Page:EB1911 - Volume 27.djvu/381

Rh glassy scoria which, after consolidation, weather to a red soft rock known as palagonite; tuffs of this kind occur in Iceland and Sicily. In the Lipari Islands and Hungary there are acid (rhyolitic) tuffs, of pale grey or yellow colour, largely composed of lumps and fragments of pumice. Over a large portion of the sea bottom the beds of fine mud contain small, water-worn, rounded pebbles of very spongy volcanic glass; these have been fioated from the shore or cast out by submarine volcanoes, and may have travelled for hundreds of miles before sinking; it has been proved by experiment that some kinds of pumice will float on sea-water for more than a year. The deep sea-deposit known as the “red clay” is largely of volcanic origin and might be suitably described as a “submarine tuff-bed.”

For petrographical purposes tuffs are generally classified according to the nature of the volcanic rock of which they consist; this is the same as the accompanying lavas if any of these were emitted during an eruption, and if there is a change in the kind of lava which is poured out, the tuffs also indicate this equally clearly. Rhyolite tuffs contain pumiceous, glassy fragments and small scoriae with quartz, alkali felspar, biotite, &c. In Iceland, Lipari, Hungary, Nevada, New Zealand, recent tuffs of this kind occur. The broken pumice is clear and isotropic, and when the particles are very small they have often crescentic, sickle-shaped, or biconcave outlines, showing that they are produced by the shattering of a vesicular glass; this is sometimes described as ash-structure. In the ancient rocks of Wales, Charnwood, the Pentland Hills, &c., similar tuffs are known, but in all cases they are greatly changed by silicification (which has filled them with opal, chalcedony and quartz) and by devitrification. The frequent presence of rounded corroded quartz crystals, such as occur in rhyolitic lavas, helps to demonstrate their real nature. Trachyte tuffs contain little or no quartz but much orthoclase and oligoclase felspar with often biotite, augite and hornblende. In weathering they often change to soft red or yellow “clay-stones,” rich in kaolin with secondary quartz. Recent trachyte tuffs are found on the Rhine (at Siebengebirge), in Ischia, near Naples, Hungary, &c. Andesitic tuffs are exceedingly common. They occur along the whole chain of the Cordilleras and Andes, in the West Indies, New Zealand, Japan, &c. In the Lake district, North Wales, Lorne, the Pentland Hills, the Cheviots and many other districts of Britain, ancient rocks of exactly similar nature are abundant. In colour they are red or brown; their scoriae fragments are of all sizes from huge blocks down to minute granular dust. The cavities are filled up with many secondary minerals, such as calcite, chlorite, quartz, epidote, chalcedony: but in microscopic sections the nature of the original lava can nearly always be made out from the shapes and properties of the little crystals which occur in the decomposed glassy base. Even in the smallest details these ancient tuffs have a complete resemblance to the modern ash beds of Cotopaxi, Krakatoa and Mont Pelée. Basaltic tuffs are also of wide spread occurrence both in districts where volcanoes are now active and in lands where eruptions have long since ended. In the British Isles they are found in Skye, Mull, Antrim and other places, where there are Tertiary volcanic rocks; in Scotland, Derbyshire, Ireland among the carboniferous strata; and among the still older rocks of the lake district, southern uplands of Scotland and Wales. They are black, dark reen or red in colour; vary greatly in coarseness, some being full of round spongy bombs a foot or more in diameter, and, being often submarine, may contain shale, sandstone, grit and other sedimentary material, and are occasionally fossiliferous. Recent basaltic tuffs are found in Iceland, the Faeroes, Jan Mayen, Sicily, Vesuvius, Sandwich Islands, Samoa, &c. When weathered they are filled with calcite, chlorite, serpentine and, especially where the lavas contain nepheline or leucite, are often rich in zeolites, such as analcite, prehnite, natrolite, scolecite, chabazite, heulandite, &c. Ultra-basic tuffs are by no means frequent; their characteristic is the abundance of olivine or serpentine and the scarcity or absence of felspar. In this class the peridotite, breccias or kimberlites of the diamond-fields of South Africa may perhaps be placed (see ). The principal rock is a dark bluish green serpentine (blue-ground) which when thoroughly oxidized and weathered becomes a friable brown or yellow mass (the “yellow-ground”). Besides olivine and augite (chrome diopside) there occur crystals of hypersthene, brown mica, garnet (Cape ruby), magnetite, ilmenite and kyanite, together with crystalline blocks of garnet, augite and olivine (which some petrographers have called eclogites). Many lumps of shale are embedded in the breccia, and some have supposed that the diamonds are due to the ultra-basic magma dissolving carbon, which subsequently crystallized as the rock cooled down. Many of the crystals are broken, and as the rock fragments also are angular, rather than rounded, the kimberlite is more properly an ultra-basic breccia than a tuff.

In course of time other changes than weathering may overtake tuff deposits. Sometimes they are involved in folding and become sheared and cleaved. Many of the green slates of the lake district in Cumberland are fine cleaved ashes. In Charnwood forest also the tuffs are slaty and cleaved. The green colour is due to the large development of chlorite. Among the crystalline schists of many regions green beds or green schists occur, which consist of quartz, hornblende, chlorite or biotite, iron oxides, felspar, &c., and are probably recrystallized or metamorphosed tuffs. They often accompany masses of epidiorite and hornblende-schists which are the corresponding lavas and sills. Some chlorite-schists also are probably altered beds of volcanic tuff. The “Schalsteins” of Devon and Germany include many cleaved and partly recrystallized ash-beds, some of which still retain their fragmental structure though their lapilli are flattened and drawn out. Their steam cavities are usually filled with calcite, but sometimes with quartz. The more completely altered forms of these rocks are platy, green chloritic schists; in these, however, structures indicating their original volcanic nature only sparingly occur. These are intermediate stages between cleaved tuffs and crystalline schists.

Tuffs are not of much importance in an economic sense. The peperino, much used at Rome and Naples as a building stone, is a trachyte tuff. Puzzuolana also is a decomposed tuff, but of basic character, originally obtained near Naples and used as a cement, but this name is now applied to a number of substances not always of identical character. In the Eifel a trachytic, pumiceous tuff called trass (q.v.) has been extensively worked as a hydraulic mortar.  TUGELA (“Startling”), a river of south-east Africa, the largest in Natal. It drains, with its tributaries, an area of about 8000 sq. m. The river valley is some 190 m. in length, the river, which has an exceedingly sinuous course is fully 300 m. long. It rises, at an altitude of nearly 11,000 ft. in the Drakensberg mountains on the eastern face of the Mont aux Sources, down which it leaps in a nearly perpendicular fall of 1800 ft.

The river, which starts its race to the ocean with a north-east course, soon bends more directly east, and, with many windings north and south, maintains this general direction across the tableland of north Natal until its junction with the Buffalo river, when it turns south. On its northern bank in its upper course are the heights of Spion Kop and Vaal Kranz, and on its southern bank, 56 m. east in a direct line from its source, is the village of Colenso, all three places being the scene of ineffectual attempts (Dec. 1899–Feb. 1900) by the British troops under General Sir Redvers Buller to dislodge the Boers who blocked the road to Ladysmith. Below Colenso are more waterfalls, and above the river is Pieter's Hill, the storming of which by the British, on the 27th of February 1900 at length led to the relief of Ladysmith. Six miles lower down the Tugela receives the Klip, which rises in the Drakensberg near Van Reenen's Pass and fiows by Ladysmith. Another northern tributary is the Sunday's river, which rises in the Biggarsberg. From the south the river is increased by several affluents, the chief being the Mooi (Beautiful) river. The Tugela-Mooi confluence is 44 m. south-east of Colenso at the base of the Biggarsberg. Seven miles farther down the Tugela joins the Buffalo river, the united stream retaining, however, the name Tugela. The Buffalo has its origin in the Drakensberg near Majuba Hill and flows south with, also, a general trend to east. In its course, which is very winding, it receives numerous tributaries, one of them being the Ingogo, a small stream whose name recalls the fight on its banks on the 8th of February 1881, between British and Boers. The chief affluents are the Ingagani (from the south-west) and the Blood (from the north-east), the last-named so called after the defeat of the Zulu king Dingaan, on the 16th of December 1838, by the Boers under Andries Pretorius, when the river ran red with the blood of the Zulus. Eighteen miles in a direct line below the Blood confluence is Rorke's Drift, or ford across the river, and some 12 m. south-east of the drift is the hill of Isandhlwana, both places rendered famous in the Zulu War of 1878–79. The junction with the Tugela is 30 m. in a direct line, farther south, the Buffalo river in that distance passing through a wooded and hilly region.

Below the confidence of the two streams the Tugela flows south-east in a deep channel between lofty cliffs, or through wild, stone strewn valleys until it reaches the narrow coast belt. Its mouth is nearly closed by a sand bar, formed by the action of the ocean. The Tugela is thus useless for navigation. About 6 m. above the mouth are two forts, Pearson and Tenedos, built by the British in 1879, during the war with the Zulus, to guard the passage of the river. Generally fordable in the winter months, the Tugela is, after the heavy rains of summer, a deep and rapid river. It is crossed, some 5 m. above the forts, by a railway bridge—the longest bridge in South Africa. From the junction of the Blood river with the Buffalo, that stream and subsequently the Tugela form the boundary between Natal and Zululand.  TUGGURT, a town in the Wadi Ghir, Algerian Sahara, 127 m. S. of Biskra. Tuggurt, which has a population (1906) of 2073, was formerly surrounded by a moat, which the French filled up. The town is entered by two gates. Just within the northern gate is the market place, which contains the chief mosque. The surrounding oasis is very fertile. It has about 9000 inhabitants and contains about 200,000 date palms. From Tuggurt a road 75 m. long leads across the desert north-east to El Wad (q.v.).