Landslips caused by the falling in of cavern roofs are nowhere more common than in the cretaceous strata, which are more liable than others to be undermined by the action of running waters. In the vast chalk-range extending from Carinthia to the Morea, they occur of all sizes, from a diameter of a few fathoms to one of many thousand feet, and are not seldom of considerable depth. They are generally funnel-shaped, sometimes elongated; and the bottom of the larger ones is generally covered with villages, orchards, vineyards, or considerable tracts of arable land. In Dalmatia, Carinthia, Carniola, and Istria, where the country consists chiefly of arid plateaux or mountain-chains, exposed to the dry north-easterly winds, the cultivation of the soil is almost exclusively confined to these depressions or dollinas, which, as a further protection against the cutting blasts, are inclosed with walls of loose stones.
Besides the funnel-shaped landslips or dollinas, there are others with perpendicular sides like walls or shafts, which are called jamas or mouths. One of these (near Breschiak) descends to a depth of 384 feet. The hares seek a winter refuge in the dollinas, and the jamas, as the favourite resort of pigeons, are also called pigeon-holes or golubinas. Many a pedestrian has lost his life by falling into a jama, particularly in former times, when fewer precautions were taken to protect the stranger against these treacherous precipices.
In the Jura Mountains there are also whole rows of cauldron-shaped depressions; and in North Jutland, where the chalk formation is likewise very extensive, a recent landslip suddenly emptied the Norr Lake, which lost itself in subterranean channels.
Effects very similar to those of an ordinary landslip are sometimes produced by the bursting of a bog. On the western confines of England and Scotland, the Solway Moss occupies a flat area about seven miles in circumference. Its surface is covered with grass and rushes, presenting a dry crust and a fair appearance; but it shakes under the least pressure, the bottom being unsound and semi-fluid. The adventurous passenger therefore, who sometimes, in dry seasons, traverses this treacherous waste, must pick his way over the rushy tussocks as they appear before him, for here the soil is firmest. If his foot slip, or if he venture to move in any other part, it is possible he may sink never to rise again.
On December 16, 1772, this quagmire, having been filled, like a great sponge, with water, during heavy rains, swelled to an unusual height above the surrounding country, and then burst. The turfy covering seemed for a time to act like the skin of a bladder retaining the fluid within, till it forced a passage for itself, when a stream of black half-consolidated mud began at first to creep over the plain, resembling in the slow rate of its progress an ordinary lava-current. No lives were lost, but the deluge totally overwhelmed some cottages, and covered 400 acres with a mass of mud and vegetable matter, which in the lowest parts of the submerged area was at least fifteen feet deep.
It may easily be imagined that in Ireland, the classic land of bogs, such phenomena are not uncommon. In the peat of Donegal an ancient log-cabin was found, in 1833, at the depth of fourteen feet. The cabin was filled with peat, and was surrounded by other huts, which were not examined. Trunks and roots of trees, preserved in their natural position, lay around these huts. There can be little doubt that we have here one instance out of many in which villages have been overwhelmed by the bursting of a moss.
In many volcanic regions we find circular cauldron-shaped depressions in the earth’s surface, which might easily be mistaken for land-slips, but which have in reality been formed by explosive discharges of confined vapours. When vents or fissures are produced by a paroxysm of volcanic energy, we can easily understand how in some cases the pent-up gases, finding a sudden outlet through some weaker part of the surface, must act like a powder mine, and scattering the rocks that surrounded the orifice, leave a deep hollow behind as a memento of their fury. The depressions thus caused bear a great resemblance to real craters, from which they are, however, distinguished by the absence of a cone of scoriæ and from their never having ejected lava.
These curious crateriform hollows are very common in the Eifel, a volcanic region in Rhenish Prussia, where, probably owing to the clayey nature of the soil, they have become reservoirs of water, or Maare, as they are called by the natives. Most of them still have small lakes at their bottom, while others have been drained for the sake of cultivation, or by the spontaneous rupture or erosion of their banks. Some of them are of considerable dimensions, such as that of Meerfeld, the diameter of which falls very little short of a mile; or the Pulvermaar of Gillenfeld, remarkable for the extreme regularity of its magnificent oval basin.
Similar lakes or Maare occur in Auvergne, in Java, in the Canary Islands, in New Zealand, and in the volcanic districts of Italy. The beautiful lakes of Albano and Nemi, which have been so often sung by ancient and modern poets, belong to this class; but Fr. Hoffmann, a celebrated German geologist, ascribes the origin of the former to a landslip caused by the falling in of the roof of a vast subterranean cavern.
