Fires have been often observed on the surface of the water. Pliny tells us that the lake Thrasimenia appeared inflamed over all its surface. Agricola relates that when a stone was thrown into the lake Denstat, in Thuringia, it appeared, as it descended in the water, like a train of fire.
In short, the quantities of pumice-stones which travellers affirm are met with in many parts of the ocean, and the Mediterranean, prove there are volcanos at the bottom of the sea, similar to those we are acquainted with, and which differ not in the least from them, neither by the matters they cast out, nor by the violence of the explosion, but solely by the rarity and shortness of the duration of their effects. From hence we may fairly infer that the bottom of the sea in every respect resembles the surface of the earth.
We shall find many connections between land and sea volcanos; both are found at the summit of mountains. The islands of Azores and those of the Archipelago are only peaks of mountains, some of which rise above the water, and others are underneath. By the account of the new islands among the Azores we see that the part from whence the smoke issued was only 15 fathoms under water, which, compared with the common depth of the ocean, proves that even this part is the summit of a mountain; as much may be said of the new island near Santorini, which could not be any great depth, since oysters were found attached to the rocks which rose above the water. It appears also that marine-volcanos have, like those of the land, subterraneous communications, since the summit of the volcano of St. George, in the island Peak, sunk at the time the new island among the Azores arose. It must also be observed, that these new islands never appear but near the old ones, and that we have no example of new islands in the high seas; we must therefore look on them as a continuation of the adjacent islands; and when ancient islands have volcanos, it is not astonishing that the ground adjacent should contain matters proper to form them, and which inflame, either by fermentation alone, or by the action of subterraneous winds.
Islands produced by the action of fire and earthquakes are but few, but there are an infinite number produced by the mud, sand, and earth, which the rivers or the sea transport into different places. At the mouth of rivers earth and sand accumulate in such quantities as to form islands of a moderate extent. The sea, retiring from certain coasts, leaves the highest parts of the bottom naked, which forms so many new islands; so likewise the sea, by extending itself on certain shores, covers the lowest parts, and leaves the highest, which appear as so many islands; and thus it is we may account for there being so few islands in the open sea, and so many bordering on the continents.
Water and fire, whose natures appear so different and so contrary, produce many similar effects, independent of the particular productions of these two elements, some of which bear so striking a resemblance as to be mistaken for each other, as glass and crystal, natural and fictitious antimony, &c. There are in nature an infinity of great effects produced by them, which are scarcely to be distinguished. Water, as has been observed, has produced mountains and formed most islands, while others owe their origin to fire. There are likewise caverns, clefts, holes, gulphs, &c. some of which owe their origin to subterraneous fires, and others to waters.
Caverns are met with in mountains, and few or none in plains: there are many in the Archipelago, and in other islands, because they are in general only the tops of mountains: caverns are formed like precipices, by the sinking of rocks, or large abysses, by the action of the fire; for to make a cavern form a precipice or abyss, we need only suppose the tops of adjacent rocks had fallen together and formed an arch, which must often happen when their bottoms are shaken and dislodged by time or earthquakes. Caverns may be produced by the same causes which produce holes, the shaking and sinking of the earth, and which causes are the explosion of volcanos, the action of subterraneous vapours and earthquakes; for they occasion caverns, holes, and hollows of every kind by their shocks and commotion.
St. Patrick's cavern in Ireland is not so considerable as it is famous; it is the same with the Dog's Grotto in Italy; and that which throws out fire, in the mountain of Beniguazeval in the kingdom of Fez. In the county of Derby, in England, there is a very considerable cavern, much larger than the famous cavern of Beauman, near the Black Forest, in Brunswick. I have been informed by a person as respectable for his merit as his name, Lord Morton, that this large cavern, called the Devil's Hole, at first presents a very considerable opening, larger than any church door; that through this opening a rivulet flows; that in advancing the vault of the cavern becomes so low, that persons who are desirous of continuing their road are obliged to lie flat in a boat and be pushed through this narrow passage, where the water almost touches the roof; but after having passed this part of the vault, the arch rises to a considerable height, and continues so for some distance, when it sinks again so low as to touch the water, and where the cavern ends. The source of the rivulet which issues from it sometimes encreases considerably: it transports and heaps up a great quantity of sand in one part of the cavern, which is formed like a kind of alley, whose direction is different from that of the principal cavern.
In Carniola, near Potpechio, is a very spacious cavern, in which is a large lake. Near Adelsperg is a cavern, in which we may travel two German miles, and where very deep precipices are to be met with.[AF] There are also large caverns and beautiful grottos under the mountains of Mendip, in Wales; mines of lead are found near these caverns, and whole oaks at fifteen fathoms deep. In the county of Gloucester there is a very large cavern, called Pen Park-hole, at the bottom of which there is thirty fathoms water, and mines of lead are also found.
[AF] See Act. erud. Lips. anno. 1689, page 558.
The Devil's Hole, and other caverns, from whence issue large springs or rivulets, have plainly therefore been formed by the water, and their origin cannot be considered as the effects either of earthquakes or volcanos.