tracts in Scandinavia and the basin of the Baltic, and probably continue eastwards under the great plains of northern Russia, for they crop up along the flanks of the Ural Mountains. In middle and southern Europe they rise again and again to the surface in mountain-regions, from which it may be inferred that they underlie vast areas in the surrounding low grounds. In North America Silurian rocks extend westwards from the mouth of the St Lawrence by the great lakes into the far northwest, and south-west by the Alleghanies into Alabama. As more or less isolated areas rise to the surface in the interior of the continent, it is probable that Silurian strata, concealed under younger formations, extend throughout the interior continental basin; and they reappear in the Rocky Mountains. The system has been recognised as entering into the formation of the Cordilleras of South America. Silurian strata have also been detected in the Himalayas and other parts of Asia, in Australia, and New Zealand.

The Silurian rocks which occur in mountainous areas are usually much indurated and dip at high angles, being frequently much contorted and dislocated. In the Highlands of Scotland and in Norway they have even been subjected to such disturbance that they have over wide areas acquired a more or less schistose or foliated character. In other low-lying regions, however, as in Russia, the strata are not indurated, and occur in horizontal or gently inclined positions.

Life of the Period. —Plant-remains consist almost exclusively of seaweeds, only a few traces of landplants having been met with. These indicate a cryptogamic flora-ferns and lycopodiaceous trees. The lower forms of animal life were represented chiefly by sponges (Amphispongia, Astylospongia, &c.). Amongst the Coelenterata were numerous forms of graptolites and corals. The former are eminently characteristic of the Silurian, and abound in certain thin bands of black shale. Those with two rows of cells are mostly confined to the Lower Silurian, while the single-rowed graptolites are upon the whole most common in the Upper Silurian. Corals abounded-certain limestones appearing to be almost wholly made up of their remains. Some common or characteristic rugose forms were Omphyma, Zaphrentis, Cyathophyllum, &c. Prominent tabulate forms were Halysites, Favosites, &c. The living Alcyonarian corals (Heliopora) were represented by Heliolites. Crinoids or sea-lilies were very numerous-their jointed stems and arms entering largely into the composition of many of the limestones. Cystideans, which attained their maximum development in Silurian times, star-fishes, and brittle-stars appear to have been less abundant. Annelid-tracks are common on the surfaces of beds, and the filled-up burrows of sea-worms frequently occur. Now and again jaws of annelids are likewise met with, and occasionally their tubular cases (Serpulites, Spirorbis) are seen attached to shells, corals, &c. Among the most characteristic Silurian fossils were the Trilobites, which made their first appearance in Cambrian seas and reached their greatest development in the Silurian age. They lived on in much diminished numbers through the Devonian and Carboniferous periods, and became finally extinct in Permian times. Some of the more widely distributed Silurian forms are Ogygia, Trinucleus, Asaphus, Illanus, Phacops, Calymene, &c. Other remarkable Arthropods were the Eurypterids (an extinct order allied to the existing kingcrabs). One of these (Pterygotus) was seven or eight feet in length. Ostracods (Beyrichia) and Phyllopods (Ceratiocaris) likewise occur in Silurian rocks. Here also we meet with the first scorpion (Palæophoneus) and the earliest insect (Palaoblattina, a form of cockroach). Various lace-corals

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(Fenestella) and other Polyzoa occur, but by far the most abundant forms amongst Molluscoids were the Brachiopods. Characteristic types are Discina, Orthis, Leptæna, Pentamerus, Rhynchonella, Strophomena. Lamellibranchs were much less numerous- -Modiolopsis, Ctenodonta, and Orthonota are examples. Amongst Gasteropods the more common genera are Euomphalus, Bellerophon, Holopella, Murchisonia, and Pleurotomaria. The Cephalopods are well represented by straight and coiled chambered cells: amongst the former is Orthoceras, many species of which are known; the latter show such types as Cyrtoceras, Lituites, Phragmoceras, Nautilus, &c. Vertebrates make their earliest known appearance in the Upper Silurian. The remains consist of bony bucklers or head-shields of ganoid fishes (Pteraspis, Cephalaspis), the defensive spines of some cestraciont, and fragments of shagreen-like skin and plates.

Physical Conditions.-The Silurian strata appear to have been deposited chiefly in shallow seas, which here and there, however, may have been moderately deep. No certain indications of true oceanic conditions have yet been met with. During the formation of the Lower Silurian strata the entire area of the British Islands, with the excep tion, perhaps, of some of the Archæan tracts of the north-west, appears to have been under water. At this time active volcanoes, forming groups of islets, were scattered over the area of what is now North and South Wales, the south-east of Ireland, and southern Ayrshire. Considerable earth-movements affected the British area at the close of the Lower Silurian period-the bed of the sea being here and there elevated, so that islands of considerable size came into existence at the beginning of the Upper Silurian period. Eventually, however, a movement of depression supervened, and the islands referred to were submerged and gradually buried under the sedimentary accumulations of the Upper Silurian sea. Judging from the geographical distribution of the Silurian strata, we are led to conclude that enormous areas of what are now our continents were during the accumulation of those rocks overflowed by shallow seas. The main land-masses of the period seem to have been grouped chiefly in boreal regions and were composed essentially of Archæan rocks. In Europe and North America alike the old land-surface lay towards the north, but the great continental ridges may have risen here and there to the surface in other places so as to form groups of islands, as in Bavaria and Bohemia in Europe, and in the region of the Colorado and Park ranges in America.

The temperature of the seas was such as to allow of the migration of closely allied and apparently identical species of molluscs, crustaceans, &c. over vast regions. In arctic lands fossils occur which are met with likewise in the Silurian rocks of Wales; more than this, many Silurian species were truly cosmopolitan, ranging from the extreme north across the equator to Australia. To have allowed of such world-wide distribution the temperature of the seas must have been singularly uniform. But while not a few Silurian species were cosmopolitan, many others appear to have had a more restricted range. Thus it may be inferred that, however uniform the climatic conditions may have been, the uniformity nevertheless was only comparative, and that even in Silurian times the oceanic areas had their distinct life-provinces. It is remarkable that in the Lower Silurian of Scotland large erratic blocks of gneiss and other crystalline rocks occur, and similar blocks are met with in the Silurian strata of the Lake Superior region. It is difficult to see how such blocks could have been transported without the agency of floating ice. See also the articles MURCHISON and GEOLOGY.

Siluridæ, or CAT-FISHES, a large family of fresh-water fishes, included in the order Physostomi. The skin is naked, or equipped with some bony scutes, never with scales; the dorsal fin is short and is occasionally absent-when present it is inserted above or in front of the ventrals; there is generally an adipose fin; the maxilla is very small; the barbels are well developed. The family is represented in most parts of the world. Europe, however, there is only one Siluroid, the Sheat-fish or Shadden (Silurus glanis), which occurs in some eastern and central regions-e.g. in the Danube and the Elbe. In size it sometimes

Silurus glanis.

In

approaches the sturgeon, and is a sluggish but very voracious animal, and has been the subject of many strange stories. In North America most of the common cat-fishes belong to the genus Amiurus; in tropical America the genus Pimelodus has many representatives (see CAT-FISH). In many species of Arius-a marine genus-common in the East Indies and on both coasts of Central America, the males hatch the eggs in their mouths. Species of Doras, common in South America, are remarkable for their habit of travelling from one pond to another. In northern Africa the most remarkable Siluroid is the Electric Cat-fish, Malapterurus electricus.

Silver (sym. Ag; equiv. 108; sp. gr. between 10:47 and 10:57). This metal was one of the earliest known, and is of a peculiar Copyright 1892 in U.S. and beautiful white colour, by by J. B. Lippincott which it can be distinguished

Company.

from all other metals or alloys, except one or two rare metals, such as lithium and indium, which are seldom seen. Silver is harder than gold, but softer than copper. It takes a very high polish, and for this reason was sometimes used for making small mirrors by the ancient Romans. It ranks next to gold in malleability and ductility, the thinnest silver-leaf produced by hammering being only Toogoo of an inch thick; and a wire of the metal can be drawn so fine that a length of 130 yards weighs only one grain. It has been usually stated that silver has a clear ringing sound when struck, but accurate observers, like Karsten and Percy, say that, on the contrary, a bar of the metal emits a dull sound on receiving a blow. Silver conducts heat and electricity better than any other metal, for which reason it is adopted as the standard represented by 100. Its melting-point has been variously stated, but appears to be about 1904° F. (1040° C.). Silver does not suffer even from long exposure to the atmosphere, except that it readily tarnishes when sulphuretted hydrogen or animal exhalations containing sulphur are present. Both nitric and sulphuric acid dissolve silver; and aqueous hydrochloric acid, as well as a solution of common salt, converts the surface of the metal into the chloride of silver. The metal is not attacked by caustic alkalies.

Oxides of Silver.-Three compounds of silver with oxygen are known. Argentic Oxide or Protoxide of Silver, AgO, is the best defined of the three. If to an aqueous solution of nitrate of silver lime-water or baryta-water be added, this oxide of

SILVER

silver is precipitated. The same brown precipitate is obtained if pure potash or soda be used as the precipitant. The protoxide of silver when mois tened absorbs carbonic acid from the air. It decomposes and loses its oxygen at 572° F. (300° C.), and ignition takes place when it is rubbed in a mortar with sulphide of arsenic or of antimony and other easily oxidisable substances. The other two oxides of silver are the Argentous Oxide or Suboxide, Ag40, and the Peroxide, Agz02.

Sulphide of Silver, Ag2S.-The strong affinity silver has for sulphur is seen from the readiness with which it blackens in an atmosphere containing sulphuretted hydrogen. A silver coin can be easily darkened by rubbing it with sulphur or by placing it in contact with vulcanised indiarubber, which contains sulphur. Sulphide of silver can be prepared by melting together silver clippings and sulphur in a covered crucible. It is also formed when sulphuretted hydrogen or a soluble alkaline sulphide is added to an aqueous solution of a salt of silver, the silver sulphide precipitating as a black powder. The tarnish on silver articles which from their nature cannot be easily rubbed with leather and rouge or brushed can be readily removed by an aqueous solution of cyanide of potassium; but as this salt is poisonous the article should be afterwards carefully washed. Silversmiths perfectly restore the original white colour to darkened silver objects by heating them in contact with carbonate of soda and nitre. Statuettes and other art objects in silver are frequently oxidised,' as it is called. This is really darkening their surface more or less by a film of sulphide produced by immersing them in a hot solution of sulphide of potassium. The prominent parts are then brightened by brushing or otherwise. The native compounds of silver and sulphur (ores) are noticed below.

The

Chloride of Silver, AgCl.-This salt is prepared by adding to an aqueous solution of the nitrate of silver either hydrochloric acid or chloride of sodium (common salt), when a thick, white, curdy precipitate of the chloride is thrown down. precipitate requires to be washed and dried in the dark, and it is then an anhydrous white powder. When fused and allowed to cool it becomes waxy and hornlike, in which state it is translucent or even transparent in thin plates. In this massive condition it is still sometimes called by the old name of horn silver or luna cornea, whether native or artificially prepared. Chloride of silver is very insoluble in water, so that an extremely small proportion of silver may be detected in water by the formation of chloride. It dissolves in ammoniawater and cyanide of potassium or sodium. When a solution of chloride of silver in ammonia-water is boiled a fulminating compound is deposited; but this can be avoided by evaporating the solution at a gentle heat, by which treatment scales of the chloride separate. As commonly prepared, chloride of silver blackens by exposure to daylight. Native chloride of silver is an important ore of the metal, and is referred to below. So also are the native compounds of silver with bromine and iodine.

Cyanide of Silver, AgCy, is obtained by precipitation as a white powder when hydrocyanic acid or cyanide of potassium is added to an aqueous solution of nitrate of silver. It is insoluble in water, but aqueous solutions of ferrocyanide of potassium, hyposulphite of soda, and ammonia and some of its salts dissolve it. It forms double salts with various metallic cyanides, one of these, the argentocyanide of potassium, being of great service in Electro-metallurgy (q.v.). Mr A. Wright, surgeon, Birmingham, first applied this salt to electroplat ing, his invention having been bought and patented by the Messrs Elkinton in 1840.