"I have heard that one of these rivers, the Silarus, now called Sele, which is not far from Pæstum, was remarked by ancient writers for petrifying all the leaves that fell into it. Whether this is the case at present, I cannot tell you; but it appears that the most rapid motion which can be communicated to water does not prevent these formations. This is remarkably exemplified in the most magnificent waterfall in Europe, that of the Velino near Terni, (a town about forty miles to the north of Rome, which you may easily find in your new map.) The river Velino rises at some distance among the Apennines, and is at first an inconsiderable stream, flowing quietly along, but receiving some tributary waters, it becomes at length a mighty torrent, sweeping away whatever obstructs its progress. About three miles from Terni it

rushes impetuously over a ledge of rocks, down a precipice three hundred feet in height; and then, emerging from the cavity that first receives it, tumbles over craggy rocks, which are indistinctly seen through the misty spray continually rising round them. In the very bed of this torrent, even amidst the thundering descent and foam of the cataract, marble is deposited, and it hardens, or crystallizes,

1 Lempriere.

MARBLE OF TIVOLI.

91

as it is called, notwithstanding the ceaseless agitation of the water.1

"And where, through hanging woods,

Steep Anio pours his floods,'

4

similar formations may probably be found, as the waters of that river are also known to be calcareous. The cascades which it forms at Tivoli almost equal those of the Velino in picturesque beauty, though the principal fall is greatly inferior in height. In the adjacent plain there is another remarkable example of the deposition of marble, resembling, in some respects, that of Pæstum. It is of a warm cream-colour, and very porous; yet so hard and imperishable, that it has been employed in building the Colosæum and St. Peter's, the pride of ancient and of modern Rome. The quarries from which the materials of those great edifices were obtained, lie in the plain, at a short distance from Tivoli. The marble is deposited in horizontal beds, from which blocks of almost any size may be raised by wedges. The deposition seems to be continually going on; but the quarries, though very extensive, are worked only to the depth of about fifteen feet, because there is no contrivance for draining off the

1 Consol. of Trav. p. 121, &c.

2 Ib. and Edinburgh Gazetteer.

water, which springs up everywhere, impeding the

progress of the workmen."

"Ah! they want some of our English machinery for that purpose."

-The

"No doubt it would be of great use." conversation was suspended for a little while, and Harry occupied himself by trimming a stick which he had cut out of the hedge. He then returned to his usual station beside his mother, and accommodated his pace to hers. "Are you not tired, dear mamma?" said he. "I believe my arm is high enough now, to be of some use to you." Mrs. Beaufoy smiled, and accepted the offer: she then said, "I think I am under an engagement, Harry, to explain something I would not touch upon the other day, because you were not prepared for it; but now that I have said so much respecting the influence of carbonic acid upon calcareous rocks, I may add, that granite is not exempt from its power. A granite rock might defy the current of a large river, and, it has been said, the washing of the sea, for ages; perhaps it may be so, but if a small spring, strongly impregnated with carbonic acid, should break forth beside it, the rock may gradually decay, and be carried by the waters of a little brook to the ocean.

1 Life of Playfair, xlv.

GRANITE OF AUVERGNE.

93

There are large districts in Auvergne, one of the central provinces of southern France, where this fact has been observed to produce a striking effect on the appearance of the country. The decay which there takes place in the rocks, has been called

la maladie du granite;' and I have heard that if you take up a piece of the rock we are accustomed to think so imperishable, it will crumble to pieces in the hand. But though carbonic acid destroys its solidity in the way I have mentioned, it does not appear to have power to dissolve granite, as it does limestone-rocks, in which great caverns and long winding passages are formed by the action of water impregnated with this acid."

"But how comes carbonic acid to be so plentiful in Auvergne? Is it a volcanic country?"

"There are not at present any active volcanoes, but the streets of Clermont are said to be paved with lava brought from a neighbouring mountain, and many of the houses are built of the same material. The beautiful fountains of that town also deposit stony matter in great abundance; and I have heard that, by the gradual accumulation of such deposits, arches have been formed over some of the rivulets; one of them so broad and strong,

1 Lyell, 211 and 217.

1

that it forms a natural bridge, over which carriages may pass. This property of the waters, and the decay of the granite, which is ascribed to the continual escape of carbonic acid from numerous fissures, seem to indicate the presence of subterraneous fire, although there has not been any volcanic eruption in France within the memory of man."?

"But in our own country, where we have nothing to fear from the action of volcanoes, I suppose there is no danger of granite decaying. I like to think of those rocks as the Pillars of the Globe."

"I am afraid," said Mrs. Beaufoy, smiling, "that imposing phrase has had more than its due influence on your imagination. Ask Dr. Forbes what he thinks of granite.”

"But I may not see him, perhaps, for a good while; at least not when he is at leisure. Mamma,

if

you know what he thinks, I wish you would be so kind as to tell me."

66

I believe that before he went into Cornwall, he had some exalted notions of the adamantine and imperishable nature of granite; but his attention being directed to the structure of St. Michael's Mount, which is a very striking object from part of the Cornish coast, his early associations respecting

1 Edinburgh Gazetteer.

2 Ure, p. 375.