Such examples would be more frequently met with, if there was anything in the external coat to tell us in what direction to slit the stone : this same specimen might have been cut in many directions without throwing any light upon its mode of formation, and the section we now see was an accidental cut in the right direction. An attentive consideration of the products of volcanoes may lead to some satisfactory conclusion ; for although agates have not been found in lavas, cavities in them are often partially or entirely filled with minerals distinct from any in the rest of the rock.
Agates are often found as loose pebbles in the beds of rivers, or in gravel, but in these cases they have been derived from the disinte gration of Awygdaloids, the base of which is very often subject to decomposition when exposed to air and moisture, and then the silicious nodules fall out. They vary in size from that of millet seed to a foot in diameter ; but one, two, and three inches in diameter are the most common.
The stones distinguished by mineralogists and lapidaries by the names of Carnelian, Calcedouy, Onyx, Sardonyx, Mocha Stone, Blood stone, Chrysopraec, and Plasma, are so closely allied to agate, that they may be conveniently described under this head. In chemical composition they are not distinguishable, except in the case of the chrysoprase by its colouring matter.—Carnelian, so called became some kinds are of a flesh colour (carnis, Latin for flesh), is that variety of a uniform colour which is of moat common occurrence ; carnelhans are never figured or striped. The colours are shades of red and yellow, the deep char red being the rarest and most valuable. The great supply of carueliena is from Japan, where they exist iu vast quantities, and they are also imported from Bombay, being collected in the province of Gurernt; but the beat, according to Niebuhr, cones from the Gulf of Cambay. Many of the antique gems are engraved In carnelian, and it is now much used for seals.—Cakedong, so called from having been early found at Calebedon (sometimes incorrectly written Chalcedon) in Ilithynia, opposite Constantinople, is also of a uniform colour, generally of a milky white or pale yellow, like turbid jelly, often with an internal wavy structure in the form of stalactites, and very generally with a peculiar mammillary surface. It is found in great abundance in the Faroe Islands, in Iceland, in Cornwall, and many places of Great Britain, as well as other countries; sometimes' in lary masses, from which cups and other vessels are formed. Pliny describes it as being found in the neighbourhood of Thebes in Egypt and as brought to Home from Carthage.—Chat is a kind of granular calcedony, and forms a transition to the rock called Hornstone.— Onyx. In this agate the silicious particles are arranged in alternating horizontal layers of opaque white and translucent blue, gray, or brown ; and because these have a resemblance to the marks on the human nail, the stone was called from the Greek word for nail, Ge'v . It was
known to the ancients, and was employed by them, as it is now, for those beautiful gems called cameos, the figure being cut out of the opaque white, the dark part forming the ground, or the contrary. It is most valuable when the contrast of colours is strong, and when the layer is thick enough to give a high relief to the object to be engraved. In the royal library at Paris, there is an antique cameo cut out of an onyx with four layers, representing the apotheosis of Augustus, eleven inches by nine, which is supposed to be the finest in existence. Agates with an onyx structure are not uncommon, particularly among calce (Ionia', but the forest are brought from India. Cameos are sold at Rome which are made from a thick shell, having different coloured layers like an onyx.—Sardonyx is a variety of onyx which is supposed by some to have received ita name from having been brought from Saralee, in Lydia. By others it has been said that the name conies from Sardo, the Greek name of Sardinia, there being some reason for thinking that the Carthaginians brought the stones from that island, and exported them during their occupation of it In this the opaque white alternates with a rich deep orange brown of considerable trans lucency, and as this is of rare occurrence the sardonyx is of greater value. The finest are brought from the east, and some antique gems are formed of them.--Mocha-Stones and Moss-Agates are semitransparent caleedony, including various ramified forma, produced by iron, mnuga nose, bitumen, and chlorite or green earth, but sometimes also, as has been proved by Daubenton and Mac Culloch, produced by the presence of real vegetable bodies, such as Conferm and mosses. The first are found in Ouzerst, but received their name from having been brought from Mocha, in Arabia—Rood-80,1re, or Heliotrope, is a green agate coloured by chlorite, with numerous bright red spots like drops of blood. It is also called oriental jasper.—Cargeoprase (from xpuaior, golden or beautiful, and Triaor, a leek) is a rare Fipple-green ealcedony, found in Silesia, which owes its colour to the presence of the metal nickeL—Plasma is another scarce green semitranspareut calcedony, but of a dark tint, which, in the opinion of Mac Culled', is coloured by chlorite.—Sard is a deep reddish-brown variety.
The great supply of the figured agates of commerce is from Ober stein, in the old Palntinato, about 30 miles cast of Treece, and 45 miles south of Coblenz. When they were used as buttons, knife-handles, &c., the trade was more extensive. than nt present They are found in many parts of Scotland, especially at the Hill of Kinnoul, near Perth, where there is an nmygdaloidal trap very full of fine specimens.