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composition. 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-See also:

lime See also:

FELSPAR, or FELDSPAR

felspar (ranging from See also:

OLIGOCLASE

oligoclase to See also:

BYTOWNITE

bytownite and even See also:

ANORTHITE

anorthite), along with one or more of the ferro-magnesian minerals, See also:

BIOTITE

biotite, See also:

HORNBLENDE

hornblende, See also:

AUGITE

augite and See also:

HYPERSTHENE

hypersthene. Both See also:

OLIVINE

olivine and See also:

QUARTZ

quartz are typically absent, though in some varieties they occur in small quantity. See also:

ORTHOCLASE

Orthoclase is more See also:

COMMON

common than these two, but is never very abundant. The andesites have mostly a porphyritic structure, and the larger felspars and ferro-magnesian minerals are often visible to the naked See also:

eye, lying in a finer groundmass, usually crystalline, but sometimes to a large extent vitreous.

When very fresh they are dark-coloured if they contain much See also:

glass, but paler in See also:

COLOUR (Lat. color, connected with celare, to hide, the root meaning, therefore, being that of a covering)

colour, red, See also:

grey or pinkish when more thoroughly crystallized. They See also:

WEATHER (O. Eng. weder; the word is common to Teutonic languages; cf. Du. weder, Dan. veir, Icel. ve8r, and Ger. Wetter and Gewitter, storm; the root is wa- to blow, from which is derived " wind ")

weather to various shades of dark See also:

brown, reddish-brown, See also:

green, grey and yellow. Many of them are highly vesicular or amygdaloidal. The older (pre-See also:

TERTIARY

Tertiary) andesites are grouped together by many See also:

German, and formerly by See also:

BRITISH

British petrologists, under the See also:

TERM

term porphyrites, but are distinguished only by being, as a See also:

RULE

rule, in a less fresh See also:

CONDITION (Lat. condicio, from condicere, to agree upon, arrange; not connected with conditio, from condere, conditum, to put together)

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 See also:

PYROXENE

pyroxene-andesites). The dacites, a term first applied by Karl Heinrich Hektor Guido Stache (b. 1833) to quartz-bearing andesite of Transylvania or See also:

DACIA

Dacia, contain See also:

PRIMARY

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 See also:

minute interstitial grains in the ground-See also:

mass; other dacites are very vitreous (dacitic-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 See also:

PLAGIOCLASE

plagioclase. The hornblende- and biotite-andesites, like the dacites, have in most cases a See also:

PALE (through Fr. pal, from Lat. palms, a stake, for paglus, from the stem pag- of pangere, to fix; " pole " is from the same original source)

pale colour (See also:

PINK

pink, yellow or grey), being comparatively See also:

rich in felspar. They resemble the trachytes both in See also:

APPEARANCE (from Lat. apparere, to appear)

appearance and in structure, but their felspar is mostly plagioclase, not sanidine. The biotite and hornblende have much the same characters in both of these See also:

groups of rocks, and are often surrounded by See also:

black See also:

BORDERS, THE

borders produced by corrosion and partial resorption by the magma. A pale green augite is common in these andesites, but See also:

BRONZITE

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, especially when they contain a small amount of olivine. They are probably the commonest types of andesite, both at the See also:

PRESENT

present See also:

time and in former geological periods. Often their groundmass consists of brownish glass, filled with small microliths of augite and felspar, and having a velvety, glistening lustre when observed in a See also:

GOOD, JOHN MASON (1764-1827)

good See also:

LIGHT

light (hyalopilitic structure). In addition to the See also:

ACCESSORY

accessory minerals, See also:

ZIRCON

zircon, See also:

APATITE

apatite and See also:

IRON [symbol Fe, atomic weight 55.85 (0=16)]

iron oxides, which are practically never absent, certain others occur which, on See also:

account of their rarity and importance, are of See also:

SPECIAL

special See also:

INTEREST

interest. Sharply-formed little crystals of cordierite are occasion-ally found in andesites (Japan,.See also:

Spain, St Vincent, See also:

Cumberland); they seem to depend on more or less See also:

COMPLETE

complete digestion of fragments of See also:

GNEISS

gneiss and other rocks in the molten See also:

LAVA

lava. See also:

Garnet and See also:

SAPPHIRE

sapphire have also been found in andesites, and perhaps have the same signification; a See also:

rose-red variety of See also:

EPIDOTE

epidote (withamite) is known as a secondary product in certain andesites (See also:

GLENCOE

Glencoe, See also:

Scotland), and the famous red See also:

PORPHYRY (IlopcPisptos) (A.D. 233-c. 304)

porphyry (porfido rosso) of the ancients is a See also:

rock of this type. Ore deposits very frequently occur in connexion with andesitic rocks (See also:

Nevada, See also:

California, See also:

Hungary, See also:

BORNEO

Borneo, &c.), especially those of See also:

gold and See also:

SILVER

silver. They have been laid down in fissures as See also:

VEINS

veins of quartz, and the surrounding igneous rocks are frequently altered and decomposed in a See also:

PECULIAR

peculiar way by the hot ascending metalliferous solutions. Andesites affected in this manner are known as propylites. The alteration is one of those See also:

POST

post-volcanic, pneumatolytic processes, so frequent in volcanic districts. Propylitization consists in the replacement of the See also:

ORIGINAL

original minerals of the andesite by secondary products such as See also:

KAOLIN

kaolin, epidote, See also:

MICA

mica, See also:

CHLORITE

chlorite, quartz and See also:

CHALCEDONY, or CALCEDONY (sometimes called by old writers cassidoine)

chalcedony, often with the retention of the igneous structures of the rocks. In microscopic characters the andesites present considerable variety; their porphyritic felspars are usually of See also:

TABULAR

tabular shape with good crystalline outlines, but often filled with glass enclosures.

Zonal structure is exceedingly common, and the central parts of the crystals are more basic (bytownite, &c.) than the edges (oligoclase). Sanidine occurs with considerable frequency, but not in notable amount. The biotite and hornblende are yellow or brown and richly pleochroic. The hypersthene is nearly always idiomorphic, with a distinct pleochroism ranging from See also:

salmon-pink to green. Augite may be green in the more See also:

ACID (from the Lat. root ac-, sharp; acere, to be sour)

acid andesites, but is pale brown in the pyroxene-andesites. The apatite is often filled with minute dust-like enclosures. In the dacites felsitic groundmasses are by no means rare, but micro-crystalline types consisting .of plagioclase and sanidine with quartz are more prevalent. The hornblende- and mica-andesites have groundmasses composed mainly of acid plagioclase with little orthoclase or glassy See also:

BASE

base (pilotaxitic groundmass). Clear brown glass with many small crystals of plagioclase and pale brown augite (hyalopilitic groundmass) is very frequent in pyroxene-andesites. Vitreous rocks belonging to all of the above groups are well known though not very common, and exhibit the perlitic, pumiceous, spherulitic and other structures, characteristic to volcanic obsidians and pitchstones. (J. S.

End of Article: ANDESITE

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