Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/199

Rh J MADMAD 181 All the islands of the group arc of volcanic origin, ami recent soundings show that they are the summits of very lofty mountains which have their bases in an abyssal ocean. The greater part of what is now visible in Madeira is of subaerial formation, consisting of an accumulation of basaltic and trachytic lavas, beds of tuff and other ejectamenta, the result of a long and complicated series of eruptions from innumerable vents. Besides this operation of building up by the emission of matter from craters and clefts there is evidence that a certain amount of upheaval in mass has taken place, for at a spot about 1200 feet above the sea in the northern valley of St Vicente, and again at about the same height on Pico Juliana in Porto Santo, there have been found fragments of lime stone accompanied by tuffs containing marine shells and echiuoderms of the Miocene Tertiary epoch. We have here proof that during or since that epoch portions at least these islands have been bodily uplifted more than 1000 feet. The fossils are sufficiently well pre served to admit of their genera and in many instances even their species being made out. That there were pauses of considerable duration whilst the island of Madeira was being increased in height is proved by several facts. The leaf bed and the accompanying carbonaceous matter, frequently termed lignite, although it displays no trace of structure, which lie under 1200 feet of lavas in the valley of St Jorge, afford proof that there had been sufficient time for the growth of a vegetation of high order, many of the leaf impressions having been identified as belonging to species of trees and shrubs which still exist on the island. It is evident, moreover, that great alterations and disloca tions had taken place in the rocks of various localities before other lavas and t nil s had been thrown upon them. There are no data for determining when volcanic action commenced in this locality, but looking at the enormous depth of the surround ing sea it is clear that a vast period of time must have elapsed to allow of a great mountain reaching the surface and then rising several thousand feet into the air. Again, considering the com paratively feeble agents for effecting the work of denudation (neither glaciers nor thick accumulations of alpine snow being found here), and then the enormous erosion that has actually taken place, the inference is inevitable that a very great lapse of time was required to excavate the deep and wide ravines that everywhere intersect the island. Nor is anything known as to the period of the cessation of volcanic action. At the present day there are no live craters, or smoking crevices, as at the Canaries and Cape Vc-rds, nor any hot springs, as at the Azores. On the slopes which descend from the central ridge to the sea, especially in the ncigbourhood of Funchal, there are many hills with conical shapes of more or less regularity, which seem to have been formed at a comparatively modern epoch. Volcanic cinders and slag are lying upon several of them, which look as if they had been thrown out of a furnace yesterday. Yet round the base of others there may be traced streams of lava flowing from a higher source, and showing that, subsequent to the con struction of these lateral cones, modern as they look, molten matter issued from higher vents, which assumed, on cooling, the character of ordinary compact basalt. If we examine the general configuration of Madeira, we shall see a mountain chain, about 30 miles in length, running east and west, and throwing off lateral ridges, that give it an extreme breadth of about 12 miles. Peaks rise about the middle to a height of more than 6030 feet ; and deep ravines, lying between the lateral ridges, strike for the most part north and south from the central ridge to the sea. In the sections afforded by the ravines, the nucleus of the island is seen to consist of a confused mass of more or less stratified rock, upon which rest beds of tuft, scoriae, and lava, in the shape of basalt, trap, and trachyte, the whole traversed by dykes. These beds are thinnest near the central axis ; as they approach the coast they become thicker and less intersected by dykes. At the centre of the island there are several summits of nearly the same altitude, and these are in some places connected by narrow walls and ridges, which are frequently quite impassable, whilst at others they are separated by ravines of great depth. On all sides are seen vertical dykes, projecting like turrets above the weathered surface of the softer beds. In various parts of the island may be seen elevated tracts of com paratively level ground. These are supposed to have been formed by the meeting of numerous streams of lava flowing from cones and points of eruption in close proximity, various ejectamenta assisting at the same time to fill up inequalities. Deep down in some of the lateral ravines may be seen ancient cones of eruption which have been overwhelmed by streams of melted matter issuing from the central region, and afterwards exposed to view by the same causes that excavated the ravines. These ravines may be regarded as^ having been formed at first by subterranean movements, both gradual and violent, which dislocated the rocks, and cut clefts through which streams flowed to the sea. In course of time the waters, periodically swollen by melted snows and the copious rains of winter, would cut deeper and deeper into the heart of the mountains, and would undermine the lateral cliffs, until the valleys became as large as we now find them. Even the Curral, which, from its rounded shape and its position in the centre of the island, has been usually deemed the ruins of a crater, is thought to be nothing more than a valley scooped out in the way described. The rarity of crateriform cavities in Madeira is very remarkable. Then; exists, however, to the east of Funchal, on a tract 2000 feet high, the Lagoa, a small but perfect crater, 500 feet in diameter, and with a depth of 150 feet; and there is another, which is a double one, in the district known as Fanal, in the north-west of Madeira, nearly 5000 feet above the sea. The basalt of which much of the outer part of the island is composed is of a dark colour and a tough texture, with small disseminated crystals of olivine and augite. It is sometimes full of vesicular cavities, formed by the expansion of imprisoned gases. A rudely columnar structure is very often seen in the basalt, but there is nothing so perfect as the columns of Staffa or the Giant s Causeway. The trachytic rocks are small in quantity compared with those of the basaltic class. The tufa is soft and friable, and generally of a yellow colour ; but where it has been overflowed by a hot stream of lava it has assumed a red colour. Black ashes and fragments of pumice are sometimes found in the tufaceous strata. The mineral contents of the rocks of Madeira are unimportant. There are no metallic ores, nor has any sulphur been found ; but a little iron pyrites and specular iron are occasionally met with. The basalt yields an excellent building-stone, various qualities of which are quarried near Cama dos Lobos, 5 or 6 miles west of Fun chal. At Porto Santo the trachytic rocks bear a much greater proportion to the basaltic than in Madeira. An adjacent islet is formed of tuffs and calcareous rock, indicating a submarine origin, upon which supramarine lavas have been poured. The older series contains corals and shells (also of the Miocene Tertiary epoch), with water-worn pebbles, cemented together by carbonate of lime, the whole appearing to have* been a coral reef near an ancient beach. The calcareous rock is taken in large quantities to Funchal, to be burnt into lime for building purposes. PORTO SANTO. This forms a single concelho and parish, about 25 geographical miles north-east of Madeira. It has a length of 6?;- geographical miles and a width of 3. A stationary population of about 1750 persons inhabits 435 houses, chiefly collected at one spot known as the Villa, where a lieutenant-governor resides. Thu island is very unproductive, water being scarce and wood wholly absent. Around the little town there is a considerable tract of pretty level ground covered by calcareous sand containing fossil land shells. At each end of the island there are hills, of which Pico do Facho, the highest, reaches the altitude of 1600 feet. Barley, but little else, is grown here, the limited requirements of the in habitants being supplied from Funchal by means of small sailing vessels. THE DESERTAS. These are three uninhabited rocks lying ?.bout 11 miles south-east of Madeira. They are not easily accessible, as they present lofty precipices to the sea on all sides. Rabbits anil goats abound on them. The archil weed grows on the rocks, and is gathered for exportation. The largest islet is 6^ miles long, and attains the height of 2000 feet. These rocks are conspicuous objects in the sea- views from Funchal. (J. Y. J. ) MADISON, a city of the United States, the county seat of Jefferson county, Indiana, is situated on the north bank of the Ohio, 90 miles below Cincinnati, and 44 above Louisville, with which it has daily steamboat communica tion. As the terminus of one of the divisions of the Jeffersonville, Madison, and Indianapolis Eailroad, Madison commands extensive means of traffic ; and its provision trade especially has attained important dimensions. Pork- packing is also carried on, and brass and iron foundries, tanneries, and flour-mills appear among the industrial establishments. The population was 8012 in 1850, 8130 in 1860, 10,709 in 1870, and 8945 in 1880. MADISON, a city of the United States, the capital of Wisconsin, and seat of justice of Dane county, lies towards the south of the State, in 43 4 N. lat. and 89 21 W. long., 75 miles west of Milwaukee. In the beauty of its situation it lias few rivals, occupying as it does the undulating isthmus between Mendota and Menona, two of the lakes which give name to the Four Lake Region, con nected with the Mississippi by Yahara or Catfish river and Rock river ; and the cool summer climate, which it owes to the fact that it stands 788 feet above the level of the sea, and 210 feet above Lake Michigan, renders it a health resort of some value, especially for consumptive patients. The State capitol, situated in the midst of a finely wooded