the relations of its fossils it has been transferred to the Silurian. The Cambrian period has two epochs, the Acadian and the Potsdam. The Canadian period has the calciferous, the Quebec, and the Chazy epochs. The Trenton period embraces the Trenton, Utica, and Cincinnati epochs; the Niagara period, the Medina, Clinton, and Niagara epochs; while the Salina, lower Helderberg, and Oriskany periods have each one epoch, correspondingly named. The lower Silurian animal fossils are sponges, radiates, mollusks, and articulates; among the last are numerous trilobites, a species of which found near Braintree, Mass., in the Acadian formation, was 20 in. long. The calciferous and Quebec epochs of the Canadian period are remarkably rich in fossils and economic products, the latter including copper and silver ores. In Newfoundland the Quebec formation reaches a thickness of 6,600 ft., the upper half being sandstone and shales and the lower half mostly limestones. The Trenton period, abounding in fossils and economic products, among which is petroleum, has its formation along the Appalachians and over a large part of the Mississippi basin, including the galena limestone of Wisconsin and other states. Trenton limestone has been found in the arctic regions, upon King William's island, North Somerset, and Boothia. The Niagara formation in North America covers a large part of the interior of the continent, and the arctic and other parts of British America, and also contains petroleum. At Niagara falls 85 ft. of limestone rest on 80 ft. of shale, and near the falls the shale is covered with 165 ft. of limestone. The Salina period includes the rocks which yield the salt brines of central New York. Through the Mississippi basin the Salina formation is for the most part absent. This formation contains numerous beds of gypsum, which are not stratified like the other rocks, and have been formed by the action of sulphuric acid upon limestone, the sulphuric acid being derived fnom sulphur springs. The Oriskany period contains no land plants in New York, but at Gaspé, province of Quebec, a small species of lycopodium or ground pine has been found. The most common animal fossils are bivalve mollusks. In Maryland there are five species of crinoids, but in New York they are rare. The rocks of both the lower and upper Silurian are widely distributed over the globe, although the lower are the most extensive. The upper Silurian in Europe, besides invertebrate fossils, contains the vestiges of the earliest fishes, some of which are of the shark tribe; so that although the Devonian is the age of fishes, they really originated in the Silurian. It was formerly thought that the Silurian formation contained the earliest vestiges of organic life, but organic remains have recently been found in older formations. (See Geology, vol. vii., p. 694, and Palæontology, vol. xii., pp. 811, 813, 816.)
SILVER, one of the precious metals, distinguished by its whiteness, its brilliant lustre when polished, its malleability, and its indifference to atmospheric oxygen. It is one of the most widely distributed of metals. Since it occurs frequently in a native state (though never chemically pure, being invariably alloyed with gold or copper, and sometimes antimony, arsenic, bismuth, quicksilver, or iron), and is easily fusible, it naturally became known to mankind in the earliest ages. The alchemists called it Luna or Diana. The Greek name ἄργυρος is from ἀργός, white, and is the source of the Latin argentum. Silver is one of the first metals named in the Old Testament, being included among the enumerated riches of Abraham. At that period, as in later times, it was used as a medium of exchange and as a material in the arts. In Solomon's reign it is said to have been so abundant as to be nothing accounted of, and the king had made it to be as stones in Jerusalem. Among other ancient nations it was also abundant. Polybius says the tiles upon the roof of the temple at Ecbatana were of solid silver, and the beams and pillars of the temple were covered with plates of silver and gold. These metals were obtained from Nubia, Ethiopia, Attica, Epirus, and the distant countries of eastern Asia. The rich Spanish silver mines were developed at an early day, and furnished the main supply of the metal for Phœnicia, Carthage, and Rome. Pliny speaks of a mine opened by Hannibal, which supplied him with 300 lbs. of silver daily, and was worked by adits reaching a mile and a half into the mountain. This was at Guadalcanal, at the foot of the Sierra Morena, in the modern province of Seville. Pure silver, in its massive state, is the whitest of metals. It takes by burnishing a brilliant lustre, though inferior to that of its white alloys with copper. When granulated by falling molten into water, it acquires a rough but exceedingly beautiful surface. Reduced from the chloride in the humid way, it appears as a gray, spongy powder. It crystallizes in cubes and octahedrons when allowed to cool from the molten condition or precipitated from solution for instance, by copper or zinc. Sometimes it is precipitated black by the galvanic current or by zinc. In hardness and strength it is superior to gold and inferior to copper; a slight alloy of copper hardens and strengthens it. In malleability and ductility it is inferior to gold only. (See Metal.) Leaves less than 1100,000 of an inch thick can be obtained by beating, and wires may be drawn out of extreme tenuity. Its chemical symbol is Ag, its equivalent 108. According to G. Rose, the specific gravity of cast silver is 10.505, of pressed or hammered silver 10.566. Other authorities give for the former 10.474, and for the latter 10.510. Lengsdorf found the specific gravity of silver wire which had been repeatedly drawn to be 10.47 before heating and 10.43 afterward. The specific heat of silver is given by Regnault as 0.057. Its heat-conducting power is greater than that of any other metal, as is also its power of reflecting
