ANORTHITE  , an important

mineral of the felspar group, being one of the end members of the plagioclase (q.v.) series . It is a calcium and aluminium silicate, CaAl2SiaOs, and crystallizes in the anorthic system . Like all the felspars, it possesses two cleavages, one perfect and the other less so, here inclined to one another at an angle of 85° 5o' . The colour is white, greyish or reddish, and the crystals are trans-parent to translucent . The hard- ness is 6-61, and the specific gravity 273 . Anorthite is an essential constituent of many basic igneous rocks, such as gabbro and basalt, also of some meteoric stones . The best developed crystals are those which accompany mica, augite, sanidine, &c., in the ejected blocks of metamorphosed limestone from Monte Somma, the ancient portion 9 of Mount Vesuvius; these are Anorthite. perfectly colourless and transparent, and are bounded by numerous brilliant faces . Distinctly developed crystals are also met with in the basalts of Japan, but are usually rare at other localities . The name anorthite was given to the Vesuvian mineral by G . Rose in 1823, on account of its anorthic crystallization . The species had, however, been earlier described by the comte de Bournon under the name indianite, this name being applied to a greyish or reddish granular mineral forming the matrix of corundum from the Carnatic in India . Several unimportant varieties have been distinguished .

(L . J .