NAPOLEONITE, also called corsite, because the stone is found in Corsica, is a variety of diorite characterized by orbicular structure. The grey matrix of the rock has the normal appear ance of a diorite, but contains many rounded lumps 1 or 2in. in diam., 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 min eral makes the pale zones while the dark ones are rich in horn blende 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 are usually separated by small areas of massive diorite. When cut and polished the rock makes a beauti ful ornamental stone.
Spheroidal structure is found in other diorites and in quite a number of granites in Sweden, Russia, America, Sardinia, Ireland, etc. 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 ; it arises evidently from in termittent and repeated crystallization of the rock-forming min erals in successive stages, a process which would be favoured by complete rest, allowing 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 crys tal, sometimes an enclosure of gneiss, etc., and often does not differ essentially in composition from the surrounding rock. When spheroids are in contact their inner zones may be distinct, while the outer are common to both individuals, having the outlines of a figure eight. This proves that growth was centrifugal, not centripetal. (J. S. F.)