ANDESITE, a name first applied by C. L. von Buch to a series of lavas, investigated by him, from the Andes, which has passed into general acceptance as the designation of a great family of rocks playing an important part in the geology of most of the volcanic areas of the globe. Not only the Andes but most of the Cordillera of Central and North America consist very largely of andesites ; they occur also in great numbers in Hungary, Japan, the Philippines, Java, and New Zealand. They belong to all geological epochs and are frequent among the Ordovician and Devonian rocks of Britain, forming the ranges of the Cheviots, Ochils, Breidden hills, and part of the Lake district. The well known volcanoes, Montagne Pelee, the Soufriere of St. Vincent, Krakatoa, Tarawera, and Bandaisan have within recent years emitted great quantities of andesitic rocks with disastrous violence.
They are typical intermediate rocks, containing on an average about 6o% of silica, but showing a considerable range of composi tion. Most of them correspond to the plutonic diorites, but others more nearly represent the gabbros. Their essential distinguishing features are mineralogical and consist in the presence of much soda-lime felspar (ranging from oligoclase to acid labradorite, though phenocrysts may occasionally be still more basic), along with one or more of the ferro-magnesian minerals, biotite, horn blende, augite, and hypersthene. Both olivine and quartz are typically absent, though in some varieties one or other may occur in small quantity. Orthoclase may be present in small amount. In trachyandesites orthoclase and plagioclase are more or less equally developed. These rocks are conspicuously repre sented in the igneous fields of the Auvergne, Siebengebirge, Canary islands, etc, The andesites have mostly a porphyritic structure and the iarger felspars and ferro-magnesian minerals are often visible to the naked eye, lying in a finer ground-mass, usually crystalline, but sometimes to a large extent vitreous. When very fresh they are dark-coloured if they contain much glass, but paler in colour, red, grey, or pinkish when more thoroughly crystallized.
The older (pre-Tertiary) andesites are grouped together by many German and, formerly, by British petrologists under the term porphyrites, but are distinguished only by being, as a rule, in a less fresh condition. Apart from this there are three great subdivisions of this family of rocks, the quartz-andesites or dacites, the hornblende- and biotite-andesites, and the augite- and hypersthene-andesites (or pyroxene-andesites). The dacites, a term first applied by Karl Heinrich Hektor Guido Stache (b. 1833) to quartz-bearing andesites of Transylvania or Dacia, con tain primary quartz and are the most siliceous members of the family; their quartz may appear in small blebs (or phenocrysts), or may occur only as minute interstitial grains in the ground-mass; other dacites are very vitreous (dacite-pitchstones). In many of their structural peculiarities they closely simulate the rhyolites, from which they differ in containing less potash and more soda, and in consequence less orthoclase felspar and more plagioclase. The hornblende- and biotite-andesites, like the dacites, have in most cases a pale colour (pink, yellow, or grey), being compara tively rich in felspar. They resemble the trachytes both in appearance and in structure, but their felspar is mostly plagio clase, not sanidine. The biotite and hornblende have much the same characters in both of these groups of rocks and are often surrounded by black borders, produced by corrosion and partial resorption by the magma. Augite is common in these andesites, but bronzite or hypersthene is comparatively rare. The pyroxene andesites are darker, more basic rocks, with a higher specific gravity, and approach closely to the basalts and dolerites, espe cially when they contain a small amount of olivine. They are probably the commonest types of andesite, both at the present time and in former geological periods.
In addition to the accessory minerals, zircon, apatite, and iron oxides, which are practically never absent, certain others occur which, on account of their rarity and importance, are of special interest. Sharply-formed little crystals of cordierite are occasion ally found in andesites (Japan, Spain, St. Vincent, Cumberland) ; they seem to depend on more or less complete digestion of f rag ments of aluminous rocks in the molten lava. Garnet and sapphire have also been found in andesites, and perhaps have the same signification; a rose-red variety of epidote (withamite) is known as a secondary product in certain andesites (Glencoe, Scotland), and the famous red porphyry (porfido rosso) of the ancients is a rock of this type. Ore deposits very frequently occur in connec tion with andesitic rocks (Nevada, California, Hungary, Borneo, etc.), especially those of gold and silver. They have been laid down in fissures as veins of quartz, and the surrounding igneous rocks are frequently altered and decomposed in a peculiar way by the hot ascending metalliferous solutions. Andesites affected in this manner are known as propylites. (J. S. F.)