These Geological Questions Can Be Solved Only So Far As They Are Directed By The Actual State Of Things, That Is, Of Facts Susceptible Of Being Verified By Observation.

Considering rocks according to the succession of eras, we find that primitive formations exhibit very few caverns.

The great cavities which are observed in the oldest granite, and which are called fours (ovens) in Switzerland and in the south of France, when they are lined with rock crystals, arise most frequently from the union of several contemporaneous veins of quartz,* (* Gleichzeitige Trummer. To these stone veins which appear to be of the same age as the rock, belong the veins of talc and asbestos in serpentine, and those of quartz traversing schist (Thonschiefer). Jameson on Contemporaneous Veins, in the Mem. of the Wernerian Soc.) of feldspar, or of fine-grained granite. The gneiss presents, though more seldom, the same phenomenon; and near Wunsiedel,* (* In Franconia, south-east of Luchsburg.) at the Fichtelgebirge, I had an opportunity of examining crystal fours of two or three feet diameter, in a part of the rock not traversed by veins. We are ignorant of the extent of the cavities which subterranean fires and volcanic agitations may have produced in the bowels of the earth in those primitive rocks, which, containing considerable quantities of amphibole, mica, garnet, magnetic iron-stone, and red schorl (titanite), appear to be anterior to granite. We find some fragments of these rocks among the matters ejected by volcanoes. The cavities can be considered only as partial and local phenomena; and their existence is scarcely any contradiction to the notions we have acquired from the experiments of Maskelyne and Cavendish on the mean density of the earth.

In the primitive mountains open to our researches, real grottoes, those which have some extent, belong only to calcareous formations, such as the carbonate or sulphate of lime. The solubility of these substances appears to have favoured the action of the subterranean waters for ages. The primitive limestone presents spacious caverns as well as transition limestone,* and that which is exclusively called secondary. (* In the primitive limestone are found the Kuetzel-loch, near Kaufungen in Silesia, and probably several caverns in the islands of the Archipelago. In the transition limestone we remark the caverns of Elbingerode, of Rubeland, and of Scharzfeld, in the Hartz; those of the Salzfluhe in the Grisons; and, according to Mr. Greenough, that of Torbay in Devonshire.) If these caverns be less frequent in the first, it is because this stone forms in general only layers subordinate to the mica-slate,* (* Sometimes to gneiss, as at the Simplon, between Dovredo and Crevola.) and not a particular system of mountains, into which the waters may filter, and circulate to great distances. The erosions occasioned by this element depend not only on its quantity, but also on the length of time during which it remains, the velocity it acquires by its fall, and the degree of solubility of the rock. I have observed in general, that the waters act more easily on the carbonates and the sulphates of lime of secondary mountains than on the transition limestones, which have a considerable mixture of silex and carbon. On examining the internal structure of the stalactites which line the walls of caverns, we find in them all the characters of a chemical precipitate.

As we approach those periods in which organic life develops itself in a greater number of forms, the phenomenon of grottoes becomes more frequent. There exist several under the name of baumen,* (* In the dialect of the German Swiss, Balmen. The Baumen of the Sentis, of the Mole, and of the Beatenberg, on the borders of the lake of Thun, belong to the Alpine limestone.) not in the ancient sandstone to which the great coal formation belongs, but in the Alpine limestone, and in the Jura limestone, which is often only the superior part of the Alpine formation. The Jura limestone* (* I may mention only the grottoes of Boudry, Motiers-Travers, and Valorbe, in the Jura; the grotto of Balme near Geneva; the caverns between Muggendorf and Gaylenreuth in Franconia; Sowia Jama, Ogrodzimiec, and Wlodowice, in Poland.) so abounds with caverns in both continents, that several geologists of the school of Freyberg have given it the name of cavern-limestone (hohlenkalkstein). It is this rock which so often interrupts the course of rivers, by engulfing them into its bosom. In this also is formed the famous Cueva del Guacharo, and the other grottoes of the valley of Caripe. The muriatiferous gypsum,* (* Gypsum of Bottendorf, schlottengyps.) whether it be found in layers in the Jura or Alpine limestone, or whether it separate these two formations, or lie between the Alpine limestone and argillaceous sandstone, also presents, on account of its great solubility, enormous cavities, sometimes communicating with each other at several leagues distance. After the limestone and gypseous formations, there would remain to be examined, among the secondary rocks, a third formation, that of the argillaceous sandstone, newer than the brine-spring formations; but this rock, composed of small grains of quartz cemented by clay, seldom contains caverns; and when it does, they are not extensive. Progressively narrowing towards their extremity, their walls are covered with a brown ochre.

We have just seen, that the form of grottoes depends partly on the nature of the rocks in which they are found; but this form, modified by exterior agents, often varies even in the same formation. The configuration of caverns, like the outline of mountains, the sinuosity of valleys, and so many other phenomena, present at first sight only irregularity and confusion. The appearance of order is resumed, when we can extend our observations over a vast space of ground, which has undergone violent, but periodical and uniform revolutions. From what I have seen in the mountains of Europe, and in the Cordilleras of America, caverns may be divided, according to their interior structure, into three classes. Some have the form of large clefts or crevices, like veins not filled with ore; such as the cavern of Rosenmuller, in Franconia, Elden-hole, in the peak of Derbyshire, and the Sumideros of Chamacasapa in Mexico.

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