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which seems to evince that they rose from beneath, having left an empty space in the direction of their ascent, into which the superincumbent bed subsided; for if a mere rupture had taken place, the descent of any substance from above would not have altered the original level of the beds. The eruptions of clay are frequent in American volcanoes, and may arise like sandstone, from the subterranean waters, which seem of far more extent and influence than is generally conceived. It ought also to be observed, if these arrects proceed in a northerly and southerly direction, or on any point of the compass from S. E. to N. W.; for such seems to be the common direction of chains of volcanoes, and of earthquakes; as perhaps in the desiccation of this globe, and the contraction at the poles, ruptures of different sizes took place in the shell, which were afterwards filled with subterranean waters, and combustible materials; while an exterior crust was gradually formed, with a distant resemblance of those on some morasses, considering the horrible chasms beneath. It is far from the intention of this work, a mere introduction to the science, to support any system; as it is of an eclectic nature, choosing the most authentic facts, and the most solid observations, from all the theories. If these ideas,

however, should appear to savour of volcanism, let it be considered that we are on dangerous ground; for we now approach the volcanic domain.

The decomposition and ruin of mountains forming one of the grandest features in the history of the earth, a few examples may be subjoined; which shall be introduced by some observations of the greatest of petralogists, upon this singular and important topic.

"Another fact, of which I discovered the Nature of decomposition. solution by examining these granites close and attentively, is that of those exfoliations which I had observed in the upper valley. It is a fact known by all mineralogists, that most rocks are softer in the interior of mountains than at their external part; and that in the air they acquire a considerable degree of hardness. It hence follows that the external part, or the edge of the vertical section of a large layer of granite, ought to harden. by contact with the air, whilst the interior of the same layer retains a certain degree of softness. And so long as the lower layers remain a little soft, the enormous weight of all those that rest upon them, must in time

Rapid.

compress them. But the external parts, hardened by contact with the air, are not susceptible of the same compression. They must then separate, and thus form the exfoliations which are observable.

"This explanation acquires the highest degree of probability, when we see some of these large plates still adhering, above and below, to the layers of which they were a part, and only separated in the middle, where they form a kind of convex arch on the external side; and the identity of the substance, as well as the parallel direction of their veins with those of rocks from which they are separated, demonstrate that they have formerly been united with them."*

sure,

The decomposition of these prodigious works of nature, the Alps, is far more rapid and incessant than might be supposed, increasing perhaps in proportion to their antiquity. The fol lowing grand and striking observation of Saus, will not fail to impress the reader with this singular truth: "I do not exaggerate when I say that we did not pass an hour, without seeing or hearing large masses of rock precipitate them. selves, with the sound of thunder, either from the sides of Mont Blanc, or the Aiguille Marbré, or from the crest on which we stood."+

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or Pleurs.

Of the ruin of mountains, one of the most Rain of Piura, ancient examples recorded is that which occasioned the melancholy fate of the town of Piura, by the Swiss called Pleurs, in the county of Chiavenna; a handsome and commercial town, which was overwhelmed by the fall of Mount Conto, in 1618; when the inhabitants, in num→ ber 2430, were crushed or buried alive under the ruins*. The manufacture of ollite, which yielded to the town a revenue of 60,000 ducats, is said by some to have led to this disaster; the quarries having been so improvidently conducted as to undermine the mountain. But other writers regard it as proceeding from those natural causes, which have occasioned the fall of other mountains, in Swisserland and other countries.

Burnet introduces his account of this melancholy event by some observations on pot-stone, or ollite, which are indeed materially connected with the subject.

"There is a sort of pots of stone, that is used not only in all the kitchens here, but almost all Lombardy over, called Lavege; the stone feels oily and scaly, so that a scale sticks to one's finger that touches it, and is somewhat of the

Bourrit, Glaciers, iii. 120.

Ollite.

nature of a slate: there are but three mines of it known in these parts, one near Chavennes*, another in the Valteline, and the third in the Grisons; but the first is much the best. They generally cut it in the mine round, of about a foot and a half diameter, and about a foot and a quarter thick; and they work it in a mill, where the chisels that cut the stone are driven about by a wheel that is set a going by water, and which is so ordered, that he who manages the chisel, very easily draws forward the wheel out of the course of the water. They turn off first the

outward coat of this stone, till it is exactly smooth, and then they separate one pot after another by those small and hooked chisels; by which they make a nest of pots, all one within another; the outward and biggest being as big as an ordinary beef-pot, and the inward pot being no bigger than a small pipkin: these they arm with hooks and circles of brass, and so they are served by them in their kitchens. One of these stone-pots takes heat, and boils, sooner than any pot of metal; and whereas the bottoms of metal-pots transmit the heat so entirely to the liquor within, that they are not insufferably hot, the bottom of this stone-pot, which is about

* Chiavenna.

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