Page:EB1911 - Volume 08.djvu/415

 dolomites, particularly those of the north of England, show a very remarkable concretionary structure. The beds look as if made up of rounded balls of all sizes from a foot or two in diameter downwards. Often they are stuck together like piles of shot or bunches of grapes. They are composed of fibrous radiate calcite crystals, which by some kind of concretionary action have segregated from the dolomitic material and grouped themselves together in this way. Other concretions from these beds resemble bunches of corals, tufts of plants, or present various strange imitative forms.

Dolomite, unlike calcite, is not secreted by marine animals to build up the hard parts of their skeletons, and it is generally agreed also that dolomite is only very rarely and under exceptional conditions deposited directly from solution in water. On the other hand, there is much evidence to show that limestones may absorb or be partly replaced by magnesium carbonate, and the double salt dolomite substituted for calcite by one of those processes which are described as “metasomatic.” Thus the Carboniferous limestones of various parts of Britain pass into dolomites along lines of joint, fissure or fault, or occasionally along certain bedding planes. At the same time the rock becomes crystalline, its minute structure is altered, its fossils are effaced, and as dolomite has a higher specific gravity than limestone, contraction results and cavities are formed. The prevalence of crystalline, concretionary and drusy structures in dolomite can thus be simply explained. The process may actually be studied in many “magnesian limestones,” in which by means of the microscope we may trace the gradual growth of dolomite crystals taking place simultaneously with the destruction of the original features of the limestone. Recent investigations in coral reefs show that these changes are going on at the present day at no considerable depths and in rocks which have not long consolidated.

All this goes to prove that the double carbonate of calcium and magnesium is under certain conditions a more stable salt than either of the simple carbonates, and that these conditions recur in nature with considerable frequency. Experiments have proved that at moderately high temperatures (100° to 200° C.) solutions of magnesium salts will convert calcite into dolomite in the laboratory, and that aragonite is even more readily affected than calcite. The analogy with dolomitization of limestones is strong but not complete, as the latter process must take place at ordinary temperatures and approximately under atmospheric pressures. No completely satisfactory explanation of the change, from the standpoint of the geologist, has as yet been advanced, though much light has been thrown upon the problem. Many limestones are rich in aragonite, but this in course of time tends to recrystallize as calcite. Magnesium salts are abundant in sea-water, and in the waters of evaporating enclosed coral lagoons and of many bitter lakes. Calcite is more soluble than dolomite in water saturated with carbonic acid and would tend to be slowly removed from a limestone, while the dolomite increased in relative proportion. Dolomite also being denser than calcite may be supposed to replace it more readily when pressure is increased. These and many other factors probably co-operate to effect the transmutation of limestones into dolomites.

Examples of dolomitization may be obtained in practically every geological formation in which limestones occur. The oldest rocks are most generally affected, e.g. the Cambrian limestones of Scotland, but the change occurs, as has already been stated, even in the upraised coral reefs of the Indian and Pacific oceans which are very recent formations. It is very interesting to note that dolomites are very frequent among rocks which indicate that desert or salt-lake conditions prevailed at the time of their deposit. The dolomite or magnesian limestone of the English Permian is an instance of this. The explanation may be found in the fact that the waters of bitter lakes are usually rich in magnesium salts which, percolating through beds of limestone, would convert them into dolomite. Among the most famous dolomites are those of the Dolomite Alps of Tirol. They are of Triassic age and yield remarkably picturesque mountain scenery; it is believed that some were originally coral reefs; they are now highly crystalline and often contain interesting minerals and ores. The galena limestone of the North American Trenton rocks is mostly a dolomite.

Dolomites furnish excellent building stones, and those of the north-east of England (Mansfield stone, &c.) have long been regarded with great favour on account of their resistance to decomposition. They vary a good deal in quality, and have not all proved equally satisfactory in practice. Part of the Houses of Parliament at Westminster is built of dolomite.

 DOLOMITES, THE, a mountain district in the South Tirolese Alps, though sometimes it is erroneously considered to form part of some other chain than the Alps. The distinguishing feature of this district is that it is composed of magnesian limestone, which rises in peaks of a most singular degree of sharpness and streaked by veins of the most startling colours. Nowadays it has become well known to tourists, who, however, keep mainly to a few great centres, though most of the more striking peaks were first ascended in the late sixties and early seventies of the 19th century by English mountaineers. Roughly speaking the Dolomite region lies between the Brenner railway from Franzensfeste to Trent (W.) and the road over the Monte Croce Pass from Innichen in the Drave valley by way of the Sexten glen and the Piave valley to Belluno and Feltre (E.). On the north it is limited by the railway line from Innichen to Franzensfeste, and on the south by the railway and road from Trent to Feltre. The highest summit is the Marmolata (10,972 ft.), but far more typical are the Sorapiss, the Cimon della Pala, the Langkofel, the Pelmo, the Drei Zinnen, the Sass Maor and the Rosengarten (see ). Among the chief tourist resorts are St Ulrich (in the Gröden valley), San Martino di Castrozza (near Primiero), Caprile and Cortina d’Ampezzo.

Besides the Dolomites included in the above region there are several other Dolomite groups (though less extensive) in the Alps. N.W. of Trent rises the Tosa group, while in Switzerland there are the Piz d’Aela group, S.W. of Bergün on the Albula Pass route, and the curious little group N. of the village of Splügen, besides other isolated peaks between the St Gotthard and Lukmanier Passes. In Dauphiné itself (the home of the geologist Dolomieu) the mountain districts of the Royannais, of the Vercors, and of the Dévoluy (all S.W. of Grenoble) are more or less Dolomitic in character.

 DOLPHIN, a name properly belonging to the common cetacean mammal known as Delphinus delphis, but also applied to a number of more or less nearly allied species. The dolphins, bottle-noses, or, as they are more commonly called, “porpoises,” are found in abundance in all seas, while some species are inhabitants of large rivers, as the Amazon. They are among the smaller members of the cetacean order, none exceeding 10 ft. in length. Their food is chiefly fish, for the capture of which their long narrow beaks, armed with numerous sharp-pointed teeth, are well adapted, but some also devour crustaceans and molluscs. They are mostly gregarious, and the agility and grace of their movements in the water are themes of admiration to the spectators when a “school of porpoises” is playing round the bows of a vessel at sea.  The Common Dolphin (Delphinus delphis).

The type of the group is the common dolphin (D. delphis) of the Mediterranean and Atlantic, which usually measures 6 to 8 ft. in length, and is thickest near the centre, where the back fin rises to