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Geological Notes. By Professor SILLIMAN, Junr.

Professor Silliman, junr., during his late visit to Europe, examined many interesting geological localities, of which he gives short notices in his Journal. Some of these we now lay before our readers.

1. Grotto del Cane and Lake Agnano.

The Grotto del Cane, or dog grotto, has been so much cited for its stratum of carbonic acid gas covering the floor, that all geological travellers who visit Naples feel an interest in seeing it. Unfortunately, like some other grottos, its enchantment disappears on a near view. It is a little hole dug artificially into the foot of a hill facing Lake Agnano. The aperture is closed by a door, and the space within is barely sufficient for one person to stand erect. Into this narrow cell a poor little dog is very unwillingly dragged and placed in a depression of the floor, where he is soon narcotized by the carbonic acid. The earth is warm to the hand, and the volume of gas given out is very constant.

Lake Agnano, as is well known, fills the bottom of an ancient crater whose walls are still to be distinctly traced. On the shore of the lake immediately opposite the Grotto del Cane, there is a constant and copious discharge of carbonic acid, and the water of the lake at that part is in great agitation from its escape. Nor is other evidence wanting of the present operation of the ancient causes which have characterised this region. At the foot of the hill, on the east, are numerous vents of steam and sulphur. Over some of these vapour baths (Sudatoriæ) have been established, which are held in some repute in cases of rheumatism. Digging a short distance into the hill, near the ruins of the villa of Lucullus, it is easy to perceive the presence of volcanic energy in the hot vapours, accompanied with sulphur, which issue copiously from the surface. The villa of Lucullus was entirely buried in lapilli and volcanic ashes, proceeding, as is supposed, from a vent in the immediate neighbourhood; but from which vent does not appear-evidently not from the one

now containing Lake Agnano, since that has not been active in the historic period.

In another cavity excavated in the hill, near to the Grotto del Cane, is an abundant flow of carbonic acid, accompanied, as is said, by ammonia; but we did not perceive any odour of this gas, nor any deposit of carbonate of ammonia or of sal ammoniac on the walls of the place.

2. Sulphur Lake of the Campagna, near Tivoli.

The celebrated sulphur lake of the Campagna, near Tivoli, the Aquæ Albulæ of the ancients, still retains its interest to the geologist, as the most remarkable of all the thermal sources of Italy. Whether we regard the constancy of the flow from it, both of water and of gas, the area of the lake, or the important part which it and similar craters appear to have played in the formation of the travertine of the Campagna-we must admit that it is worthy of attentive consideration.

In the days of Father Kircher, this lake was described as being over one mile in circuit. Now, the lake is not over 500 or 600 feet in diameter, and its area is constantly diminishing. The baths of Agrippa, which are now at a short distance from the shore, shew the extent to which this contraction has taken place in that direction, since they were erected in the waters of the lake, which were much esteemed by the ancient Romans for their hygienic effects. These baths were frequented by Augustus, and enlarged by Zenobia; the latter spent her Roman servitude in Tivoli; and in allusion to this, they are still called "Bagni della Regina." At present, the waters are conducted from this lake by a canal artificially cut in the travertine by Cardinal Ippolito d'Este, former Governor of Tivoli, for a distance of two miles. This canal is nine feet wide and four feet deep, and there is a constant discharge of the surplus waters through it in a very rapid current, the volume of which is remarkable when we notice the small size of the source.

The strong odour of sulphuretted hydrogen announces the existence of lake a long time before the traveller reaches vas examined by Sir Humphrey Davy, who

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found it to contain one volume of carbonic acid, and less than a half volume of sulphuretted hydrogen. Tracing the canal from where it escapes, the "Via Appia Nova," up to the lake, a mile and a quarter, the observer is much struck with the copious escape of large volumes of gas from the whole surface of the lake, which is such as to convey the idea of ebullition. A stone cast into the water is soon followed by a very violent escape of gas, causing the whole adjoining area to boil with great energy, and a loud sound of effervescence. The water is very clear, a slight milky, or opaline appearance it has, being due, as we thought, to the reflection of the white sides and bottom of the lake. Its temperature is 80° F., and it has, when drunk from a glass, an agreeable pungent taste of free carbonic acid, with no more of sulphuretted hydrogen than is pleasant. It is perfectly clear in the glass, and effervesces briskly. The escape of the gas does not appear to be confined to any particular part of the bed of the lake, but takes place equally over all its surface. The grass and reeds which grow luxuriantly at its margin, are constantly encrusted with carbonate of lime from the water, and it is from this cause that the area of the lake appears to have been contracted, and not from any partial drainage, which is evidently inexplicable, since the level of the lake is now only a few inches below that of the adjoining and level Campagna. Breaking off a mass and clods from the shore, it was found that the roots of the plants were also encased; while the stone walls of the fields adjoining were wholly composed of loose blocks of travertine, made up entirely of casts of the stems of growing grapes and aquatic plants. It required but casual observation of the travertine in other and distant parts of the Campagna, to detect the same species of plants imbedded in every part of its mass. Blocks of travertine in the Colosseum at Rome and other ancient buildings, also shewed us the same evidence. A large space on the Campagna, near the sulphur lakes, lately fell in, owing to the cavernous and unsupported nature of the rock; and it was easy to see, in the freshly fractured portions of the rock, the same stems of aquatic plants, such as now grow on the margin of the water.

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