NAPOLEONITE  , also called Corsite because the

stone is found in the island of Corsica, a variety of diorite which is characterized by orbicular structure . The grey matrix of the stone has the normal appearance of a diorite, but contains many rounded lumps 1 or 2 in. in diameter, which show concentric zones of light and dark colours . In these spheroids also a distinct and well-marked radial arrangement of the crystals is apparent . The centre of the spheroid is usually white or pale grey and consists mainly of felspar; the same mineral makes the pale zones while the dark ones are rich in hornblende and pyroxene . The felspar is a basic variety of plagioclase (anorthite or bytownite) . Though mostly rounded, the spheroids may be elliptical or subangular; sometimes they are in contact with one another but usually they are separated by small areas of massive diorite . When cut and polished the rock makes a beautiful and striking ornamental stone . It has been used for making paper-weights and other small ornamental articles . Spheroidal structure is found in other diorites and in quite a number of granites in various places, such as Sweden, Russia, America, Sardinia, Ireland . It is by no means common, however, and usually occurs in only a small part of a granitic or dioritic mass, being sometimes restricted to an area of a few square yards . In most cases it is found near the centre of the outcrop, though exceptionally it has been found quite close to the margin . It arises evidently from intermittent and repeated crystallization of the rock-forming minerals in successive stages .

Such a

process would be favoured by complete rest, which would allow of supersaturation of the magma by one of the components . Rapid crystallization would follow, producing deposits on any suitable nuclei, and the crystals then formed might have a radial disposition on the surfaces on which they grew . The magma might then be greatly impoverished in this particular substance, and another deposit of a different kind would follow, producing a zone of different colour . The nucleus for the spheroidal growth is sometimes an early porphyritic crystal, sometimes an enclosure of gneiss, &c., and often does not differ essentially in composition from the surrounding rock . When spheroids are in contact the%r inner zones may be distinct while the outer ones are common to both individuals having the outlines of a figure of eight . This proves that growth was centrifugal, not centripetal . Many varieties of spheroids are described presenting great differences in composition and in structure . Some are merely rounded balls consisting of the earliest minerals of the rock, such as apatite, zircon, biotite and hornblende, and possessing no regular arrangement . Others have as centres a foreign fragment such as gneiss or hornfels, with one or more zones, pale or dark, around this . Radial arrangement of the crystals, though often • very perfect, is by no means universal . The spheroids are sometimes flattened or egg-shaped, apparently by fluxion movements of the magma at a time when they were semi-solid or plastic . As a general rule the spheroids are more basic and richer in the ferromagnesian minerals than the surrounding rock, though some of the zones are often very rich in quartz and felspar .

Graphic or perthitic intergrowths between the minerals of a zone are frequent . The spheroids vary in width up to 1 or 2 ft . In some cases they contain abnormal constituents such as

calcite, sillimanite or corundum, (J . S .