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2016年07月12日
Caves formed by the Sea and by Volcanic Action.—Caves in Arenaceous Rocks.—Caves in Calcareous Rocks of various ages.—Their Relation to Pot-holes, “Cirques,” and Ravines.—The Water-cave of Wookey Hole.—The Goatchurch Cave.—The Water-caves of Derbyshire.—Of Yorkshire.—The Ingleborough Cave.—The Rate of Deposit of Stalagmite.—The Descent into Helln Pot.—The Caves and Pots round Weathercote.—The Formation of Caves, Pot-holes, and Ravines.—Caverns not generally formed in line of Faults.—Of various Ages.—Their Filling-up.—The Cave of Caldy.—The Blackrock Cave.—Great quantity of Carbonate of Lime dissolved by Atmospheric Water.—The Circulation of Carbonate of Lime.—The Temperature of Caves.—Conclusion.

Caves formed by the Sea and by Volcanic Action.

In this chapter we shall treat of the origin of caves and of their place in physical geography. The most obvious agent in hollowing out caves is the sea. The set of the current, the tremendous force of the breakers, and the grinding of the shingle, inevitably discover the weak places in the cliff, and leave caves as the results of their work, modified in each case by the local conditions of the rock. Caves formed in this manner have certain characters which are easily recognized. Their floors24 are very rarely much out of the horizontal, their outlook is over the sea, and they very seldom penetrate far into the cliff. A general parallelism is also to be observed in a group in the same district, and their entrances are all in the same horizontal plane, or in a succession of horizontal and parallel planes. In some cases they are elevated above the present reach of the waves, and mark the line at which the sea formerly stood. From their generally inaccessible position sea-caves have very rarely been occupied by man, and the history of their formation is so obvious that it requires no further notice. Among them the famous Fingal’s Cave, off the north coast of Ireland, and that of Staffa, on the opposite shore of Scotland, hollowed out of columnar basalt, are perhaps the most remarkable in Europe.

In volcanic regions also there are caves formed by the passage of lava to the surface of the ground, or by the imprisoned steam and gases in the lava while it was in a molten state: but these are of comparatively little importance so far as relates to the general question of caves, from the very small areas which are occupied by active volcanoes in Europe. They have been observed in Vesuvius, Etna, Iceland, and Teneriffe.
Caves in Arenaceous Rocks.

Caves also occur sometimes in sandstones, in which case they are the result of the erosion of the lines of the joints by the passage of subaërial water, and if the joints happen to traverse a stratum less compacted than the rest, the weak point is discovered, and a hollow is formed extending laterally from the original fissure. The massive25 millstone grit of Derbyshire and Yorkshire present many examples of this, as for instance in Kinderscout in the former county. The rocks at Tunbridge Wells also show to what extent the joints in the Wealden sandstones may become open fissures, more or less connected with caves, on a small scale, by the mere mechanical action of water. M. Desnoyers gives instances of the same kind in the Tertiary sandstones of the Paris basin, which have furnished remains of rhinoceros, reindeer, hyæna, and bear. Caverns, however, in the sandstone are rarely of great extent, and may be passed over as being of small importance in comparison with those in the calcareous rocks.
Caves in Calcareous Rocks of various ages.

It has long been known that wherever the calcareous strata are sufficiently hard and compact to support a roof, caves are to be found in greater or less abundance. Those of Devonshire occur in the Devonian limestone; those of Somerset, Nottinghamshire, Yorkshire, Derbyshire, and Northumberland, as well as of Belgium and Westphalia, in that of the carboniferous age. In France also, those of Maine and Anjou, and most of those of the Pyrenees and in the department of Aude, are hollowed in carboniferous limestone, as well as the greater part of those in North America, in Virginia, and Kentucky. The cave of Kirkdale in Yorkshire, and most of those in Franconia and in Bavaria penetrate Jurassic limestones, which have received the name of Hohlenkalkstein from the abundance of caverns which they contain. They are developed on a large scale in the Swiss and French Jura, and in some cases afford passage to powerful streams,26 and in others are more or less filled with ice, thus constituting the singular “glacières” that have been so ably explored by the Rev. G. F. Browne.22

The compact Neocomian and Cretaceous limestones contain most of the caverns of Périgord, Quercy, and Angoumois, and some of those in Provence and Languedoc, those of Northern Italy, Sicily, Greece, Dalmatia, Carniola, and Turkey in Europe, of Asia Minor and Palestine.

The tertiary limestones, writes M. Desnoyers,23 offer sometimes, but very rarely, caves that have become celebrated for the bones which they contain, such as those of Lunel-Viel, near Montpelier, those of Pondres and Souvignargues, near Sommières (Gard), and of Saint Macaire (Gironde). The same may also be said of the calcaire grossier of the basin of Paris.

Certain rocks composed of gypsum also contain caverns of the same sort as those in the limestones. In Thuringia, for example, near Eisleben, they occur in the saliferous and gypseous strata of the zechstein, and are connected with large gulfs and cirques on the surface, which are sometimes filled with water. In the neighbourhood of Paris, and especially at Montmorency, they contain numerous bones of the extinct mammalia. M. Desnoyers points out their identity, in all essentials, with those in calcareous strata, and infers that they have been produced in the same way. Some of them may have been formed by the removal of the salt, which is very frequently interbedded with the gypsum, by the passage of water. In Cheshire the pumping of the27 brine from the saliferous and gypseous strata produces subterranean hollows, which sometimes fall in and eventually cause depressions on the surface, such as those which are now destroying the town of Northwich, and causing the neighbouring tidal estuary to extend over what was formerly meadow land. This explanation, however, will not apply to those in the neighbourhood of Paris, because there is no trace of their ever having contained salt.
The Relation of Caves to Pot-holes, “Cirques,” and Ravines laser 1064.

The caverns hollowed in calcareous rocks present features by which they are distinguished from any others. They open, for the most part, on the abrupt sides of valleys and ravines at various levels, being arranged round the main axis of erosion just as branches are arranged round the trunk of a tree—as, for example, in Cheddar Pass. The transition in some cases from the valley to the ravine, and from the ravine to the cave, is so gradual, that it is impossible to deny that all three are due to the same cause. The caves themselves ramify in the same irregular fashion as the valleys, and are to be viewed merely as the capillaries in the general valley system, through which the rainfall passes to join the main channels. Very frequently, however, the drainage has found an outlet at a lower level, and its ancient passage is left dry; but in all cases unmistakeable proof of the erosive action of water is to be seen in the sand, gravel, and clay which compose the floor, as well as in the worn surfaces of the sides and the bottom.

In all districts in which caves occur are funnel-shaped28 cavities of various sizes, known as “pot-holes” or “swallow-holes” in Britain, as “betoires,” “chaldrons du diable,” “marmites de géants,” in France, and as “kata-vothra” in Greece, in which the rainfall is collected before it finally disappears in the subterranean passages. They are to be seen in all stages; sometimes being mere shallow funnels, that only contain water after excessive rain, and at others as profound vertical shafts, into which the water is continually falling, as in Helln Pot, in Yorkshire. The cirques, also, described by M. Desnoyers, belong to the same class of cavities, although all those which are mentioned by the Rev. T. G. Bonney,24 at the head of valleys, and in some cases hollowed in shale and igneous rocks, are most probably to be referred to the vertical, chisel-like action of streams flowing under physical conditions, that resemble those under which the cañons of the Colorado, or of the Zambesi, are being excavated, and in which frost, ice, and snow have played a very subordinate part.

The intimate relation between pot-holes, caves, ravines, and valleys will be discussed in the rest of this chapter, and illustrated by English examples; and then we shall proceed to show that the chemical action of the carbonic acid in the rain-water, and the mechanical friction of the sand and gravel, set in motion by the water, by which Professor Phillips explains the origin of caves, will equally explain the pot-holes and ravines by which they are invariably accompanied.

29
The Water-Cave of Wookey Hole, near Wells, Somerset incorporate hong kong offshore company.

Caves may be divided into two classes: those which are now mere passages for water, in which the history of their formation may be studied, and those which are dry, and capable of affording shelter to man and the lower animals. Among the water-caves, that of Wookey Hole25 is to be noticed first, since its very name implies that it was known to the Celtic inhabitants of the south of England, and since it was among the first, if not the first, of those examined with any care in this country, Mr. John Beaumont26 having brought it before the notice of the Royal Society in the year 1680.

The hamlet of Wookey Hole nestles in a valley, through which flows the river Axe, and the valley passes insensibly, at its upper end, into a ravine, which is closed abruptly by a wall of rock (Fig. 1), about two hundred feet high, covered with long streamers and festoons of ivy, and affording scanty hold, on its ledges and in its fissures, to ferns, brambles, and ash saplings. At its base the river Axe issues, in full current, out of the cave, the lower entrance of which it completely blocks up, since the water has been kept back by a weir, for the use of a30 paper-mill a little distance away. A narrow path through the wood, on the north side of the ravine, leads to the only entrance now open.27 Thence a narrow passage leads downward into the rock, until, suddenly, you find yourself in a large chamber, at the water level. Then you pass over a ridge, covered with a delicate fretwork of dripstone, with each tiny hollow full of water, and ornamented with brilliant lime crystals. One shapeless mass of dripstone is known in local tradition as the Witch of Wookey, turned into stone by the prayers of a Glastonbury monk. Beyond this the chamber expands considerably, being some seventy or eighty feet high, and adorned with beautiful stalactites, far out of the reach of visitors. The water, which bars further entrance, forms a deep pool, which Mr. James Parker managed to cross on a raft (see Appendix I.) into another chamber, which was apparently easy of access before the construction of the weir Engineered wood hong kong. It was in this further chamber that Dr. Buckland found human remains and pottery.
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