops of horses and oxen enclosed in these natural parks.”
The Organ Mountains of Epailly (in the department of the Haute Loire, in France) and of Bart (in the Corrèze), and the Colonnades of Chenavari (in the Ardèche), belong to the basaltic formation, rendered so remarkable by its frequent arrangement in prismatic columns of extreme regularity. Basalt also gives birth to chains which resemble vast walls, and sometimes appears in the form of pyramids, plateaux, or simple mamelons.
Of the columnar arrangement the Palisades, on the banks of the river Hudson, may be particularized as a noble example; but a still grander spectacle is presented on the river Columbia, west of the Rocky Mountains, where the waters pour through a valley walled on either side with tier upon tier of pillars, to the height of fully a thousand feet.
The Trachytes, massive rocks of excessive roughness, occasionally appear in the shape of cones, at times in that of domes or enormous balloons, and at times as cupolas with spire-like points, like minarets. The chalks, the sandstones, the diorites, have all their characteristic aspect, and give to the mountains where they dominate, and to the landscapes which surround them, an easily recognizable physiognomy. And, finally, everybody knows the particular configuration affected by the volcanic mountains.